cytology

CYTOLOGY

HISTORY OF THE CELL

            The term cell means ‘empty box’ or ‘empty space’ which is surrounded by the wall. The cell was first discovered and named by Robert Hooke in1665 when he investigated the thin slices of cork (bark) of tree by using compound microscope. He observed that the cork was composed of box like compartments which were termed as cells. He remarked that it looked strangely similar to cellules or small rooms which monks inhabited, thus deriving the name. However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared under the microscope. The microscope failed to show the internal parts of the cell and succeeded to show only the outer cell wall. The cell wall observed by Hooke gave no indication of the nucleus and other organelles found in most living cells. The first man to witness a live cell under a microscope was Anton van Leeuwenhoek, who in 1674 describe the algae spirogyra.

 

What is common in all cells?

Although cells are diverse, all cells have certain parts or mechanisms in common. These are;

      All cells consist of nucleic acid which contain hereditary materials DNA or RNA.

      The basic structure of membranes and their functions is the same in all cells

      The mechanisms of aerobic respiration and synthesis of nucleic acids and proteins is similar in all cells

 

INTRODUCTION

The term cytology is derived from two Greek words; ‘cyto’ means cell and ‘logos’ means study. Thus cytology can be defined as the study of structure, organization and functions of the cell. Cytology more commonly known as cell biology that is the study of the cell structure, cell composition and interaction of cells and their environment in which they exist. The term cytology can also refer to cytopathology which is the study which analyses the cell structure of diagnose the disease.

 

What is a cell?

            Cell is the basic unit of structure and organization in living organisms or Cell is the basic unit of life of living organisms which carries genetic material and metabolic activities are carried out. Cell is the smallest living unit of an organism which control all life processes and make up the structure therefore it is the basic unit of life. Cell is structural and functional unit of all living organisms. A cell is the unit of protoplasm (cytoplasm and nucleus) enclosed by the cell membrane. In other words a cell may be defined as the basic unit of life which consist of the protoplasm (nucleus and cytoplasm) surrounded by plasma membrane. Protoplasm is the living part of the cell which composed of nucleus and cytoplasm.

            Cells are self-contained units and more or less self-sufficient unit. They are surrounded by plasma membrane (cell membrane) and have nucleus (in eukaryotic cells) or nuclear area where nuclear material are located (in prokaryotic cells). Most of cells are microscopic, sizes ranges normally from 10µm to 20 µm. They show remarkable diversity, in shape and size. They are basically spherical in shape although there are some variations when they are modified by the process of cell differentiation to suit their functions.

            Cell may exist as independent unit of life as in bacteria and certain protoctist such as amoeba, plasmodium, trypanosome etc. They may form colonies or tissues as a fungi, plants and animals. The structure of an organism is composed of cells thus the cell is the building block of an organism. Also the function which carried by an organism (life process) occurs in the cells. These are reasons why cell is said to be basic unit of life.

           

Study questions;

1.     Why cell is said to be a basic structural and functional unit of life?

2.    Why the cell is said to be the self-contained unit?

Answers

1.     Cell is the basic structural unit of life because structure of an organism is composed of cells. In other words cell is the building block of an organism. Cell is the basic functional unit of life because life processes which are carried by an organism they occur in the cells.

IMPORTANCE OF STUDYING CYTOLOGY

      The study of cytology provides detailed knowledge about the cell

      The study of cytology is useful in urine cytology test to revel problems like cancer in the urinary system

      Cytology help in the study of floral (plants) and fauna (animals) cells, since cells are the most fundamental building block of living things.

      The study of cytology help us to know about the chemical composition of the cell and their functions in the cell

      It help us to know metabolic chemical processes take place in the body by studying the chemical constituents of the cell (biochemistry)

 

CELL THEORY

            One of the most important concept in biology is that the cell is the basic unit of structure and function in living organisms. This is known as cell theory.  The cell theory proposes that nucleated cells are the basic structure of plants and animals. This concept was observed and published separately, first by the Belgian botanist, Matthias Schleiden, in 1838, and then by the German zoologist, Theodor Schwann, in 1839. Their work demonstrated that cells form the basic unit of life of plants and animals. Rudolf Virchow concluded that all living organisms are the sum of single cellular units and that cells multiply.  

            By definition Cell theory-is the collection of ideas of long time proposed and agreed by various biologist attempting to describe the meaning of a cell, origin of the cell, function and nature of the cell.

 

Main ideas/ cell doctrines/ tenants of the cell theory

1.       All living organisms are made up of one or more cells

2.      The cell is smallest/fundamental unit of structure and function

3.      All new cells arise from pre-existing cells by cell division

4.      Cells contain the hereditary material of an organism which is passed from cell to cell during cell division (from parent to daughter/offspring cells)

5.      All energy flow processes (metabolic & biochemistry) occur within the cell.

6.      All cells are essentially the same in chemical composition

 

 

Challenges to main ideas of cell theory

01.   All living organisms are made up of one or more cells

Challenge; Viruses exist as living organisms when they are inside the host cell but viruses are not a cell.

02.  New cells arise from pre-existing cells by the process of cell division

Challenge; Failure to state the origin of the first cell which gave rise to the existing cells.

03.  Cells contain hereditary materials of an organism which is passed from one generation to another

Challenge; Organelles such as chloroplast and mitochondria have proved to contain genetic material (circular DNA) although they are not cell.

04.  By definition: A cell is a basic unit of life or  a cell is a basic structural and functional unit of all living organisms

Challenge; Existence of virus a living organism while virus is not a cell and it does not contain cells. Also virus can exist as living organisms but can not carry out metabolism by its own.

 

Exceptions of cell theory

The following does not confirm to the cell theory

A: Virus; Virus lack protoplasm which is the essential point of the cell and where metabolism takes place.

B: Prokaryotic cells; They lack the true nucleus hence lack protoplasm. Prokaryotic cell like bacteria and blue green algae lack well organized nucleus envelop, chromatids (nucleolus) are directly in contact with the cytoplasm.

 

PROPERTIES OF A CELL

01.     A cell is structural and functional unit of living organism

02.    A cell is self-containing/duplicating i.e. dividing itself

03.    New cells are formed by dividing of pre-existing mature cell. Once they are formed, they grow to a certain size and age, from there, they can divide or die.

04.    A cell is made up of living matter called protoplasm which include nucleus and cytoplasm enclosed by the membrane.

05.    A cell is capable of growing and repairing by itself

Note; Cytoplasm contain nucleus and cell organelles. An organelle-Is a distinct part of the cell which has a particular structure and function. Cell organelles are separated by metabolic activities in a coordinated manner. Examples of cell organelles are mitochondria, lysosome, Golgi apparatus, endoplasmic reticulum (E.R) and ribosomes. Nucleus contain genetic material in form of nucleic acid (DNA) which is responsible for transmission of hereditary characters and mechanisms of protein synthesis.

 

Cell size

The size of the varies from small (microscopic) to very large sized. Cell exist in a great variety of size and shape. The smallest size have a diameter ranging from 0.1  0.3 micrometer (0.000001m). 1µ=1/1000000 meter.

 

Examples of different cells with their size

Cell	Size (µm)
Amoeba Human check cell Red blood cell Staphylococcus bacteria Mycoplasma (a bacterium without cell wall)	300 60 7.5 1 0.15

Smallest cells can only be seen with electronic microscope. Cells with relatively large diameter can be seen with light microscope but very large cells such as egg cell of ostrich and other vertebrates, reptiles, amphibians and fishes can be seen with necked (normal) eyes. Some unicellular or one cell organisms such as plasmodium or euglena have definite shape. Even within human body cell exhibit a great range of shape. Cells near surface of the skin are flattened so as to cover the body.

Red blood cells lose nucleus and become biconcave irregular shaped disk when they mature to provide large lumen for the cell to carry large volume of oxygen. Nerve cells are long and thin over a meter in length like electric wire for transmission of nerve impulse. Such biological diversity in shapes of cells relate to their functions.

 

FACTORS LIMITING/DETERMINE THE SIZE OF THE CELL

1.    Relationship between surface area to volume ratio. The volume of a cell reflects the amount of a living materials in it. The chemicals required to keep the cell and material s must enter and leave the cell surface. It follows that the cell must be small. The smaller the size of the cell, the large the surface area to volume ratio for the exchange of materials across the cell surface membrane. As the cell increases in size, its surface area decreases while volume increases. Hence decreasing the ability of the cell to exchange materials between it and environment. In this way most of cells are limited to small size to keep them alive

2.   Nuclear control. All cells are controlled by nucleus, and capacity of nucleus to control the activities of the cell growth to largest, will not be easy for the nucleus to control the activities. If the cell grows to large size having one nucleus it may either divide or die if its nucleus loses its control. Some large and complex cells such as paramecium have two nuclei (macronucleus and micronucleus). These help to carry their activities easily. The macronucleus is polyploidy and controls metabolic activities while micronucleus is diploid and controls reproduction and formation of macronucleus during cell division. Large cells with one nucleus are inactive e.g. unfertilized egg cells.

3.   Cell function/rate of metabolism/cell metabolism. Small sized cells are more active and their rate of metabolism is rapid. To maintain survival of the cells their size are usually maintained smaller and hence survival of the whole organism. A small cell has large surface area to volume ratio to increase the rate of metabolism.

Note; The eggs of birds like an ostrich can grow as large as large possible because their food, the yolk are already inside. They don’t require food from the environment.

 

SIGNIFICANCE OF THE CELL BEING SMALL IN SIZE

01. Cells are microscopic (smaller) so as to maintain a large surface area to volume ratio so that essential substances such as oxygen, nutrients can move inside the cell and waste products such as carbon dioxide exit out across their cell surface membrane fast enough to meet their need.

02.                    A smaller sized cell can be commanded or controlled more efficiently. Only one nucleus can control the activities of a small more effectively than the large sized cell.

03.                    Because of its greater relative surface area, it has a greater opportunity to communicate with its environment.

04.                   To simplify distribution of materials. Small sized cells makes easy the distribution of materials like nutrients through it. Also collection and removal of wastes out of the cell becomes easy hence it ensures the cell’s efficiency.

 

WAYS IN WHICH CELLS INTERACT WTH THEIR ENVIRONMENT THROUGH PLASMA MEMBRANE

01. Passage of water; membrane is freely permeable to water. Water enters the cell from the environment by osmosis.

02.                    Passage of bulk materials; cell membranes sometimes engulf liquids (a process known as pinocytosis) and solids and even other cells (a process known as phagocytosis) as the foreign substance from the environment.

03.                    Selective transport of substances; membranes are very specific about which substances they allow to enter and leave the cell. A cell membrane is selectively permeable membrane. Only small sized ions are allowed to enter and leave the cells.

04.                   Reception of information; membrane acts as receptors. They are sensitive to chemical message. They allow binding of chemical providing information to pass into the cell.

05.Physical connection with other cells; when forming tissues, membranes make special connection with each other.

 

Study questions

01.         Why there is diversity in the size and shape of the cell?

02.        What factors limit cells to their microscopic sizes?

03.        How does the size of the cell affect its surface area?

04.        How would an over large cell solve this problem of low surface area to volume ratio?

05.        Why cells never grow large?

06.        Why studying cytology is important?

07.         Analyze the main ideas of the cell theory

08.        What are the short comings of the cell theory?

09.        The cell is regarded as the basic unit of life. Why?

 

Answers

01. Most of things in the everyday life, in the world, are designed to perform particular function. Therefore the diversity on the size and shape of the cells relate to their functions. E.g. skin cells they are flattened because their function is to cover the body. Nerve cells are long to enable them to transmit the nerve impulse just like in telephone wires. Cells therefore are modified in ways that meet the special needs of the organisms.

02.               Factors that limit the size of the cell

       Nucleus control

      Cell function

      Relationship between surface area to volume ratio

03.               As the size of the cell increases this means even its volume increases and hence this will lead to small surface area to volume ratio but when the size of the cell is small this means it will have small volume leading to large surface area to volume ratio.

04.              The problem of a cell to be overlarge can be solved through nuclear division to form two nucleus within a big cell micronucleus and meganucleus (macronucleus) or through cell division to form two cells.

05.Different regions of the cell need to communicate with each other and its environment in order for cell as whole to function effectively and efficiently. Protein and organelles are being synthesized and materials are continuously entering and leaving the cell. The larger the cell the longer it takes for substances to diffuse between plasma membrane and the centre of the cell. For this reason an organism is made up of many relatively small cells have an advantage over one composed of few large cells.

 

TYPES OF CELLS

All living organisms are made up of cell. Though cells perform similar function of maintaining life, they differ greatly in structure, behaviour and function. Basically there are two types of cells; which are prokaryotic cells and eukaryotic cells.

            Prokaryotic cell; the word prokaryotic is derived from two Greek words which are pro means ‘before’ and karyo means ‘nucleus’.  Therefore prokaryotic means before nucleus. By definition; prokaryotic cell; Is the type of cell which lack true nucleus and internal membrane bound organelles. They are simply cells which lack membrane bound nucleus and have few cell organelles which also lack internal membranes.  The few organelles present are not bounded by a membrane. They have ring or circular DNA with no histone protein. This DNA with no histone lie free in the cytoplasm in a region called nucleoid. Example of prokaryotic cell are cells of bacteria and blue green algae.

            Characteristics of prokaryotic cell

1.       Lack true nucleus

2.      There is no nucleus bound membrane (nuclear envelop). Nuclear material lie free on the cytoplasm in the region known as nucleoid.

3.      They have small 70’s ribosomes

4.      They have few cell organelles which lack internal bound membranes

5.      They lack mitochondria, chloroplast, endoplasmic reticulum, lysosomes, Golgi apparatus, nucleolus, true flagella etc (it has simple flagella).

6.      They are relatively small in size (with average diameter of 0.5µm to 10µm.

7.       They have a single chromosome with a ring or circular DNA which is necked and is not associated with histone.

8.      Prokaryotic cell have cell wall covering the cell membrane. They are made up of complex material known as peptidoglycan or murein.

Peptidoglycan is a polymer consist of polysaccharide and amino acids that form a mesh like layer outside the plasm membrane of prokaryotes. The cell wall acts as an additional barrier against exterior forces.

9.      Most of prokaryotes can secrete slim coating material outside the cell wall. This layer of slim material is referred to as capsule shell. The capsule serves for protection.

10.   If cilia and flagella are present, they are simple and lack 9+2 microtubules arrangement.

Diagram of prokaryotic cell

 

Outside prokaryotic cell; prokaryotic cell is lined with plasma membrane which is similar to that of eukaryotic cell except that there is few types of phospholipids. In some prokaryotic cells (bacteria) outside the cell wall possess a slime capsule which gives them extra protection against ingestion by the phagocyte and prevent them from dying out (desiccation). Some prokaryotic cells (bacteria) have flagella and cilia which are used for movement. These are simple and lack 9+2 microtubule arrangement. They are simple than those of eukaryotic cell which are made of microtubules arrangement in (9+2) pattern.

 

Note; The plasma membrane of prokaryotic cell serves the following.

(i)                 Protect the inner parts of the cell

(ii)              Allow exchange of materials in and out of the cell

(iii)            Offers a site of respiration (in mesosomes) as they lack mitochondria.

(iv)             Offers a point of photosynthesis as they lack chloroplast (in photosynthetic membrane

(v)               It offers a site for nitrogen fixation in the soil. Plasma membrane is able to absorb the atmospheric nitrogen which is then fixed into the soil.

Inside the prokaryotic cell; prokaryotic cell is filled with cytoplasm but there is no true nucleus bound membrane. There is circular DNA which is concentrated in the certain area in the cytoplasm called nucleoid. In addition there is a smaller ring or circular DNA called plasmid that provide resistance to antibiotics together with slime capsule outside the cells. Cytoplasm also contain food storage particles, enzymes and small ribosomes 70’s as cell organelles for protein photosynthesis.  There are small folds of plasma membrane; photosynthetic membrane which carry out photosynthesis to make their own food substances using the pigment called bacteriochlorophyll. This pigment is similar to that of eukaryotic cell but chemically simpler than the chlorophyll of plants (eukaryotic cell). Prokaryotic cells are important in study of biochemistry and molecular biology on account of their:-

·         Simple structure

·         Simple mode of reproduction and transmission of genetic information

·         Rapid growth

 

Physiological features of prokaryotic cell

·      Cytoplasmic streaming is absent as stagnant. Cytoplasmic streaming is the movement of materials from one part of the cell to another.

·      They respire anaerobically due to the absence of mitochondria. They respire using mesosomes.

·      Prokaryotic cells carry out nitrogen fixation, trapping air nitrogen and convert it into nitrate (NO₃).

·      In prokaryotic cells exocytosis are not observed.

·      They reproduce asexually by binary fission. Cell division is neither mitosis nor meiosis.

 

EUKARYOTIC CELL

The word eukaryotic is derived from two Greek words which are euko means ‘true’ and karyo means ‘nucleus’. Thus eukaryotic means true nucleus. By definition, Eukaryotic cell; Is the type of cell which possess true nucleus and membrane bound organelles. Nucleus is enclosed by an envelope (two membranes). It has nucleus bound membrane and many cell organelles are bounded by internal membrane. Eukaryotic cells are regarded as true cells. Eukaryotic cells are found in all animals, plants, fungi and protoctist.

Characteristics of eukaryotic cells

1.       They have true nucleus

2.      Nucleus is enclosed by envelope (two membranes)

3.      They have linear DNA which is associated with histone protein

4.      They have many organelles which are bounded by internal membranes. These are such as mitochondria, endoplasmic reticulum, lysosomes, Golgi apparatus and chloroplasts in plants.

5.      They have large ribosomes (80’s) for protein synthesis

6.      If flagella and cilia are present, they are complex made of microtubules arranged in 9+2 pattern e.g. in trypanosome and euglena their flagella ae more complex than those of prokaryotic cells

7.       Cell wall is present in some but made up of cellulose or chitin.

Diagram of eukaryotic cell

Structure of eukaryotic cell

Outside of eukaryotic cell; the cells of eukaryotes are bounded by cell membrane (plasma membrane). In animal cells and some protoctist, the plasma membrane is the outer covering of the cells. But in plant cells, fungi e.g. protozoa and some protoctist such as green algae e.g. spirogyra, plasma membrane is enclosed by a tough rigid structure called cell wall. Cell wall of plants and green algae is made up of cellulose while that of fungi is made up of chitin. In animal cell and some protoctist, plasma membrane has a role of protection of inner parts and acting as an exchange surface where materials exchange in and out of the cell. In some eukaryotes, outside have flagella and/or cilia which are used as locomotory structure. These structures are complex made up of microtubule arranged in 9+2 pattern. E.g. paramecium which locomote by using cilia, trypanosome which locomote using flagella also euglena has flagella.

Inside of eukaryotic cell; cytoplasm contain a number of cell organelles which are bounded by membrane. Cell organelles are structures in the cytoplasm responsible for carrying out metabolic reaction of cell function. The organelles present inside eukaryotic cells can be categorized into three groups; double membranous organelles e.g. nucleus, mitochondria and chloroplast, single membranous organelles e.g. microbodies, Golgi apparatus, endoplasmic reticulum and lysosomes, non-membranous organelles e.g. ribosomes. Further details on cell organelles will be discussed later in this chapter.

Physiological features of eukaryotic cell;

·         They possess mitotic apparatus, therefore the cell division is by mitosis and meiosis

·   They always show cytoplasmic streaming. Cytoplasmic streaming is the movement of materials/organelles within the cytoplasmic matrix. Cytoplasmic matrix (hyaloplasm/cytosol), is an aqueous material which is a solution or colloidal suspension of many vital cellular chemicals. These include simple ions such as sodium, phosphate and chlorides, organic molecules such as amino acids, ATP, and nucleotides and storage materials such oil droplets. Many biochemical processes include glycolysis occurs within the cytoplasm.

·   They respire both aerobically and anaerobically. Aerobic respiration takes place in the mitochondria.

·         Exocytosis and endocytosis are observed

·         Both sexual and asexual reproduction takes place

Examples of eukaryotic cells are animal cell, plant cell, cells of fungi and protoctist e.g. Amoeba, plasmodium, paramecium, Euglena, trypanosome and green algae.

Difference between prokaryotic and eukaryotic cell

Feature	Prokaryotic cell	Eukaryotic cell
Organisms	Bacteria	Animals, plants, fungi and protoctist
Cell size	Small in size with average diameter 0.5-10µm	Large in size with average diameter 100-1000µm
Form	Mainly unicellular	Mainly multicellular except protoctist (many of which are unicellular)
Cell division	Mostly binary fission; no spindle	Mitosis, meiosis or both; spindle formed
Genetic material	DNA is circular and lie free in the cytoplasm (no true nucleus)	DNA is linear and contained in nucleus
	DNA is necked (not associated with protein or RNA to form chromosomes)	DNA is associated with proteins and RNA to form chromosomes
Protein synthesis	70’s ribosomes (smaller); No endoplasmic reticulum present	80’s ribosomes (larger); Ribosomes may be attached to endoplasmic reticulum
Organelles	Few organelles	Many organelles
	Non are surrounded by an envelope (two membranes)	Envelop bound organelles are present e.g. nucleus and mitochondria
	Internal membranes scarce; if present usually associated with respiration or photosynthesis	Great diversity of organelles bound by single membranes e.g. Golgi apparatus, lysosomes, vacuoles, microbodies and endoplasmic reticulum
Cell wall	Rigid and contain polysaccharide with amino acids; Murein is main strengthening compound	Cell walls of plants and fungi rigid and contain polysaccharide; cellulose and chitin are main strengthening compound in pant and fungi cell walls respectively
Flagella	Simple, lacking microtubules; extracellular (not enclosed by cell surface membrane); 20 nm diameter	Complex with “9+2” arrangement of microtubules; intracellular (surrounded by cell surface membrane); 200 nm diameter
Respiration	Mesosomes in the bacteria, except cytoplasmic membranes in blue-green bacteria	Mitochondria for aerobic respiration
Photosynthesis	No chloroplast, takes place on membranes which show no stacking	Chloroplasts containing membranes which are usually stacked into lamellae or gran.
Nitrogen fixation	Some have ability	None have the ability

Similarities between prokaryotic cell and Eukaryotic cell

·         Both possess a surface membrane enclosing the cytoplasm and acting as a site for exchange of materials in and out of the cell

·         Both possess a cytoplasm where organelles are suspended

·         Both possess ribosomes as the site for protein synthesis

·         Both possess DNA for self-replication

 

ANIMAL CELL AND PLANT CELL

Animal cell; this is an eukaryotic cell characterized by the following;

·         It has no definite and irregular shape

·         It has centrioles for spindle apparatus formation during division

·         It has lysosomes which contain hydrolytic enzymes for digestion

·         It stores carbohydrate in form of glucose

·         It has relatively large nucleus centrally positioned.

 

Structure of animal cell; structurally an animal cell is bounded by the outer surface membrane. This encloses the living part of the cell, cytoplasm, where many cell organelles are suspended. The nucleus is situated anywhere in the cytoplasm but usually around the centre of the cell. Cytoplasmic inclusions such as glycogen granules and vacuole if present, are small and temporary, they may be sometimes absent.

Structure of animal cell as can be seen under electronic microscope.

 

Plant cell; this is an eukaryotic cell which is characterized by the following features.

·      It has fixed (defined) and regular shape

·      It has cell wall made up of cellulose enclosing the cell surface membrane

·      It has plastids such as chloroplasts and leucoplasts. Chloroplast is a site for photosynthesis. Chloroplast contains green pigment called chlorophyll, which is important in absorption of sunlight using for photosynthesis.

·      It has large permanent vacuole bounded by single membrane called tonoplast.

·       It has small nucleus located at the edge (peripheral) site of cytoplasm

·      It has plasmodesmata in the cell wall joining cytoplasm of  adjacent cell

·      It has middle lamella joining the cell wall of adjacent cell

    Structure of plant cell; generally, plant cell is extremely bounded by a non-living cell wall made up of cellulose. Beneath the cell wall is the cell surface membrane which enclose the cytoplasm. In the cytoplasm there are various organelles. Of prominent organelles is the vacuole which occupies a large portion of cytoplasm. The cell wall is porous, it has pits and plasmodesmata. Pits allow movement of materials across the cell wall. It has also middle lamella which joins one cell and another cell. The shape of the cell is fixed and regular.

Structure of the plant cell in electronic microscope

 

Differences between animal cell and plant cell.

Criteria	Plant cell	Animal cell
Size	Plant cells are large in size	Animal cells are generally smaller in size
Cell wall	Cell wall made up of cellulose is present outside the cell membrane	The cell wall is absent. The outermost membrane is plasma membrane
Centrosome (centriole)	Except a few plant, cell centrosome is absent	In some of animal cells centrosomes is generally present
Chloroplast	The photosynthetic cells are green due to presence of chloroplast	The chloroplasts are usually absent from animal cell except Euglena.
Vacuole	Plant cell generally have a large centrally placed vacuole. The protoplasm and the nucleus are present near the cell wall.	Animal cell mostly do not have vacuoles. If present they are small throughout the cell and temporary.
Golgi apparatus	Plant cells have many simpler units of golgi apparatus called dictosome	Animal cell have a single highly complex and prominent golgi apparatus.
Cytokinesis (cell division)	Cytokinesis occurs due to the formation of cell plate	This result to the formation of furrow in the centre of the cell. (cytokinesis is by constriction of plasma membrane)
Food storage	Store food in form of starch	Store food in form of glycogen

Similarities between plant cell and animal cell

·      Both have true nucleus which is bounded by an envelope i.e. two membranes.

·      Both have membrane bound organelles such as mitochondria and nucleus.

·      Both have large ribosomes (80’s) for protein synthesis

·      Both have linear, double stranded helical DNA associated with histone protein

·      Both have cytoplasm through which organelles are suspended

·      Both have cell membrane enclosing the protoplasm which controls the exchange of material in and out of the cell.

Features present in animal cell but absent in plant cell	Features present in plant cell but absent in animal cell
Centrioles, microvilli, pinocytotic vacuoles are more commonly seen in animal cells	Cell wall with middle lamella and plasmodesmata, chloroplasts, large central vacuole (animal cells do possess small vacuoles)

COMPARTMENTALIZATION AND DIVISION OF LABOUR

Eukaryotic cell are far larger and more complex than prokaryotic cells and contain many organelles. The eukaryotic cell has been therefore compared to a factory where although different machines and people have different jobs, all are working together with one purpose. Efficient in eukaryotic cells is improved by division of labour, this is cell specialization. Cell specialization is the straining out of jobs in such a way that in the cell each organelle has its own role involving its own specialized structure and chemistry. For example mitochondrion is the powerhouse of the cell providing energy in form of ATP from the reactions of respiration.

The cell as whole is in effect divided up into compartments, the process known as compartmentalization. Cell compartmentalization refers to the way organelles in eukaryotic cell live and work in separate areas within the cell in order to perform their specific functions more efficiently. This compartmentalization is often achieved by membranes so just as a cell surface membrane controls exchange of materials between the cell and its environment.  Each membrane bounded organelles can have its own particular unique set of chemicals and chemical reactions. Cell organelle; is the distinct part of the cell which has a particular structure and function. All the organelles are contained within the cytoplasmic matrix, sometimes known as cytosol/hyaloplasm.

Cell differentiation is the modification of cells in their shapes and structures, so as to perform a specific function. It is the process in which cells and other parts of the organism become different from one another and also different from their previous condition and acquire ability to perform their special functions. Examples of differentiated cells are male and female gametes, nerve cell, muscle cells, podocytes (excreting cells), and blood corpuscles e.g.  WBCs, RBCs and platelets. Specialized cells work best when organized in groups called tissues. Specialized tissues such as blood, muscle tissues, nervous tissues, conducting tissues (xylem and phloem in plants). Cell differentiation occur during embryonic development.

Question; Eukaryotic cells are compartmentally and highly specialized discuss.

 

ADVANTAGE OF MEMBRANE BOUND ORGANELLES IN EUKARYOTIC CELLS

1.      Many metabolic activities involving enzymes being embedded in a membrane; As the cells become larger the proportion of membrane area to cell volume is reduced. This proportion is increased by the presence of organelle membranes.

2.     Increase the rate of metabolic reaction; this is due to the fact that organelles contain enzymes for a particular metabolic pathway within organelles means that the product of one reaction will always be in closer proximity to the next enzyme in the sequence.

3.     The rate of any metabolic pathway inside the organelle can be controlled by regulating the rate at which membrane surrounding the organelle allow the first reactant to enter.

4.    Potentially harmful reactants or enzymes are isolated inside the organelles so they won’t damage the rest of the cell. Example lysosomes have enzymes enclosed or isolated/restricted from damaging the rest of the other cell organelles.  But if these enzymes could be free without being isolated they could be digesting other cell organelles

5.     Membrane of organelles act as site of exchange of materials just like cell surface membrane that encloses the cell. They allow exchange of materials in and out of the cell organelle, hence regulating the internal composition. E.g. Nuclear membrane allows the regulation of different materials in and out of nucleus.

6.    Membrane of organelles help to offer protection in the inner parts of the organelles. For example nuclear membrane protect DNA and other nuclear material

7.     Membrane of organelles forms a continuous system of communication between one organelle and the other. It makes organelles to be logically proximity such that their functions are very near to each other. For example; an organelle which produces protein, ribosome,  is very near to that which transport the protein out of the cell, endoplasmic reticulum, which is very near to or connected to a nuclear membrane and so on to the surface of the cell membrane.

8.    Excretion of enzymes. The process of enzymes secretion is increased by the presence of membrane. The membrane provide large surface area for secretion of enzymes.

9.    Membrane of organelles help to compartmentalize the cell so that each reaction takes place in one organelle.

 

STRUCTURES, FUNCTION AND ADAPTATION OF DIFFERENT PARTS OF THE CELL

 

1.      CELL WALL

Cell wall is rigid outer layer that surrounds the plasma membrane of plant, fungi, algae and bacterial cell. It is found in plants, fungi, algae and bacterial cells, but absent in animal and protozoan cell e.g. Amoeba. Cell wall of plant is made up of cellulose as strengthening material, cell wall of fungi is made up of chitin, cell wall of algae is made up of cellulose and cell wall of bacteria is made up of peptidoglycan which is the polymer of polysaccharide and amino acids.

PLANT CELL WALL

A plant cell wall is built in series of steps. A cell wall is differentiated into three layers which are; middle lamella, primary layer and secondary layer.

Middle layer; two newly formed cell consist of a middle layer, the one which makes a portion between them. The structure contains a glue substance, a pectin that help to hold the cell together.

Primary cell wall; each cell forms a primary cell wall on its side of lamella. The structure is composed of cellulose, a fibrous material. It is slightly elastic. The elasticity of cellulose allow the cell wall to stretch as cell grows. A primary cell wall is secreted by cytoplasm. It is the result of fusion of Golgi vesicles. This cell wall is the first to be formed after cell division in plants.

Secondary cell wall; when the cell is completely grown in plant that have woody stem and a secondary cell wall is formed. This wall is composed of cellulose and lignin substance that stiffen the cellulose. Woody materials consist of mainly rigid secondary cell wall. Cell wall does not form a continuous layer. It is interrupted by minute pores called plasmodesmata. Plasmodesmata are thin strands of cytoplasm that passes through the cell wall and allow movement of materials between adjacent cells.

Generally; cell wall is composed chemically of carbohydrate (polysaccharide) called cellulose. In addition it has also lignin, pectin, suberin, joining together with magnesium and calcium. Plant cell wall is a porous structure. It has pitches (small pores) which allow free movement of materials. In this way, cell wall does not prevent osmosis.

Structure of plant cell wall; 

a) primary cell wall during cell development

 b) secondary cell wall after maturity of the cell.

 Difference between primary cell wall and secondary cell wall

Primary cell wall	Secondary cell wall
Primary cell wall lies internal to middle lamella	Secondary cell wall lies internal to primary cell wall
It is found in the growing cell during cell division	It is found in cells only when they have matured
It is found in all plant cells	It is found in certain cells only
It is elastic and capable of expansion	It is rigid and incapable of expansion except in collenchyma cells
Cellulose microfibrils are shorter, wavy and loosely arranged	Cellulose microfibrils are longer, closely arranged in parallel and straight
Primary cell wall does not have thickening materials	Secondary cell wall may have thickening materials like lignin and suberin

Function of a cell wall in plants

1.       It gives the shape of the cell i.e. regular fixed shape.

2.      It protects the inner parts of the cell against mechanical damage and entry of pathogens. E.g. cell wall of root epidermal cells are impregnated with suberin that form casparian strips. The presence of casparian strip in the endodermal cells of the roots prevent movement of water into the endodermal cells through cell wall (apoplast pathway). This help to prevent entry of pathogens into endodermal cells and xylem tissues.

3.      Cell wall is fairly rigid and resistant to expansion and therefore allows development of turgidity when water enters the cell by osmosis. This contributes to the support of all plants and is the main source of support in herbaceous plants and organs such as leaves, which do not undergo secondary growth. The cell wall also prevent the osmotic bursting of the cell when exposed to dilute solution.

4.      Assist in cell division in plants by forming cell plate.

5.      Cell wall develops a coat of waxy cuticle on exposed epidermal surfaces reducing water loss and risks of dehydration. Cork cell walls undergo impregnation with suberin which serves a coating function after secondary growth.

6.      It helps to control cell growth. Due to orientation of cellulose microfibrils limits and helps to control cell growth and shape because the cell’s ability to stretch is determined by their arrangement.

7.      Some cell wall are modified as food reserves as in storage of hemicellulose in some seeds.

8.     It is the pathway for movement of water and mineral salts. Water and mineral salts move along interconnected (held together) by middle lamella. This movement is called apoplast movement. The cell wall also possess minute pores called plasmodesmata forming living connection between cells and allowing the protoplasts (cytoplasm) to be linked. This enables transport from cytoplasm of one cell to another through plasmodesmata, a movement called symplast movement. Cell wall of xylem vessels and sieve tubes are adapted for long distance translocation of materials through the cells.

9.      Cell wall provide mechanical strength and skeletal support to individual cells and for the plants as whole. Extensive lignification increases strength in some walls of shrubs and trees.

10.  Cell wall of transfer cells develop an increased surface area and the subsequent increase in surface area of the cell surface membrane increases the efficiency of transfer by active transport.

 

Adaptations of cell wall to its functions

1.       Presence of cellulose and lignin impregnation help to increase mechanical strength hence provide support against mechanical stress.

2.      Presence of middle lamella adjoining neighboring cells and the pores through plasmodesmata connecting adjacent cytoplasm of cells, facilitate movement of water and salts between cells hence increase survival of the cell.

3.      Presence of suberin impregnation on epidermal cell and waxy cuticle on the epidermal cells of leaves help to prevent excessive loss of water hence reduce risk of dehydration and also prevent the cell from the risk of infection.

4.      Presence of cellulose which is rigid and hard in nature to help the cell to maintain the shape of the cell.

5.      Presence of cellulose and lignin material increase rigidity. It act as a pressure vessel preventing excess water to enter the cell by osmosis. This ensures long survival of the plants even if they are subjected to hypotonic solution.

 

1.      CELL MEMBRANE

Cell membrane is an external thin structure enclosing a mass of protoplasm of a cell. The cytoplasm of the cell is surrounded by cell surface membrane. Cell surface membrane is the actual outermost protoplasmic layer of the cell. It is universal characteristic of all cells and it serves to separate and protect a cell from its surrounding environment. The cell surface membrane also known as plasma membrane or cytoplasmic membrane or cell membrane or plasmalemma, is believed to be formed by the cytoplasm as its own outer margin and it is made mostly from a double layer lipids (phospholipids).

The cell surface membrane has been known longer and studied more intensively than internal cell membranes. Thus the cell have many membranes besides the outer one. The membrane is said to be semi-permeable in that it can either let a substance or molecule or ion to pass through freely, pass through limited or not pass through at all. The cell membrane is able to control in and out movement of materials because embedded in the phospholipid bilayer of the cell membrane are variety  of protein molecules that act as the channels and pumps that moves different molecules into and out of the cell.

       In animal cell and some protozoans e.g. amoeba, it is an outer boundary but in plant cell, fungal cell, algae cell and bacterium cell it is located beneath the cell wall. Other membranes include nuclear membrane (envelop), tonoplast, which encloses a vacuole of plant cell and the membrane of various cell organelles such as endoplasmic reticulum (E.R), Golgi apparatus, mitochondria, chloroplasts, and lysosome.

 STRUCTURE OF THE MEMBRANE

There are two models that describes the structure of the cell;

(i)                Daniel Davison model

(ii)              Fluid mosaic model

Daniel Davison model; according to Daniel Davison model the membrane structurally is made up of two layers; phospholipid bilayer and two protein layer. Phospholipid bilayer is sandwiched between two protein layers. A unit membrane has protein-lipid-protein. Protein molecules layers is located outside the membranes. The protein layer is continuous and membrane therefore static and lack pores. The lipids are associated with phosphate group forming phospholipids bilayer. Phospholipid molecules have got two parts, the polar head (hydrophilic, water loving) and non-polar tail (hydrophobic, water hating).

Strength of Daniel Davison model

(i)           It true that the membrane is chemically composed of protein and phospholipid bilayer.

(ii)         It is also true that phospholipids have polar head and non-polar tails.

 

Weakness of the model

(i)                It is not true that the membrane is static. The membrane is always over changing/dynamic in structure.

(ii)              Daniel Davison fails to indicate the presence of pores in protein layer in the membrane. It is not true that the protein layer is continuous without pores. Protein layer in the membrane is ideal always not continuous and here are pores within a protein molecules and between adjacent protein molecules.

(iii)           It does not indicate presence of carbohydrate whereas membrane always contain carbohydrates branching from lipids and proteins which serves as receptor sites.

(iv)            It does not indicate the presence of cholesterol.

 

Fluid mosaic model; this model was proposed by J. singer and Nicolson (1972) to modify the Daniel Davison model. According to this model, the membrane has got three layers namely lipid, protein and carbohydrate which branches from protein and lipid molecules. The protein molecules (mosaic) are floating over the surface of phospholipids bilayer (fluid), just like boats can float on water, hence the name fluid mosaic model (the scattered protein resembles a mosaic but since the phospholipid layer is fluid, the protein forms fluid mosaic pattern).

Protein molecules are arranged in mosaic pattern which floats in a fluid lipid layer. The fluid mosaic model membrane confirms that the membrane is made up of phospholipid layers, carbohydrate molecules which form branches from lipid and protein molecules. Protein does not form continuous layer covering between sides of the membranes as proposed by Daniel Davison. The membrane is regarded as dynamic over changing structure in which protein floats on the bilayer of phospholipids. The lipids also move about. Membrane also consist of cholesterol (type of lipid which increase fluidity of the membrane). The experimentation on viscosity suggest that a membrane is of fluid consistence and not at all. Thus there is a considerable sideways movement of lipids and protein within the membrane. Due to the presence of fluid (fluidity) and the mosaic arrangement of protein molecules, this model of membrane structure is called fluid mosaic model. This model is found to be applied to all biological membranes in general as it is seen as a dynamic over changing structure. There are hydrophilic pores (polar pores) which occur within a protein (the channel protein) or between adjacent proteins. These pores allow passage of polar molecules such as glucose and water through the membrane, a hydrophobic interior. In the absence of these pores, polar molecules would be prevented from entering the membrane by the lipids. Fluid mosaic model is therefore a widely accepted model for structure of cell membrane. It is based in phospholipid bilayer which forms a framework of the membrane. Phospholipid bilayer is fluid. Proteins are randomly floating on it. The scattered protein resembles a mosaic but since the phospholipid layer is fluid, the protein forms fluid mosaic pattern.

 

 

FEATURES OF THE FLUID MOSAIC MODEL OF THE MEMBRANE STRUCTURE.

·           It is a thin structure about 7nm thick

·           The basic structure is the phospholipid bilayer

·           The hydrophilic phosphate heads of phospholipids face outward into the aqueous environment inside and outside of the cell

·           The hydrocarbon tail face inwards and create a hydrophobic interior

·           The phospholipids are fluids and move about rapidly by diffusion in their own layer

·           Some of the fatty acid tails are saturated and some are unsaturated. Unsaturated tails are bent and fit together more loosely. Therefore the more unsaturated the tail are, the more fluid the membrane is.

·           Most protein molecules float about in the phospholipid bilayer forming a fluid mosaic pattern

·           The proteins stay in the membrane because they have regions of hydrophobic amino acids which interact with the fatty acid tails to exclude water. The rest of the protein is hydrophilic and faces into the cell or out into the external environment, both of which are aqueous.

·           Some proteins penetrate only part of the way into the cell membrane while others penetrate all the way through.

·           Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoproteins and glycolipids respectively.

·           Membrane also contain cholesterol. Like unsaturated fatty acids, cholesterol disturbs the close packing of phospholipids and keeps them more fluid. This can be important for organisms living at low temperatures when membranes can solidify. Cholesterol also increases flexibility and stability of membranes. Without it, membrane break up.

·           The two sides of the membrane can differ in composition and function.

 

Please refer to diagram (fig 5.16 b) and the question 5.3 from BS page 142

 

Membrane lipids; the great majority of lipids in the biological membrane is phospholipids and cholesterol. The most common phospholipid in the membrane is phosphatidyl ethanolamine.

 

From the structure above, phospholipids are amphipathic or amphiphilic compounds; that are compounds with both water loving (hydrophilic) and water hating (hydrophobic) regions. The large non polar part of the phospholipid do not dissolve/form reaction with water but associate easily with other fatty materials. The phosphorus containing region of the phospholipid is electrically charged hence very hydrophilic (is able to interact with water). Thus the phosphorus containing region is polar (charged) and is known as polar head while the two fatty acids of the phospholipid are non-polar (not charged) and are known as non-polar tails. The cell membrane is phospholipid bilayer, so because it is made up of two layers of phospholipid molecules. The phospholipid bilayer membrane is formed when the fatty acids of two layers are towards the centre and the polar heads (charged region) facing outside separating two aqueous watery regions. When the phospholipids are exposed to water, they arrange themselves into two layered sheet (a bilayer) with all of their tails pointing towards the centre of the sheet. The centre of this bilayer contains almost no water and excludes molecules like sugars or salt that dissolve in water but not in oil. Certain kind of membrane protein are involved in the process of fusing two layers together. Cholesterol, which help to strengthen the bilayer and decrease its permeability, also help to regulate the activity of a certain integral protein.

 

FUNCTION OF EACH COMPONENT OF CELL MEMBRANE

(i)    Membrane lipids

Lipid constitutes about 45% of the total chemistry of the cell membrane. There are three types of lipids;

(a)  Phospholipids (lipids associated with phosphate group)

(b)  Glycolipids (lipids associated with carbohydrate)

(c)   Cholesterol (steroids) act like a plug of lipids by reducing even further the escape of polar molecules through the membrane.

 

A)   Phospholipids; there are two layers of phospholipid molecules hence phospholipid bilayer. They exist as fluids in the membrane and they are always in constant motion. Phospholipids makes the basic structure of the membrane. They acts as the flame work since they are in fluid state they help to determine the fluid nature of the membrane. Phospholipids make the outer and inner layers of the membrane and this acts as barrier to the entrance of some molecules. Phospholipids therefore affects the fluidity and permeability of the membrane.

 

Functions of phospholipids

(i)           Provide basic structure of the membrane.

(ii)         Determine the fluidity of the membrane.

(iii)       Allow the passage of soluble substances across the membrane.

 

B)   Glycolipids; these are formed when carbohydrate molecules attach on the lipid molecules (in this case phospholipid). In other words glycolipids is formed when carbohydrate molecule attach on phospholipid molecule. Glycolipids are found only on the external surface of the cell membrane.

 

Function of phospholipids

(i)           Acts as recognition sites (receptor) for recognizing external stimuli.

(ii)         Helps to make the membrane more stable i.e. strengthen the membrane

 

C)   Cholesterol; are lipid (steroids) which are embedded in the phospholipid bilayer.

 

Function of cholesterol

(i)           They make the phospholipids bilayer stronger and more flexible

(ii)         They make the membrane less fluid at high temperature and more fluid at low temperature.

(iii)      They make the membrane less permeable to water soluble substances e.g. glucose

(iv)       They maintain the fluidity of the membrane as they keep a part of phospholipid molecule, cholesterols disturb the packing of the phospholipids.

 

In the absence of cholesterols

(i)           The membrane would be too fluidly and would burst just like soap bubble at high temperature.

(ii)         The membrane would solidify and collapse at low temperature.

 

(ii)           Membrane proteins

Protein consist about 45% of the total chemical substances of the membrane. Embedded on or within the phospholipid bilayer of the membrane are variety of protein molecules that act as channels and pumps to move different materials into and out of the cell. Two types of proteins (according to the location) are found in the cell membrane.  These are;

1.                  Intrinsic proteins and

2.                 External proteins

 

Intrinsic membrane protein; These are proteins which penetrate into the phospholipid bilayer and may even extend from one side to the other. The proportion of the protein molecules that resides within the lipid bilayer is very hydrophobic (has a high content of hydrophobic amino acids)

Extrinsic membrane proteins; these are proteins located on the outside of the phospholipid bilayer and are attached to the rest of the membranes by weak (noncovalent) bonding with the intrinsic protein or with hydrophilic amino acids.

NOTE;

●     Membrane proteins are free to move laterally through the lipid bilayer. Thus a lipid bilayer is especially a film of oil structures immersed in it would be relatively free to float about it.

●     Also protein composition of the membranes of various organelles differ because the membrane perform different biochemical reactions i.e. respiration, photosynthesis etc. Also lipids and proteins found in the cell membrane differ from those found in the internal membranes.

●     Lastly some proteins may be confined to one part of the cell surface rather than scattered evenly about

●     According to fluid mosaic model the protein layer is not continuous. It is a collection of protein globules that float over the phospholipid bilayer.

 

 Roles of proteins in the membranes

1.    Maintain the structure of the membrane

2.   Act as enzyme and for some reactions within the membrane such as microvilli on epithelial cells lining on some parts of the gut containing digestive enzymes in their cell surface membrane.

3.   Act as recognition sites that is act as receptor site for chemical signals from other parts of the body. These proteins in the membrane receive chemical signals from the cell's external environment and respond by regulating a certain intracellular processes. Thus proteins acts as receptor sites in the membrane. A receptor molecule is a molecule usually found on the surface of cell membrane. It receives chemical signals from outside the cell, when such external substance (may be an enzyme, hormone, pharmaceutical drug or toxins) attach to the receptor protein they direct a cell to do something i.e. divide, die or to allow specific substance to enter or exit the cell.

4.   Act as energy transducers and electron carrier

5.   They form channel or pathways through which materials pass across the membrane. These are involved in the selective transport of polar molecules and ions across the membrane by facilitated diffusion and active transport. In other words act as pump for moving materials through or across the membrane.

6.   Protein such as antigens acts as cell identity marker. They are glycoproteins that is protein with branching carbohydrate side chains like antennae. There is an enormous member of possible shapes to these side chains, so each type of cell can have its own specific maker. This enables cell to recognize other cells and behave in an organized way. For example during the development of tissues and organs in multicellular organisms and recognition of foreign antigens so as to be attached by immune system.

7.    Proteins associate with carbohydrate to form glycoprotein. The hydroxyl groups of the carbohydrates increase the hydrophilic nature of the membrane

 

3.        Membrane carbohydrate

The carbohydrates are attached to either lipid or protein molecules and they are confined to the outside of the plasma membrane, where they protrude into the cell’s environment. Carbohydrate branch from lipid (phospholipids) and protein forming glycolipids and glycoproteins respectively. They branch like antennae which extend outside the membrane. They act as recognition sites for molecules.

 

Roles of carbohydrates in the membrane

·  Carbohydrates acts as receptor sites in the membranes in other words carbohydrates act as recognition sites for foreign substance and cells, thus carbohydrates are important in all sorts of recognition reaction at the cell surface.

 

 

STRENGTH OF FLUID MOSAIC MODEL

1.      It is true that the membrane is made up of protein, lipid and carbohydrate.

2.     It is true that protein layer is not continuous

3.     It indicate the presence of pores in the  membrane

4.    It indicate that the membrane is not static rather it is dynamic ever changing structure. This is true because the membrane is always in constant motion.

 

GENERAL FUNCTION OF THE CELL SURFACE MEMBRANE.

1.    It controls materials entering and leaving  the cytoplasm (passage of materials in and out of the call) or Restricts entry and exit  of polar molecules and  ions

2. It acts as a receptor site for external changes (stimulus), proteins have very specific shapes, and this makes them ideal as receptor molecules for chemical signals between cells. Examples;

·  Hormones are chemical messengers which circulate in the blood but only bind to specific target cells which have the correct receptor sites.

·  Neurotransmitters, the chemicals which enable the nerve impulses pass from and one nerve cell to the next, also fit into specific receptor proteins in nerve cells.

3. It separates cytoplasm and its inclusion from external environment (separate content of  the cell from external environment)
4. It allow interaction with the other cells (media of  interaction between cells)
5. Cell membrane makes separate compartments inside the cell which specialize it in specific metabolic pathways e.g. photosynthesis in chloroplasts and respiration in mitochondria.
6. Act as the site of occurrence of different metabolic reactions e.g. light reaction of photosynthesis on chloroplasts and oxidative phosphorylation of respiration in mitochondrion takes place in the membranes.
7. Cholesterol acts like a plug, reducing even further the escape or entry of polar molecules through the membrane.
8. Channel proteins and carrier proteins. These are involved in selective transport of polar molecules and ions across the membrane e.g. facilitated diffusion and active transport.
9. It is used in energy transfer system in photosynthesis and respiration that exit in the membrane of chloroplasts and mitochondria.
10.  Glycolipids have branching carbohydrate side chains and are involved in cell-cell   recognition. They may act receptor sites for chemicals signals with glycoproteins they are involved in sticking the correct cells together in tissue.
11. Antigens. These acts as cell identity markers or name tags. They are glycoproteins that is protein with branching carbohydrate side chain like antennae. There is enormous number of possible shapes to these side chains, so each type of cell can have its own specific markers. This enables cell to recognize other cells and to behave in an organized way. For example during development of tissues and organs in multicellular organisms. It also means that foreign antigens can be recognized and attached by immune system.
12. Enzymes. Proteins sometimes act as enzymes for example microvillus epithelial cell lining some parts of the gut containing digestive enzymes in the cell surface membrane.

 

Adaptations of the cell membrane to its functions

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Study questions

1.      Name the major chemical constituents of the cell membrane structure and state the function of each.

2.     Discuss the structure of the membrane of Singer & Nicolson model and point out its strength and weakness.

3.     (a) Give two functions of glycoproteins in the cell membrane

(b) Briefly explain why the cell surface membrane are described as having fluid mosaic structure

(c) Outline the differences between Daniel Davison and fluid mosaic model of membrane structure

 

THE PROTOPLASM

The protoplasm is the living part of the cell which is made up of nucleus and cytoplasm. The nucleus and cytoplasm together determine the life of the cell. It is enclosed by the cell surface membrane.

Qn:- Cytoplasm without nucleus is useless and nucleus without cytoplasm is short lived. Explain. Answer; This statement implies that both nucleus and cytoplasm determine the life of the cell. The nucleus controls all what is presented in the cytoplasm whereas the cytoplasm support the nucleus.

 

CYTOPLASM

Cytoplasm is slightly viscous transparent fluid which is often granular. It is made up of soluble part called cytosol and insoluble part made up of living and non-living structure. The living part of cytoplasm is made up of structures of different shapes and sizes called cell organelles. The non-living structure of cytoplasm called cytoplasmic inclusion they include starch grain, glycogen granule, oil droplet and crystal of substance to be excreted. The chemical substance such as ATP, proteins do not dissolve completely forming physical solution.  They rather remain dispersed as colloid. This account for the colloidal nature of cytoplasm.

Function of cytoplasm

·         It is the site of metabolic activities. Most of chemical reactions takes place in the cytoplasm e.g. glycolysis.

·         It is stores biochemicals of life such as starch, glycogen, oil droplet etc.

·         It is the media where's intensity of various chemical substances takes place e.g. protein synthesis and fatty acids.

·         Forms the ground substance located between cell organelles

·         It is where cell organelles are suspended which perform different activities such as mitochondria where aerobic respiration takes place, ribosomes where protein synthesis takes place.

·         It facilitates back transport of materials between adjacent cells (cytoplasmic streaming).

 

STRUCTURE AND FUNCTIONS OF CELL ORGANELLES AND CELL INCLUSIONS (SUBCELLULAR UNITS).

Living part of cytoplasm

A. Cell organelles

An organelle is a specialised subunit within a cell that has a specific function and is usually separated enclosed within its own membrane. Both eukaryotic and prokaryotic cells have organelles, but organelles in eukaryotes are generally more complex.

Types of cell organelles

There are several types of cell organelles in the cell. Cell organelles can be categorized into three major groups which are;

a)     Double membranous organelles search is mitochondria, nucleus and chloroplast.

b)     Single membranous organelles such a smooth and the rough endoplasmic reticulum, large permanent vacuole, Golgi apparatus, peroxisomes/microbodies, centrioles and lysosomes.

c)      Non membranous organelles includes ribosomes and microtubules.

 

Double membranous organelles

1. Nucleus

Nucleus is central core of eukaryotic cell. It is the largest structure of cell organelles. Normally a cell contain only one nucleus however in some protozoan such as paramecium there are two nuclei namely macronucleus and micronucleus. The nucleus is found in eukaryotic cell except in mature phloem sieve tube element and a red blood cell (RBCs) of animals. Nucleus is nicknamed as ‘brain of the cell’ or ‘control centre of the cell’ because it control all activities of the cell.

Structure of nucleus

The nucleus is typically spherical or oval in shape bounded (enclosed) by a double membrane (envelope known as nuclear envelope). The nuclear pores allow exchange of materials between the nucleus and cytoplasm e.g.  The entry and exit of messenger RNA (mRNA) or newly made ribosomes can leave through nuclear pore and molecules such as proteins and nucleotides needed for the manufacture of ribosomes and DNA. The outer membrane is continuous with endoplasmic reticulum that is covered with ribosomes for tearing out protein synthesis. Inside the nucleus there is a matrix called nucleoplasm or nuclear sap-is the jelly like fluid which contains one or more nucleoli together with chromatin, it also contain soluble materials of the nucleus e.g. ions, proteins, nucleotides and enzymes. The nucleoplasm containing chromatin and nucleoli (singular nucleolus).

            A chromosome is very long DNA molecules and the associated histone protein that carry portion of hereditary information (genes) of an organism. In other words chromosomes is a thread-like structure which carry unit of inheritance (genes). A gene can be defined is the region of DNA that controls hereditary characteristics. It is usually corresponds for a sequence used in the production of a specific protein or DNA. A gene carries biological information in a form that must be copied and it transmitted from each cell to its progeny.

Diagram showing the structure of nucleus

Nucleolus

Nucleolus this is the round body without a membrane which looks like a large dark spot when you received by a microscope within the nucleus. A nucleus may contain up to four nucleoli but within each species the number of nucleoli is fixed. The function of nucleolus is to synthesize ribosomal RNA or ribosomes (the protein producing organelles). It is strains intensely because of large amount of DNA and RNA it contains. The densely straining core of nucleolus is made up of the DNA from one of the several chromosomes. This contain many copies of genes that code for the RNA needed to make ribosomes (ribosomal RNA or rRNA). During nuclear division nucleoli seem to disappear but this is because the DNA disperses. They reassemble after division. Around the central core of nucleolus is a less dense region where ribosomal RNA is beginning to be folded and the combined with proteins to make ribosomes. The partly assembled ribosomes move out through the nuclear pores into the cytoplasm where assembly is completed.

 

Function of nucleus

1.       It contains DNA which is organized into genes controlling all activities of the cell e.g. growth, cell division, protein synthesis etc. (save as the information processing and administrative centre of the cell).

2.      Contains the genetic material in form of chromosome which control inheritance of characteristics.

3.      It is involved in production of ribosomes (ribosomal RNA) and messenger RNA.

4.      Nuclear division in the basis of cell replication and hence reproduction.

5.      All genetic variations are caused by changes in the genetic material present in the nucleus. These genetic variation contribute to evolution.

 

Adaptation of nucleus to its functions

1.       It has nuclear envelope which is perforated by nuclear pores which allows the exchange of materials between the nucleus and cytoplasm e.g. exit of messenger RNA and ribosomal subunits, and entry of ribosomal proteins, nucleotides and other molecules.

2.      The nucleus houses chromosomes containing DNA. DNA holds heredity information and instructions for cell growth, development, and reproduction.

3.      Presence of DNA associated with protein molecules to form chromosomes responsible for carrying genetic material from parents to offspring.

4.      It is covered with rough endoplasmic reticulum responsible for protein synthesis. This also allows the transfer of materials as well.

5.      The nuclear envelope help to maintain the shape of the nucleus.

6.      The nucleolus contains nucleolar organizers, the parts of chromosomes carrying the genes for ribosome synthesis. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These subunits join together to form ribosomes during protein synthesis.

7.      The nucleoplasm supports the nucleus to hold its shape. It also provides a medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported throughout the nucleus to its various parts.

8.     Nucleoplasm contain enzymes which catalyse transcription of DNA into messenger RNA used for protein synthesis.

 

2.  Mitochondrion (plural Mitochondria).

Mitochondrion in the sausage or rod-shaped cell organelle bounded by two thin membrane (double membrane) which form an envelope. The membranes are separated by narrow space, the intermembrane space. The inner membrane is folded into a number of shelf like structures called cristae. The cristae increases its surface area, providing space for the components of respiratory chain which are located in the membrane. Mitochondria are most important organelles in which aerobic respiration takes place. Cristae are the site for the oxidative phosphorylation and electron transport chain (respiratory chain). The inner membrane encloses a cytoplasm like structure called matrix. The matrix is the site of Krebs cycle reaction and thus mitochondria matrix contains most of the enzymes of the Krebs cycle. Fatty acids oxidation also takes place here. The matrix also contains a few ribosomes (70's) for protein synthesis, circular DNA molecule which is capable of self-replication, the phosphate granule and ADP (adenosine diphosphate).

Note;

Mitochondrial ribosomes have the same size at the bacterial ribosomes (prokaryotic cell) and mitochondrion is shape the perfect tree to maximize its effects. The organelle is therefore nicknamed powerhouse of the cell.

 

Compartments of mitochondria

Mitochondria is composed of five compartments that carry out specialised functions. These are as follows;

1.       Outer membrane; this is an outer membrane that encloses the entire organelle. It's protein to phospholipid ratio is similar to that of eukaryotic plasma membrane. It contains large number of integral proteins call the parents which form channels that allow molecules to diffuse from one side of the membrane to the other. Disruption of the outer membrane permits proteins in the intermembrane space to leak onto cytosol reading to a certain cell death called apoptosis.

2.      Intermembrane space; this is the space between the outer and inner membrane, also known as perimitochondrial space. Since the outer mitochondrial membrane is permeable to small molecules the concentration of molecules like ions and sugar in the intermembrane space is the same as the cytosol. However large proteins must have a specific signaling sequence to be transported across the outer membrane. One protein that is localised to the intermembrane space is cytochrome oxidase.

3.      Inner membrane; it is rich in phospholipids, lacking integral proteins. Inner membrane contains five complexes of proteins, namely; cytochrome reductase, cytochrome oxidase, flavoprotein, NADH₂ dehydrogenase and ATP synthase.

4.      Cristae; they are formed when the inner membrane folds to form shelf like cristae projecting into the matrix. They are studded with proteins including ATP synthase. Cristae increases the surface area for chemical reaction to occur.

5.      Matrix; this is viscous than a cytoplasm. It contains some enzymes that catalyzes the oxidation of pyruvate and other small organic molecules. Also contain mitochondria DNA and ribosomes. Mitochondria proteins vary depending on the tissue and the species. Example in human 615 distinct types of proteins have been identified from cardiac mitochondria. In the matrix pyruvic acid is oxidized by NAD⁺ and FAD⁺ producing NADH+H⁺ and FADH+H⁺. Also decarboxylated to produce three molecules of carbon dioxide and ATP molecules.

Note; in eukaryotes, glycolysis occurs in cytoplasm while the remaining two stages of respiration takes place in the mitochondria.

Diagram showing the structure of mitochondria.

Distribution of mitochondria in cells of a multicellular organism.

Mitochondria are present in all eukaryotic cells and are the major sites of aerobic respiration within cells. The number of mitochondria per cell varies considerably depends on the type of organism and the nature of the cell. Cells with high energy requirements process large numbers of mitochondria, for example liver cells contain upward of 1000 mitochondria list active service process far fewer. Mitochondria are often packed close together in a cell like spermatozoa (sperm cell), contractile fibrils in muscles and areas where active transport or absorption occurs e.g. in ileum, nephron, roots epidermal cell, companion cells etc. If the number of mitochondria in the cell is fewer, that is cell not getting enough energy to survive, more mitochondria can be created.

LOCATION

Mitochondria are largely found in does body cells whose energy demand is high. Example;

·      Nerve cell for active transportation of sodium and potassium ions across the axon membrane by sodium-potassium pump.

·      Cardiac muscle for pumping blood in the heart.

·      Skeletal muscle cell for movement.

·      Nephron for active transportation of molecules from glomerular filtrate into the blood capillaries.

·      Sperms for beating their flagella during movement.

·      Companion cells in plants where large amount of energy is to be generated for active loading of sieve tube.

 

Function of mitochondria

1.         It the site for aerobic respiration and it is where ATP is synthesized. Mitochondria produce chemical energy in the form of adenosine triphosphate (ATP). This is the main function of mitochondria thus they convert potential energy of food molecules into ATP. A complete aerobic oxidation respiration of one molecule of glucose yield 38 ATP molecules. 36 of these are made available in the mitochondria. That's why mitochondria is referred to as powerhouse of the cell or power plant of the cell.

2.        It is involved in protein synthesis due to the presence of circular DNA and ribosomes.

3.        Controls heredity due to the presence of DNA material.

4.        Help in plants in the synthesis of rubber.

5.        It help in the process of antibody formation in animals.

6.        Help with the metabolism of calcium.

7.        It help in the control of water in the cytoplasm.

8.       Act as storage path of phosphate (phosphate granule).

9.        It form a middle piece of spermatozoon during maturation.

10.    Help in yolk formation during development of ovum (egg).

11.     Mitochondria involved in controlling cell cycle and cell growth.

12.    Mitochondria involved in cellular differentiation. Some cells differentiate by processing either many or few mitochondria.

Note; with all the functions listed above the major function of mitochondria is to generate energy in the form of ATP. It is referred to as powerhouse of the cell. Most of chemical bond energy originally present in respiratory substrate is made available to the tissues by mitochondria.

 

Adaptations of mitochondria to the functions it perform

1.       It has permeable double membrane which allow diffusion of materials in and out. They allow entry of oxygen and acetylene coenzyme A and also exit of carbon dioxide and waste formed during respiration process.

2.      Presence of cristae increasing the surface area for the enzyme attachment hands increase the rate of respiration.

3.      Matrix consisting of different enzymes for the process of ATP formation such as Krebs cycle.

4.      It has the DNA (circular DNA) RNA and ribosomes which play a role in self-replication and protein synthesis.

5.      It is remembering a flexible. This enables it to move place where energy is needed.

6.      It has phosphate granule and ADP for formation of ATP.

7.      It has small size which gives it large surface area to volume ratio to allow rapid uptake and release of materials.

8.     It is enclosed by two membrane. Discount to separate it from other process in the cytoplasm.

9.      Presence of integral proteins in the membrane which form channels for the transportation of materials across the mitochondria membrane.

10.  The matrix is an aqueous (fluidly) allowing enzymes to work effectively.

 

Endosymbiotic hypothesis

According to social endosymbiotic theory (SEST) mitochondria is considered as prokaryotic cell that has invaded in eukaryotic cell. The theory suggests that mitochondria and plastids (chloroplasts) represent a formerly free living bacteria that were taken inside another cell as an endosymbiotic (an organism that lives within the body or cell of another organism). Thus mitochondria and chloroplast descended from bacteria that somehow survived endosymbiotic in another cell and become incorporated into the cytoplasm. Mitochondria are thought to be original derived from proteobacteria whereas chloroplasts are assumed to originate from cyanobacteria (blue green algae).

 

Biochemical evidence to prove that mitochondria and chloroplast is developed from proteobacteria and cyanobacteria respectively are (features of prokaryotic cell possessed by mitochondria and chloroplasts are;)

1.       The circular and naked DNA which are free in the matrix of mitochondria and stroma of chloroplast are organized with genes similar to that of proteobacteria and cyanobacteria respectively. Also the genes in mitochondria and chloroplast are different from that of cell’s nucleus.

2.      Mitochondria and chloroplasts have smaller and fewer ribosomes of 70's type similar to those from proteobacteria and cyanobacteria respectively in case of structure and size.

3.      New mitochondria and chloroplast are formed through a process of binary fission similar to the means of asexual reproduction common in bacteria.

4.      They are surrounded by two membranes which show differences in composition from other membrane of the cell. For example crops are surrounded by to remove repeat bilayer membranes that you have sought to correspond to the outer and inner membrane of the ancestral cyanobacteria/gram-negative wall.

5.      Some proteins increase the in the nucleus are transported to the organelles. This shows increasing dependence on the eukaryotic us to sell after forming endosymbiosis. 

Comparison of prokaryotes (chloroplast and mitochondria) with eukaryotes

Criterion	Prokaryotes	Eukaryotic
DNA	Circular DNA not contained in chromosomes not contained the nucleus	Linear DNA content in chromosome Contain DNA nucleus
Ribosomes	Small 70's	Large 80's
Sensitivity to the antibodies	Protein synthesis is inhibited by chloramphenicol not cycloheximide	Protein synthesis is inhibited by cycloheximide not chloramphenicol
Average diameter	Prokaryotic cell; 0.5 to 1 µm Chloroplast; 1 to 10 µm Mitochondrion; 1 µm	Eukaryotic cell 10 to 100 µm

Symbiotic nature of mitochondria

Mitochondria is considered as an independent cell within another cell.  This is because;

   It has its own DNA Hennessy able to control hereditary and self-replication.

   It has its own ribosomes and hands capable of synthesizing its own protein.

   It has enzymes and energy system of its own for different chemical reactions taking place in mitochondria.

   It has double membrane as that of the cell (a phospholipid bilayer membrane).

   It has matrix analog to cytoplasm of the cell.

 

Question;

1.       What makes a mitochondria act as independent cell within a cell?

2.      Explain the statement mitochondrion is a bacterium and cell within a cell.

3.      Why should be the diameter of a mitochondria remain fairly constant equal the length is so variable?

 

3. Plastids

Are the organelles of spherical shape found in the plant cell and few eukaryotic cell or are the spherical shaped pigment containing organelles found in the plant cell and a few eukaryotic cells. They develop from structure known as protoplastids found in the meristematic regions.

 

Types of plastids.

There are three types of plastids, classified depending on where in the plant body the plastid develop and their function. These are; leucoplast, chromoplast and chloroplast.

 

Leucoplast

These are plastids without any pigment. They are mainly found in the storage organs like seeds, roots and young leaves. The main function of leucoplast is the storage of food. Depending on what type of food is a stored, leucoplasts are classified as follows;

      Amyloplast which store starch; they commonly found in roots.

      Elaioplast (Lipidoplast, Oleoplast): They store lipids and oils e.g. castor endosperm, tube rose, etc.

      Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Diagram showing the types of plastids

 

Chromoplast

They contain non photosynthetic pigment main red, orange or yellow commonly known as carotenoid. They are found in fruits like orange, tomatoes and also in flowers.

 

Function of chromoplast

1.    They identify ripeness of fruits

2.   They help in the process of pollination since they bring coloured flowers and fruits which attract insects and birds for pollination and seed dispersal or they import colour to flowers and fruits thus attract insects and birds for pollination and seed dispersal.

 

Chloroplast

This is the plastid containing a green pigment called chlorophyll. It is mainly found in leaves, young stem or green part of the plant. It is also found in green algae and euglena. It is a photosynthesis cell organelle found in the cytoplasm of plant cell and the cells of mention the organisms.

Structure of chloroplast

Structurally of chloroplast in spherical, oval or disc-shaped organelle with green pigment (chlorophyll) containing plastids found in the almost all member of kingdom plantae (mesophyll cell and bundle sheath cell) and in certain algae or other members of kingdom protista. Cyanobacteria (blue-green bacteria) and other photosynthetic bacteria member of kingdom monera have no chloroplast but they still have the photosynthetic pigment embedded in specialised membrane. Chloroplast is bounded by a double membrane known as a chloroplast envelope (inner and outer membrane). The two-membranes encloses a fluid-filled space called stroma. Within the stroma there is parallel running membrane called thylakoids. Each thylakoid consist of pair of membrane. Chloroplast always containing chlorophyll and other photosynthetic pigments located in the system of membrane within the stroma called thylakoid membranes running through a ground substance of stroma. One pair of the thylakoids can be distinguished into two regions; grana and integranal region. The interval between one grana and another is called integranal lamella. The integranal lamella together with the grana holding chlorophyll in most suitable position for photosynthesis. The stroma contain enzymes for Calvin cycle reactions, circular and naked DNA, fewer and smaller ribosomes of 70’s type, chemicals like sugar, organic acids and starch grain. The enzymes present in the stroma are involved in the secondary stage of photosynthesis, during carbon dioxide fixation, which takes place in series of cyclic reactions collectively called Calvin cycle (the light independent reactions).

Diagram showing the structure of chloroplast

 

Functions of chloroplast

1.    The main function of chloroplast is to carry out photosynthesis producing sugar [carbohydrate] and the other substances from water the carbon dioxide using light energy captured by chlorophyll (green pigment). Chloroplast function as the site for photosynthesis process whereby complex organic food substances (sugar) are made from simple inorganic molecules of carbon dioxide and water in the presence of sunlight. The light energy is converted into chemical energy and the store in the food materials.

2.   They give plant leaves their colour due to the green pigment they contain.

3.   Carries self-replication, this help to increase the number of chloroplast.

4.   It act as the temporary storage of end product of photosynthesis. The product of photosynthesis is glucose. Once glucose is formed some may be used within the chloroplast, some are converted into sucrose which diffuse out of chloroplasts into the phloem for translocation to different parts of the plant body.

 

Roles of the thylakoid membrane

1.    To hold chlorophyll in most suitable position for photosynthesis. The memory system is the site of the light dependent reactions in photosynthesis. The membrane are covered with the chlorophyll and the other pigment enzymes and electron carrier. The system consists of many flattened, fluid-filled sacs called thylakoid which form stacks called grana at intervals, with integranal lamella between the grana.

2.   They make a system of internal membranes in the stroma suitable for light dependent reaction to take place.

 

Roles of the stroma

1.    It is where the second stage of photosynthesis (light dependent reaction) takes place in which carbon dioxide combine with hydrogen from water to form glucose the process called carbon dioxide fixation (Calvin cycle reaction). The structure in the get line containing soluble enzymes particularly those of the Calvin cycle and the other molecules such as sugar and organic acids.

2.   Excess carbohydrates from photosynthesis is sometimes seen stored is grains of starch. Spherically lipids droplets are often associated with membrane. They become larger as membrane breakdown during aging presumably accumulating lipids from membrane.

Note;

      The double membrane of chloroplast play the role of exchange of materials. The outer membrane is a permeable and the inner membrane is semi permeable. The membrane therefore allow the exit of photosynthetic products such as glucose and oxygen gas (as by product) and the entry of photosynthetic raw material (carbon dioxide and water).

      Chloroplast membranes include envelope (double membrane) and the thylakoid (lamella and grana) inside stroma. Function of membrane is to isolate one organelle from another and hence efficiency in organelles functions and protect its own chemical function.  Lamella and grana (thylakoids) contain chlorophyll electron transport and enzymes which carry out the first stage of photosynthesis (light reaction). Chloroplast membrane act as reservoir of chlorophyll, carotenoids etc.

 

Adaptation of chloroplast to its function.

1.       Presence of chlorophyll for trapping sunlight energy to facilitate the process of photosynthesis.

2.      Presence of grana and lamella (a system of internal membranes) in the stroma. It help to hold chlorophyll in most suitable position for maximum absorption of sunlight. This ensure maximum photosynthesis.

3.      Presence of permeable outer membrane which help to allow material in and out and a semi-permeable membrane which selectively allows materials to enter and leave the stroma help to ensure supply of materials for maximum photosynthesis.

4.      Presence of circular DNA and ribosomes help in the formation (synthesis) of protein.

5.      Presence of stroma which has ability of to store food e.g. starch temporary in bundle sheath cells to ensure continuous of photosynthesis.

 

Prokaryotic nature of chloroplast.

Like is a prokaryotic cell, the chloroplast has the following features

      It is very microscopic

      It has circular DNA lacking histone protein coat and lying freely in the stroma i.e. naked DNA

      It has fewer and smaller ribosomes of 70’s type.

 

Similarities between chloroplast and mitochondria.

      Both replicate themselves because the DNA is present (circular DNA).

      Both have fewer and smaller ribosomes 7 things carry out protein synthesis

      Both are double membranous cell organelles. They have an outer and inner membrane

      Both can generate energy; mitochondria in form of ATP and chloroplast in a form of carbohydrate

      Both are microscopic cell organelles

 

Question 1; Mitochondria, chloroplast and bacteria shape and structure resemble one another. Explain.

Answer. The way in which mitochondria, chloroplast and bacterial shape and structure resemble to one another are as follows;

1.    They are all microscopic, relatively rod, spherical or oval shaped structure.

2.   They all have circular and naked DNA that lie freely in the ground substance (cytoplasm/stroma/matrix)

3.   There have fewer and smaller ribosomes of 70’s magnitude.

 

Question 2. If mitochondria where to perform the role of chloroplasts, what modification would they require?

Answer; Mitochondria organelle that are concerned with production of energy in the form of ATP, a process known as aerobic respiration where has chloroplast an organelle is concerned with the production of food substance from carbon dioxide and water using light energy from the sun to by chlorophyll a and b process known as photosynthesis.

If mitochondria were to perform the role of chloroplasts, it require the following modification.

1.    The outer and inner membrane should be permeable to allow for the photosynthetic raw material such as water and carbon dioxide to enter.

2.   There must be development of chlorophyll for trapping light (sunlight).

3.   They must lose the cristae giving room for the gran, thylakoid and integranal lamella which provide surface area for holding and packing chlorophyll in a proper position for maximum absorption of sunlight.

4.   The matrix you should be converted into stroma and become equipped with photosynthetic enzymes such as ribulose bisphosphate carboxylase (RUBISCO) in C₃ plants and phosphoenol pyruvate carboxylase (PEPCASE) in C₄ plants facilitates carbon dioxide fixation (Calvin cycle) and regeneration of carbon dioxide acceptor.

5.   They must be equipped with the components of photophosphorylation instead of oxidative phosphorylation in the internal membrane system.

 

Study questions

1.    (a) Draw a well labelled diagram of chloroplast.

(b)    From where diagram above mention the components and their functions of the structures of parts involved in the light and dark reaction of photosynthesis

Model answers

   Thylakoids which are neatly arranged to form grana contain green pigment (chlorophyll) for light absorption needed to drive photosynthesis process. Also thylakoids contain phospholipids, other pigments and electron acceptors necessary for light reaction of photosynthesis.

   Stroma contain enzymes to catalyse chemical reactions during carbon dioxide fixation such as ribulose bisphosphate carboxylase (RUBISCO) and phosphoenol pyruvate carboxylase (PEP).

 

Differences between chloroplast and mitochondria.

Chloroplast	Mitochondria
(i)             Sites of photosynthesis and are known as photosynthetic organelle of the cell.	Sites of aerobic respiration and they are known as powerhouse of the cell
(ii)           Present in plant cell only green parts of the plant.	present in all cells in both plant and animals
(iii)        Contain chlorophyll and impact green colour to the plant.	Contain enzymes for oxidation of food do not impact any colour.

Common structures of plastics

   The plastics are bound by two membranes

   Plastids containing their own genetic material DNA and protein synthesizing machinery like RNA and ribosomes.

   They are capable of multiplication by fission like process.

 

Differences between three types of plastids

Criteria	Chloroplast	Chromoplast	Leucoplasts
Physiological process	Carry photosynthesis	Non-photosynthetic	Non-photosynthetic
Pigment	Contain chlorophyll (green pigment) and carotenoid pigment	Contain fat soluble carotenoid pigments like carotene and xanthophyll. They have red, yellow or orange pigments	Colorless and they lack any pigment (although they have the ability to develop pigment when required)
Location	Found mainly in leaves and green stem	Found in fruits and flowers	Abundant in storage organs e.g. roots, seeds and young leaves
Role	They trap light energy for photosynthesis	Give colour to the fruit and flowers thus attract  insects and birds for pollination and seed  dispersal	They are used to store food in storage organs e.g. Amyloplast used to store starch,

SINGLE MEMBRANEOUS ORGANELLES.

1.   Endoplasmic reticulum. (ER)

Endoplasmic reticulum is the network of folded membrane and flattened sacs found throughout the cytoplasm of eukaryotic cells except the red blood cells of mammals, eggs and embryonic cells. Also endoplasmic reticulum is described as an extension of the outer nuclear membrane serving a variety of functions in a cell. It originates from the outer membrane of the nucleus to which it often remain attached. We can assume that the endoplasmic reticulum is the flexible and mobile since it occupies much of the cytoplasm. It is a system of flattened membrane bounded sacs called cisternae forming tubes and sheets.

It is found in all eukaryotic cells but absent in prokaryotic cells. It is mainly found in abundance in lipid and steroid secreting cells like liver cells, cells of the dermis of the skin, kidney and heart, cells of the testes, intestines, leukocytes and the adrenal cortex cells. Endoplasmic reticulum is the site of enzymatically controlled reaction of the cell biochemistry. The outer surface of some endoplasmic reticulum carries numerous ribosomes. The presence of ribosomes gives the granular appearance and in these conditions ER is described as rough endoplasmic reticulum. RER is the site of synthesis of proteins which are packed up in the membranous vesicles and either moved within the cell or dissipated from it. Smooth endoplasmic reticulum (SER) does not have this coating of ribosomes. Smooth endoplasmic reticulum is concerned with lipid metabolism.

Diagram showing the structure of endoplasmic reticulum

 

 

Types of endoplasmic reticulum; there are two types of endoplasmic reticulum which differ in structure and function, these are:-

1. Smooth endoplasmic reticulum

2. Rough endoplasmic reticulum

 

Smooth endoplasmic reticulum (SER)

It is the type of endoplasmic reticulum where the cisternae lack ribosomes i.e. the surface of the membrane is smooth because of the absence of ribosomes attached on it. It is consequently abundant in the cells producing lipids related secretions such as the sebaceous gland of the mammalian skin and cell secreting steroids. The space inside the endoplasmic reticulum is called lumen. ER is very extensive, extending from the cell membrane throughout the cytoplasm and forming a continuous connection with the nuclear envelope. The muscle cells have specific type of smooth endoplasmic reticulum called sarcoplasmic reticulum.

Functions of smooth endoplasmic reticulum

1.    Carbohydrates and lipids synthesis; SER synthesizes phospholipids and steroids and the cells which secrete these products such as those in the testes, ovaries and skin oil glands have a great number of smooth endoplasmic reticulum.

2.   To carry out carbohydrate and steroid metabolism during drug detoxification and to attach receptors on the cell membrane proteins. Example in liver cells the smooth endoplasmic reticulum produce enzymes that detoxify compounds.

3.   SER serves as a transitional area for the vesicles that transport endoplasmic reticulum products to the various destinations.

4.   Smooth endoplasmic reticulum regulates calcium ions concentration in the muscle cells and assists in the contraction of muscle cells

5.   Smooth endoplasmic reticulum provides a large surface area for enzymatically controlled chemical reactions.

 

Rough endoplasmic reticulum

Is the type of endoplasmic reticulum where this is the cisternae touched by ribosomes i.e the surface is attached by ribosomes gives it is roughness on the membrane. The ribosomes bound to the surface of the membrane by complex protein known as translocon. However the ribosomes bound to the endoplasmic reticulum at any one time are not a stable part of these organelles as they are constantly being bound and released from the membrane. Ribosomes binds to the endoplasmic reticulum only when it begins to synthesize the protein destined for secretory pathway. After protein synthesis a ribosome may be released back into the cytosol.

Function of rough endoplasmic reticulum

1. Rough endoplasmic reticulum manufactures membrane and secretory proteins.

2. In white blood cells the rough endoplasmic reticulum produce antibodies.

3. In pancreatic cells the rough endoplasmic reticulum produce insulin.

4. Also rough endoplasmic reticulum manufacture lysosomal enzymes.

 

`          The rough endoplasmic reticulum and smooth endoplasmic reticulum are usually interconnected and the proteins made by the rough endoplasmic reticulum move into the smooth endoplasmic reticulum to be transferred to other locations. Although there is no continuous membrane between endoplasmic reticulum and Golgi apparatus, membrane-bound vesicles shuttle protein between two organelles. Another method of transport out of the endoplasmic reticulum involves area called membrane contact site where the membrane of endoplasmic reticulum and other organelles are held together allowing the transfer of lipids and other small molecules. It is consequently most abundant in that cells which are rapidly growing or secreting enzymes. Damage to a cell often results in an increased formation of endoplasmic reticulum in order to produce the proteins necessary for the cell repair.

 

NOTE: The smoothness a roughness of endoplasmic reticulum is due to the presence or absence of ribosomes on the sides of their membranes. Some cells have only smooth endoplasmic reticulum (SER) these are retinal cells, sebaceous gland cells and the corpus luteum cells.

Relationship between nucleus, endoplasmic reticulum and Golgi apparatus.

 

General functions of endoplasmic reticulum

1.         Provide a large surface area for chemical reactions enzymatic e.g. smooth endoplasmic reticulum involved in the reaction of glycolysis.

2.        Provide structural skeleton for maintenance of cellular shape (give mechanical support by forming a network in the cytoplasm).

3.        RES provide surface area for attachment of ribosomes for protein synthesis. They are concerned with production and storage of protein molecules before they are used inside the cell or secreted to the exterior.

4.        Provide a pathway for the transportation of materials inside the cell and the cell surface membrane for secretion.

5.        It is concerned with synthesis of lipids and steroids. This occur in a smooth endoplasmic reticulum.

6.        It is involved in detoxification of drugs and harmful by-products. This occur in the smooth endoplasmic reticulum in liver.

7.        It help to synthesise membrane for various purposes

8.       It is involved in the formation of Golgi bodies and modification. This occurs in the smooth endoplasmic reticulum.

9.        It transport proteins made by ribosomes through the cisternae.

10.    They are involved in collecting and storing the synthesized materials

11.     They form a route for the movement of materials between the cytoplasm and the cell nucleus.

12.    Endoplasmic reticulum is associated with muscle contraction by release and uptake of calcium ions (Ca²⁺)

 

Major functions of endoplasmic reticulum

Rough endoplasmic reticulum

      It is concerned with the transport of proteins which are made by ribosomes on its surface.

      Modification of proteins transported in it.

Smooth endoplasmic reticulum

      Synthesis of lipid and steroids

      Formation of Golgi bodies

      Regulates calcium ions concentration in the muscle cell and assist in the contraction of muscle.

 

Summary of endoplasmic reticulum

Major functions of smooth endoplasmic reticulum are to synthesise lipids including fatty acids and glycerol, phospholipids and steroids. Different types of cells make particular kind of lipids in animal for examples of the ovaries and tests synthesise the steroid hormones. They are rich in a smooth endoplasmic reticulum, liver cells have large amount of smooth endoplasmic reticulum with the additional kind of functions.

Golgi apparatus/Dictyosomes

Golgi apparatus also called Golgi body or Golgi complex or packing department of the cell is bounded sacs in tubes called cisternae which are continuous being formed at one end of the stack. The sacs are fluid-filled and pinch off smaller membranous sacs called vesicles at their ends. There is normally only one Golgi apparatus in each animal cell, but in a plant cell is there may be in large number of stacks known as dictyosomes. Dictyosomes are net-like flat, membrane bound cavity structures called cisternae which compose the Golgi apparatus.

Golgi apparatus is associated with smooth endoplasmic reticulum and various in position, size and shape according to the functional state of the cell. Golgi apparatus is the site of synthesis of biochemical. These are packed into swelling at the margin of each sac, which become pinched off as vesicles. The Golgi apparatus also collects proteins and lipids made in the endoplasmic reticulum by the fusion of vesicles pinched off from endoplasmic reticulum (RER and SER) with its own flattened sacs. In the Golgi apparatus additional substances are and the products are packaged into fresh vesicles, which are then cut off and moved to other parts of the cell. Here the secretions are deposited.

All proteins produced by the endoplasmic reticulum are passed through the Golgi apparatus in the strict sequence. They pass first through the cis-golgi network which returns to the endoplasmic reticulum any protein wrongly exported by it. They then pass through the stalks of cisternae which modifies the proteins and lipids undergoing transport and add labels which allows them to be identified and sorted at the next stage, the trans Golgi network. Here the proteins and lipids are sorted and send to their final destinations.

 

In general, the Golgi act at the cells post office receiving sorting and delivering proteins and lipids. Golgi apparatus is highly or largely found in cells located in the body parts which involved in the production concentration and packing of secretions e.g. glandular cells (glands).

Location/position. It is well developed in the secretory cells and neurones and it is small in muscle cells.

 

Function of Golgi apparatus

• Golgi bodies pack secretions. Golgi apparatus are numerous in cells that synthesise and secrete large amount of substances (enzymes, hormones and mucus). Example the abundant in plasma  cells, antibody secreting cells of the immune system where either after packaging the vesicles bud-off immediately move towards the plasma membrane to release the content into the extracellular space (constitutive secretion) or after packaging the vesicles bud-off and are stored in the cells until a signal is given for the their release (regulate secretion).
• Assemblage of materials such as lipoproteins and glycoproteins (a process known as glycosylation). The components which are synthesized else where in the cell, for example lipids are made in the smooth endoplasmic reticulum and proteins are made in the ribosomes in a rough endoplasmic reticulum but they are both assembled to form lipoproteins in the Golgi apparatus.
• Golgi body synthesizes large number of different macromolecules. For example, Golgi apparatus play an important role in synthesis of proteoglycans which are which molecules present in the extracellular matrix of animals. Proteoglycan is a complex of polysaccharide and the protein from rough endoplasmic reticulum.
• Golgi apparatus modifies received proteins from rough endoplasmic reticulum. For example Golgi apparatus adds a mannose-6-phosphate to protein destined for lysosome. Here vesicles from the endoplasmic reticulum fuse with the network and subsequently the proteins are modified as they travel through the stalk then they are packed and sent to their destination. In this aspect Golgi apparatus can be thought of a similar to post office. Another modification that buy gold body is going cold surface molecules pass through of a similar to post office. Another modifications done by Golgi body is when Golgi sulphate molecules pass through its lumen, they become negatively charged in the presence of sulphotransferase enzymes.
• Synthesis of cell components in plants such as cellulose and hemicelluloses in production of new cell wall.
• Golgi bodies involved in storage and transportation of lipids
• The Golgi apparatus has a putative role in apoptosis. Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Events leading to apoptosis including bulging of plasmalemma, cell shrinkage, nuclear fragmentation, chromatin condensation, and DNA fragmentation. For example differentiation of fingers and toes in the developing human embryo occurs because cells between the fingers apoptose, the result is that the digits as separate. Apoptosis is caused by the Bcl-2 family member localised in the Golgi bodies. Also the Golgi anti-poptotic protein resides in the Golgi and protects cells from apoptosis.

Note; Necrosis in the death of a cell by external factors

• Producing glycoprotein such as mucin in secretions by adding the carbohydrate parts to the protein.
• Golgi apparatus involved in the transformation of spermatids to mature spermatozoa.
• Transportation of materials produced by itself and other organelles from the site of production to the other parts of the cell. E.g. enzymes from endoplasmic reticulum to where they needed for action.
• Formation (making) of lysosomes. The Golgi vesicle are modified to form lysosomes where hydrolytic enzymes (digestive enzymes) are produced. Lysosomes are spherical cell organelles found in an animal cell only.
• Storage of substances and materials secreted (manufactured) in the cell.
• It helps to add extra length of the cell membrane which is necessary during division.
• Involved in lipid metabolism in rats/mice.

 

Note.

Ø  Golgi apparatus performs two major functions

1.       Processing of materials and sub-structures and transportation of materials synthesized by itself or others.

2.      Golgi apparatus derived from the cristenae are used as packaging structure of materials which then transport the material to other parts of the cell or surface cell membrane to be discharged out.

 

Ø  Proteins received by the Golgi apparatus from the ER have short carbohydrate chains added to make them glycoproteins (membrane proteins). This carbohydrates antenna can be remodeled in the Golgi apparatus possibly to become markers that direct the proteins to their correct destination

 

Secretion of inactive enzymes by reverse pinocytosis

The function of Golgi apparatus is to transport and chemically modified the materials contained within it. It is particularly important in secretory cells, a good example being provided by pancreas. Here specialised cell secrete the digestive enzymes in inactive form from pancreatic juice into the pancreatic duct along which the passes to the duodenum. The digestive enzymes secreted by the pancreas are synthesized in inactive form so that they do not attack and destroy the cell that make themselves. E.g. trypsinogen which is converted into active trypsin in the duodenum. Details of the pathway have been confirmed by using radioactively labeled amino acids. This get used by the cell to make proteins. Because they are radioactive, they can be traced as they pass through different cell organelles.

 

Diagram showing pancreatic inductee secreting the digestive enzymes

 

Summary

The Golgi apparatus performs several functions in close partnership with endoplasmic reticulum. Serving as molecular warehouse and finishing factory, of the materials manufactured by the ER one side of the Golgi stalk saves as a receiving clock for transporting vesicles produced by the ER.

Example; Golgi apparatus receives transport vesicles containing glycoproteins molecules. E.g. it takes in the materials and then modifies them chemically. One of the function of this chemical modification seems to be mark and sort the molecules into different batches for different destination. Molecules may be moved from sac in the Golgi by transport vesicles or according to the recent research entire sac may be moved from the receiving to the shipping, modifying their protein cargo as they go. The shipping side of the Golgi stalk services as the deport from which finished products may become part of the membrane of itself or part of another organelles such as lysosomes. Golgi apparatus consists of a stalks of membranes which make up flattened sacs of cisternae and the associated hollow vesicles. The protein and lipids produced by the endoplasmic reticulum are passed through the Golgi apparatus strict sequence. The Golgi modifies these proteins often adding non-protein components such as carbohydrates to them. It also label them allowing them to be accurately sorted and sent to their correct destinations. Once sorted, the modified proteins and lipids are transported in vesicles which are regularly pinched-off from the ends of Golgi cisternae. The vesicles move to the cell surface  where they fuse with the membrane and releases their content to the outside.

 

Functions

1. Google patents act as the cell’s post office by receiving, sorting and delivering proteins and lipids.
2. Adding carbohydrates to proteins to form glycoproteins such as mucin.
3. Produces secretory enzymes  such as those secreted by the pancreas.
4. Secrete carbohydrates such as those used in making cell wall in plants
5. Forms lysosomes.

 

Vesicles

A vesicle is the small bubble within a cell and thus type of organelle enclosed by a lipid bilayer. Vesicle can form naturally, for example during endocytosis or prepared artificially, when they are called liposomes. When is enclosed by only one phospholipid bilayer it is called a unicellular vesicle or otherwise they are called multilamellar. The phospholipid bilayer enclosing the vesicle is similar to that of the plasma membrane. The vesicles can fuse with the plasmalemma to release their content outside of the cell but also they can fuse with other organelles within the cell.

Function of vesicles

1. Vesicles organize (collect) cellular substances.
2. Vesicles act as chemical reaction chambers
3. Vesicles are involved in metabolism, transportation, buoyancy control and enzyme storage

 Types of vesicles

a)     Transport vesicles; this move molecules between locations inside the cell example proteins from the rough endoplasmic reticulum to the Golgi body are transported by Golgi vesicles.

b)     Secretory vesicles these contain materials that are to be exported from the cell example synaptic vesicles.

c)       Gas vesicles used by bacteria and planktonic microorganisms, possibly to control vertical migration by regulating the gas content and thereby buoyancy

d)      Matrix vesicles these are located within the extracellular space or matrix

e)      Vacuoles these are vesicles which containing mostly water. Contractile vacuoles are found in a certain protoctist (paramecium) are used to take water from the cell to avoid busting due to osmotic pressure. Large central vacuole in a plant is used to control osmotic pressure, nutrient and pigment storage.

f)       Lysosome these vesicles are involved in cellular digestion, destroying the damaged organelles (autophagy) and death of the whole sell (Stores hydraulic enzymes).

 

LYSOSOMES

The term lysosomes comes from two Greek words ‘lysis’ and ‘soma’. Lysis means split or break up and soma means body. Lysosome is the cell organelle, spherical sac bounded by a single membrane which contain digestive or hydrolytic enzymes such as proteases, lipase and nucleases which break down proteins, lipids and nucleic acid respectively. Lysosome is the cell organelle mainly deal with breakup of body materials only and/or any ageing organelles. Lysosome are found in eukaryotic cells except plant cells. They are also absent in prokaryotic cells. In animals, there are abundant in animal cells which have phagocytic (engulfing foreign bodies) and digestive activities. In a plant cells the large vacuole may act as lysosome, although body is similar to the lysosome of animal cells sometimes seen in the cytoplasm of plant cells.

The enzymes contained within lysosomes are synthesized on rough endoplasmic reticulum and transported to the Golgi apparatus. Golgi vesicles containing the processed enzymes later bud off to form the primary lysosomes.

 

Function of lysosomes

1.    Lysosomes are involved in digestion of large molecules like food because they are filled with digestive enzymes. They digest carbohydrate (polysaccharides), lipids, proteins and nucleic vacuole into the simpler monomers. Food can be taken from outside of the cell into the food vacuole (endocytosis). These food vacuoles fuses with lysosomes which breakdown the component so that they can be consumed by the cell. This form of cellular eating is called phagocytosis.

2.   Lysosomes digest excess or worn-out organelles the process called autophagy. Here the lysosome either fuse autophagic vacuole and burs to release  its concentrated enzymes which digest the content/organelle itself or the lysosome act as a scavenger ingesting and digesting the worn out solar party. After the death of the cell, the lysosome are responsible for it is complete breakdown a process called autolysis. To release their enzyme outside the cell exocytosis in order to break down other cell e.g. reabsorption of tadpole tails during metamorphosis.

3.   Lysosomes kill cells that you are no longer wanted such as those in the tails of the tadpole or in the web from the fingers of three to six months of old fetus. When a tadpole is changing into an adult frog, the lysosomes in its tail cells cause breakdown of structure and the tail is absorbed into the frog's body. Also when the cells are injured or die the lysosome aid their disintegration. That's why lysosomes are frequently nickname as ‘suicide bags’ or ‘suicide sacs’ due to their autolysis. Sometimes it is called garbage disposal unit.

4.   Lysosomes also engulf and digest invading bacteria and viruses or any cellular debris. It digest materials which they cell consumes from the environment. In a case of white blood cells, this may be bacteria or other harmful materials. In protozoan it is the food which has been consumed by phagocytosis. Materials are broken down within the lysosome, useful chemical are absorbs into the cytoplasm and the any debris is engested by the cell by exocytosis.

 

Lysosomes are especially abundant in a secretory cells and in phagocytic white blood cells. Lysosome have many functions or concerned with the breakdown of structures or molecules. They involved in digestion process within the cell and also involved in secretion of digestive enzymes. In short lysosomes play party in autophagy, autolysis, endocytosis and exocytosis.

Note; the single membrane of the lysosome prevent the leakage of lysosome contents into the cell cytoplasm because enzymes may digest even the cell and that's why it is sometimes known as suicide bags.

 

Functions of lysosome

1.    Digestion of materials taken in by endocytosis. Endocytosis in the bulk transport of materials through membranes into the cell. Lysosomes may fuse with the vesicles or vacuoles formed by endocytosis, releasing the enzymes into the vacuole and digesting the materials inside. This material might be taken in for food as in the food vacuole of some protozoans or for defensive purposes as in case of phagocytic vacuole formed by white blood cells when engulfing bacteria. The products of digestion are absorbed and assimilated by cytoplasm of the cell living undigested remains. The vacuole usually migrate to the cell surface membrane and release its contents.

Endocytosis in the process by which a cell takes (engulfs) materials and digesting the materials being engulfed. E.g. when a white blood cell (phagocyte) engulf a bacteria the membrane invaginates to form a porch like structure called a phagocytic vacuole. The bacterium gets engulfed and the lysosome release it is content and digest and kill the bacterium.

 Endocytosis explains the concept of cell eating and cell drinking in two ways.

·         Phagocytosis (cell eating); materials taken up is solid form. During this process the cell takes in solid particles, e.g. bacterium or any foreign. The membrane invaginates to form a porch like structure called a phagocytic vacuole containing a solid the particles. The particle gets engulfed and digested by the lysosomes contents (enzymes) release in to eat.

·         Pinocytosis (cell drinking); is the taking in liquid particles by cell during this process in a cell takes in liquids. In this case the membrane infolds and captures liquid particles in the porch called pinocytic vesicles which in turn forms pinocytic vacuole.

 

Thus the process by which solid particles are taken in (pinocytosis) by a cell is what called cell eating and the process by which fluid substance are taken in (pinocytosis) by a cell is what called cell drinking.

 

2.   Autophagy; is the process by which unwanted structures within the cell are engulfed and digested within lysosome. They are firstly enclosed by a single membrane, usually derivered from smooth endoplasmic reticulum and this structure then fuses with lysosomes to form out autophagic vacuole in which they unwanted material is digested. This is the part of the normal turnover of cytoplasmic organelles, old once being replaced by new ones.

3.   Release of enzymes outside the cell (exocytosis). Exocytosis is the process whereby waste products are removed from the cell. Sometimes the enzymes of lysosomes are released from the cell. This occur during the replacement of cartilage by bone during the development. Similarly, bone may be broken down during remodeling of bone that can occur in response to injury, new stress and so on. A sperm contain a special lysosome called the acrosome. This release it’s enzyme outside the cell to digest a path through the layers of cells surrounding the egg just before fertilization.

4.   Autolysis; in the self-digestion of a cell by releasing the contents of lysosome within the cell. In such circumstances lysosomes have sometimes been apply the name suicide bags. Autolysis is normally process in some differentiation processes and may occur throughout a tissue. As when a tadpole tail is reabsorbed during metamorphosis. Another example occurred in the uterus, during pregnancy the uterus grows much larger to accommodate the growing baby. After birth it gradually returns to its normal size by self-digestion (autolysis) out of many of the cells. Autolysis also occur in the muscles which are not exercised.

 

Microbodies/peroxisomes

Microbodies are small spherical organelle bounded by a single membrane commonly found in in eukaryotic cell. It contains several types of enzymes but mainly catalyse enzymes. They are formed by endoplasmic reticulum and commonly found in highly metabolizing cells like liver cells and replicate by fission. In animals peroxisomes appear to be confined to the cells of the liver and kidney. In plants, special microbodies/peroxisomes called glyoxysomes occurs is variety of cells including cotyledons. Peroxisomes in plant cells are often packed with crystallized materials.

Catalase enzyme contained in the microbodies catalyse the breakdown of toxic byproduct produced from by chemical reaction within the cells organisms. The major metabolic toxic byproduct is hydro peroxide (H₂O₂). When this compound is produced during metabolic activity since it is toxic, it is quickly broken down into water and oxygen which is shown as effervescence.

Microbodies/peroxisomes are numerous in cells of the liver and some seeds of plants e.g. pea seeds. When a piece of liver dropped into a beaker or test tube containing hydrogen peroxide the liver cell release catalyse enzymes which catalyse the decomposition of H2 O2 to water and oxygen gas which is shown as effervescence.

 

Structure of peroxisome

 

The process of breaking down of toxic byproducts produced from metabolism in the cells is called detoxification. The role of catalase enzyme is to speed up the rate of decomposition of hydrogen peroxide into water and oxygen. Microbodies in plants are the site of glycolate cycle (photorespiration).

 

Function of microbodies

1.  Release catalase enzyme which catalyze the composition of toxic hydrogen peroxide produced from metabolic process into water and oxygen gas.

2. Leaf peroxisomes are site of glycoxylate cycle in C3 plants (photorespiration in c3 plants)

3. They are also concerned with the lipid metabolism a conversion of fat into carbohydrates and in the breakdown of purines within the cell. For example in plants glyoxysomes are centres of glycoxylate cycle, a conversion of fat into carbohydrate especially during germination.

4. Peroxisomes breakdown the very long chain of fatty acids through oxidation. 

 

NOTE;

If catalase enzymes are not produced or are scantly produced, it may lead to accumulation of hydrogen peroxide and therefore an individual may die because hydrogen peroxide is a toxic byproduct.

 

Plasmodesmata

Plasmodesmata (singular plasmodesma), are living connection between neighboring plant cells which run through very fine pores in the walls. The cell surface membrane of neighboring cells are continuous and line the pores.

 

Advantages of plasmodesmata

1.    It enables connection and coordination between cells since molecules and ions do not have ability to cross a cell surface membrane.

2.   Plasmodesmata form the sieve plate pores of phloem sieve tube.

 

Vacuole

Vacuole is a fluid-filled sac which is bounded by a single membrane. It is found in almost all eukaryotic cells. In animal cells it is relatively small and temporary such as phagocytic vacuole, pinocytic vacuole, autophagic vacuole, food vacuoles and contractile vacuole. In plant cells vacuoles is large and permanent, occupies greater proportional of the cytoplasm.

In young developing plants, vacuole is very small but as the plant grow to the maturity the vacuoles become very large. Small vacuole is known as cutieles. The membrane bounding the vacuole is known as tonoplast and the fluid found in the vacuole is called sap. The sap is made up of mineral salts, sugar, amino acid, waste (e.g. tennis) and gases (such as CO2 and O2) and sometimes are pigments such as anthocyanins.

Structure of vacuole

 

Advantage of vacuole

1.    It acts as an organ of osmoregulation and excretion in aquatic organism such as Euglena, paramecium, amoeba and sponges.

2.   It act as a temporary storage of waste such as tennis. This may accumulate in the vacuole of leaf cell and are removed when the leaf fall.

3.   They are occasionally contains hydrolytic enzyme and so perform functions similar to those of lysosome. After the cell death tonoplast like all cell membranes, loses its partial permeability and the enzymes escape causing autolysis.

4.   In plant it contains pigments in solution (anthocynins) includes red blue and purple and other related compounds which are shaded of yellow and red which are responsible for colour in flowers fruits and leaves this may colour petals to attract pollination insects and fruits to attract animals for disposal.

5.   They support herbaceous plant and herbaceous part of woody plant by providing an osmotic system which creates a pressure potential.

6.   It acts as temporal area for storing materials like water, sugar (glucose), amino acid, organic acid, oxygen, mineral salts and secondary products of metabolism.

7.    Vacuole help to increase the surface area to volume ratio. It helps in water movement in cell, which create osmotic pressure important in cell expansion during growth.

8.   Some of the dissolved substances, act as food reserves which can be utilised by cytoplasm when necessary for example; minerals salts, sugar and amino acid may act as a temporal store.

9.   Waste products and certain secondary products of plant metabolism may accumulate in vacuoles. For example crystals of waste calcium oxalate and secondary products like alkaids and tannins may after motoetion from conception bye harbivous. (to be corrected). Latex e.i. milky (liquid) may accumulate in the vacuoles as in dandelion stems. The latex of the rubber tree contain the chemical needed for rubbers synthesis.

 

NON-MEMBRANEOUS STRUCTURE

1.      Ribosomes

Are tiny (smaller) organelles about 20 nm in diameter, non-membranous found in all eukaryotic and prokaryotic cells. They are found on surface of the endoplasmic reticulum and other freely in the cytoplasm, matrix of mitochondria and stroma of chloroplast in eukaryotic cell and freely in the cytoplasm in prokaryotic cell. Ribosomes are connected of two subunits one large and another smaller subunit.

Ribsosomes are composed of two subunits one large ants another small subunit but ribosomes is made from complex of RNA, and protein thus it therefore a rebonuclear proteins. Two subunits exist independently only coming together when they are performing function. During protein synthesis the smaller subunits birds to the messenger RNA (mRNA) while the large subunit expose the codons, hence translation (convention of base sequence of mRNA) into amino acids of protein. Ribosomes are sometimes referred to as organelles; but they use.

Formation of ribosomes start in the nucleus and ends in the cytoplasm eukaryotic cells. In the nucleus, ribosomal formation is initiated in the chromosomes. The loop nucleus organelles, DNA codes for the production of ribosomes RNA (ribosomal RNA) is there for used to form ribosomes in the cytoplasm. Ribosomes are made of ribosomal RNA (rRNA) and protein sedimentation of ribosomes. Sedmentation of ribosomes, when these organelles from eukaryotic cell were centrifuged has revealed two types of ribosomes.

70's- ribosomes occur in prokaryotic cells, mitochondria and chloroplast.

80's-ribosomes which occur in eukaryotic cell.

Ribosomes are made of roughly equal amount of RNA and proteins. The RNA is termed ribosomal RNA and is made in nucleoli. During protein synthesis in ribosomes amino acid are joined together by one to form polypeptide chains.

The ribosomes act as the binding site where the molecules involved can be precisely positioned rela­­­tive to each other. These molecule include messenger RNA (mRNA) which carrier the genetic instruction from the nucleus, transfer RNA (tRNA) which brings about the required amino acids to the ribosomes and the growing polypeptides chain.

In eukaryotic cells there are two population of ribosomes namely free ribosomes and ER bound ribosomes. All ribosomes have identical structure but some are bound to the ER by the protein that they are making, such protein are usually secreted. An example of a protein made by the ribosomes is hemoglobin in young red blood cells. During protein synthesis the ribosomes move along the thread-like mRNA molecule. Rather than one ribosome at a time passing along mRNA, the process is carried more efficiently by the number of ribosomes moving along m RNA molecule. Chains of ribosomes are called polyribosomes or polysomes (which occur in groups).

 

The distribution of RNA and protein molecules.

 

NOTE that;

S-Stands for sredberg unit whose is related to the rate of sedimentation in a centrifuge. The greater the number the greater the rate of sedimentation. Therefore the ribosomes called 70’s is relatively small compared to the ribosomes celled 80’s which is slightly large.

 

Function of ribosomes

1.    Act as machinery instrument for protein synthesis in the cytoplasm. Synthesis of protein (polypeptides) takes place on the surface of the ribosomes where the amino acids are brought and condensed/polymerized together to form polypeptide chains.

2.   Ribosomes are also used to transport proteins produced by themselves and by other organelles.

 

CYTOSKELETON

This is the complex network of fibrous proteins. The proteins fibres of the cytoskeleton are dynamic system constantly being formed and disassembled. The cytoskeleton consist of protein that support the cell, holds organelles in place in the cytoplasm and a nap of the cell to change its shape.

Cytoskeleton is found in eukaryotic cell and it consist of; microtubules, microfilament (actin filaments) and intermediate filament.

 

1.       Microtubules

These are thin cylindrical tubes, non-membranous found in all eukaryotic cells, either single or in groups. Microtubules are straight unbranched, hollow cylinders which are usually short in length. They occur in most plant and animals cells. Microtubules are involved in the movement of cytoplasmic components with the cell. They also occur in centrioles, in the spindle, in cilia and flagella and in the basal bodies. Microtubules are made up of proteins. They help to maintain the shape of the cell and act as routes along which organelles can move. They are hollow tubes made up of fibrous proteins. They are rigid and act as supporting structure in the cell forming a cytoskeletal (intracellular) skeleton.

 

Function of microtubules

1.       They provide skeleton support (internal skeleton-cytoskeleton) that helping the cell to maintain their shape.

2.      They help in the transportation of materials within the cytoplasm by providing the routes along which materials move.

3.      They bring about movement of chromosomes during nuclear division by spindle fibre. In the spindle during cell division and within the centrioles from which the spindle is formed. They help to draw chromosomes or chromatids to opposite poles.

4.      They are part of the structure of centrioles, cilia, flagella, and basal bodies and hence involved in movement.

 

Adaptation of microtubules to its function

1.    They have ability to be formed rapidly and reform finally they disappear after their job m to be completed.

2.   They are along gated so that they can support the transportation of the cell organelles e.g. Golgi body.

3.   They are stiff so that they support the cell.

 

Distribution location of microtubules

They are found more at the following regions of the body; the cortex of the minister meristematic tissue, at the cilia e.g. Paramecium, at the flagellum e.g. Euglena, at the centriole, nerve processes, at the basal body, at the mitotic apparatus to form a framework along which the cell wall of plants is laid down.

 

2.  Microfilaments (Actin filaments)

Microfilaments are very thin strands about 6nm in diameter. They are hard, not hollow which are usually made up of actin protein. They also contain small portion of myosin protein. They are of two types, thin and thick. The thick ones are made up of protein called myosin and the thin ones are made up of proteins called actin. They can readily assemble and disassemble and useful for cell movement. They often occur in sheets or bundles just below the cell membrane and at interphase between stationary and moving cytoplasm streaming is taking place.

 

Function of microfilaments

1.    They are involved with cell motility (whole cell and within the cell). Responsible for cytoplasmic streaming example when the cytoplasm moves about. It can be observed in movement of protozoans e.g. Amoeba when moving or engulfing.

2.   Involved in muscle contraction especially in striated muscle due to interaction of actin and myosin filaments e.g. skeletal and cardiac muscles

3.   They help to bring about the cleavage of animal cells (cell division).

4.   Also involved in the formation of vesicles by pinching off of the infolded portion of the cell membrane.

 

3.  Intermediate filaments

These are structures intermediate between microfilaments and microtubules. There are about 10nm diameter and largely found at the base of the microvilli. They are intermediate in size between the actin filaments and microtubules. Intermediate filaments support the nuclear envelope and plasma membrane and take part in the formation of cell to cell junction. They are made up of different proteins in different specialized cell types. They are very abundant in skin cell and nerve cells in human. In the skin the intermediate filament is made up of protein keratin which gives mechanical strength to the skin cells.

 

Functions of intermediate filaments

In nerve cells, intermediate filaments are called microfilaments which consist different proteins and they are important for nerve conduction.

 

MICROVILLI

Microvilli (singular microvillus) are finger-like projections (extension) about o.6 µm in length on the membrane of certain cells, such as those of intestinal epithelium and kidney tubules epithelium. They increase surface area for absorption. These should not be confused with much larger villi which are multicellar structures. Microvilli contain bundles of actin and myosin filaments. Actin and myosin are proteins found in muscles which cause muscle contraction. At the base of the microvillus actin and other filaments join up with filaments from neighboring microvilli to form a network of filaments. The system as whole allows the microvilli to remain upright and retain their shape, which still allowing their movement through the interaction between actin and myosin (similar to the muscle contraction). The microvilli can just be seen with a light microscope as fringe across the top of the cell called a brush border.

The increase in surface area also improves the efficiency of digestion in the gut because certain digestive enzymes are attached to their surface.

 

Function of microtubules

1.    The function of microvilli is to increase the surface area for absorption of materials.

2.   Improve the efficiency of digestion in the gut because a certain digestive enzymes are attached to their surface e.g. Pancreatic amylase, trypsin and lipase.

 

Note; Plants lack microvilli because of their rigid cell walls impose restrictions on extension of the plasma membrane.

 

SECRETORY VESSELS OR GRANULES

Many cells are involved in secreting a variety of substances such as cell concentrate and store the products in secretory vesicles. Secretary vesicles arise from face of Golgi apparatus. They release their contents by the process of exocytosis.

 

STORAGE GRANULES

Every cell contains a limited store of food energy. This store may be in the form of soluble material such as sugar found in the vacuoles of plant cells. It also occur in insoluble form as grains or granules within cells or organelles. Starch grains occur within chloroplast and cytoplasm of the plant cell. Starch may also be stored in specialised leucoplast called Amyloplast. Glycogen granules occur throughout the cytoplasm of animal cells. They store animal starch or glycogen. Oil or lipid droplets are found within the cytoplasm of both plant and animal cells.

 

CENTRIOLES /CENTROSOMES

Centrioles are cylindrical organelles found in animal cells. Centrioles are small hollow cylinders that occur in pair in most animal cells. They are short cylinder heaven (9+3) pattern of microtubule arrangement i.e. they contain group of microtubules each of which has three microtubules. The region of the location in the cytoplasm is known as centrosome. In centrosomes (poorly defined structure which initiates the development of microtubules) the two centrioles lie at right angle to each other. Before an animal cell divides the centrioles replicates then each pair becomes part of a separate centrosomes. During cell division the centrioles migrate to opposite poles of the cell where they synthesise the microtubules of the spindle. During cell division the centrosome move apart so that each new cell has its own centrosome.

Plant cells have the equivalent of a centrosome but it does not contain centrioles. But absent in flowering plants. Animal cells contain a pair of centrioles in the cytoplasm which usually lie at right angle to each other due to the nuclear membrane.

 

FUNCTION OF CENTRIOLE

1.    They are used in the formation of spindle apparatus during cell division are the rabbit spindle fibres.

2.   They are concerned with formation of cilia and flagella.

3.   This are responsible for the production of microtubules.

 

CILIA AND FLAGELLA

Cilia and flagella organelles that have identical structures although flagella are longer while cilia are usually shorter and move numerously. Both are around 0.2 µm in diameter. Cilia are up to 10 µm long. They are found in limited number of cells but are nevertheless of greater importance.

They are outgrowths from the cell which can beat either in one direction (cilia) or like a wave (flagella). The organelle can be used for locomotion of single cell or to move fluids over the surface of the cells, as cilia do when they move mucus through the respiratory tract.

At the base of every cilium and flagellum is a basal body. Basal bodies are identical in structures to centrioles and are probably made by replication of centrioles. Like centrioles, they are also seem to act as MTOCs (microtubules organizing centres) because cilia and flagella contain characteristic “9+2” arrangement of microtubules. In cilia and flagella, microtubules undergo sliding motions which are responsible for the beating movements. Note that bacterial flagella are simpler than eukaryotic flagella and do not have basal bodies.

Diagram showing the tructure of cilia and flagella

 

FUNCTION OF CILIA AND FLAGELLA.

1.    Flagella and cilia are locomotory structures (used for locomotion of a single cell organisms) e.g. cilia on protozoans such as paramecium and flagella in euglena, trypanosoma and bacteria.

2.   In the respiratory track cilia are used to trap dust particles taken together with inhaled air.

3.   In the urinary bladder and tubules contain cilia which flow urine in one direction.

4.   In the oviduct (fallopian tube) cilia are used to transport the egg (ovum) into the uterus.

5.   In the brain ventricles cilia contain the movement of celebralspinal fluid.

6.   They are also used to acquire food e.g. feeding current generated by paramecium in its oral groove.

7.    They are used to sense the environment e.g. sensory hair cells.

 

Differences between cilia and flagella.

Cilia	Flagella
They are short.	They are long.
They are numerous.	They are few in number per cell

NON-LIVING PART OF THE CYTOPLASM.

Cytoplasmic inclusion

There are non living structures of the cytoplasm. They are also known as ergastic bodies. Example of non-living (cytoplasm inclusion) structures in the cytoplasm are food granules and crystals of various substances mainly waste products of metabolism.eg; Calcium oxalate which are found in citrus fruits calcium carbonate and silica. Food granules are such as glycogen in animal cell and fungi starch in plants chlorophytes (green algae) and euglena.

 

CELL DIFFERENTIATION

Cell differentiation is the process whereby a single cell or group of cell undergo change in their structure so as to become more specialized. It is therefore the process which leads to the less specialised cell to become specialised to perform a particular function. During this process generic or a stem cell develops into a specific type of cell. When the cells differentiate it may lose or acquire some features which enable it to perform the functions effectively developed into multicellular embryo then into more complex multi system of distinct cell type of fetus.

The structure of a differentiated cell is always related to the function performed by that cell.eg; in mammals’ red blood cells has no nucleus and it has a disc or oval shape. This structure is related to the function it perform. Absence of nucleus enables the cell to obtain large lumen so as to pick more hemoglobin for carrying large volume of oxygen.

 

CELLL POTENCY (POWER)

Cell potency; Is the term describes a stem cells ability to differentiate to cell types.

 

Significances of cells differentiation

1.    Help an organism to be as effective as possible at carrying out its job properly

2.   It leads the less specialised cell to become more specialised cell in the body to perform specific function.

3.   It bring about division of labour in the multicellular body because during cell differentiation each cell is assigned a function to perform.

4.   It allows a single celled organism to become a multicellular organism e.g. a zygote.

5.   It leads to the formation of tissues organs and systems.