Cytology
This is the branch of biology which deals with the study of structure, function and behavior of the cell. The cells are the smallest structures that show all the features of living things. Each cell is a complex system consisting of many different building blocks enclosed in membrane bag. There are unicellular and multi-cellular organisms.
In 1865 a British scientist Robert Hooke coined the term "cell" while looking at the nonliving tissue known as cork under the light microscope.
Since there were several definitions put forward by different scientists to explain the cell such as;-
Cell is the smallest living organism like amoeba
Cell is smallest unit in the body of living organism
A cell is the basic unit of life.
The basic meaning of the cell which combines all the three statement above is;
A cell smallest functional and structural unit of living organism. Therefore cells are the building blocks of organisms.
The Cell Theory.
After the discovery of a cell by Robert Hooke two other scientist Theodor Schwann (a zoologist) and Mathias schriden (a botanist) using the idea of Hooke put forward the cell theory which consists of the following tenets;
All living organisms are composed of one or more cells.
Cell is the basic unit of life
All new cells come from pre-existing cells by cell division.
All metabolic activities of an organism take place within the cell.
Cells contain hereditary material which are passed from generation to another or parent to offspring
1.1.3 Type of Cell
According to the structure and organization there are two fundamentally different types of cells. These are:-
Prokaryotic cell and
Eukaryotic cell
Prokaryotic Cell
The term prokaryotic come from two Greek words which are “Pro”- means before and “karyo” means nucleus.
Therefore Prokaryotic cell is the cell whose nuclear materials are not enclosed by nuclear membrane. In other word Prokaryotic cell can be defined as the cell which has no true nucleus.
Prokaryotic cells are smaller than eukaryotic cells of about 1 micron in diameter and have simpler structure .The organisms possessing prokaryotic cells are called prokaryotes, including all bacteria. Prokaryotes are single cellular organisms, but note that being a single cell does not mean that an organism is a prokaryote.
Structure of the Prokaryotic Cell.
All the prokaryotic cells consists of the following parts in common
Capsule (or Slime Layer). A thick polysaccharide layer outside of the cell wall whose roles are;-
Used for sticking cells together.
Used as a food reserve.
Used for protection against desiccation and chemicals, and
Used for protection against phagocytosis.
Cell Wall. Made of murein, which is a glycoprotein
There are two kinds of cell wall, which can be distinguished by a Gram stain, these are;-
Gram positive bacteria have a thick cell wall and stain purple, and
Gram negative bacteria have a thin cell wall with an outer lipid layer and stain pink.
Cell membrane. Made of phospholipids and proteins, like eukaryotic membranes.
Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles
Ribosomes. These are smaller (70s) type.
Nucleoid. The region of the cytoplasm that contains DNA, which is not surrounded by a nuclear membrane. Nucleoid contains a circular molecule of DNA which is the cells genetic material, or genome.
Mesosome. A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis.
Flagellum. A rigid rotating helical-shaped tail used for propulsion.
Diagram: a typical structure of a prokaryotic cell.
Characteristics of Prokaryotic cells
They have no well organized nucleus
They lack nuclear membrane
They have 70s ribosome and free
Respiration take place in folding of the cell surface called mesosome
Cell division is by simple fission
No spindle are formed during cell division because their lack centrioles
Many of them under goes anaerobic respiration
They are small cell in size
Eukaryotic Cell
Eukaryotic originated from two Greek words which are “Eu”- true and “karyo”- nucleus
Therefore Eukaryotic cell is defined as the cell whose nuclear material is bounded by nuclear membrane.
Eukaryote is an organism with true nucleus. Eukaryotic cell contain a large number of organelles. Organelle is a distinct part of a cell which has a particular structures and functions.
Differences between prokaryotic and eukaryotic cells
Types of Eukaryotic cell
There are two different types of Eukaryotic cell namely;-
Plant cell and
Animal cell
Basically plant cells are very similar to animal cells but plant cells have more structures than animal cells. The organelles and structures which are common to both plant and animal cells are:-
Plasma membrane
Nucleus
Mitochondria
Endoplasmic reticulum
Golgi apparatus
The chief differences between animal and plant cells are the presence of cell wall, chloroplast and a large vacuole in plant cells.
Figure; Diagram of an Animal cell
Figure; The diagram of a Plant Cell
The differences between plant cell and Animal cell
Components of Eukaryotic cell and their functions
Cell Wall
This is the semi-rigid external covering of the cell of the plant cell made up of cellulose and micro fibrils.
Both plant cells and fungi are surrounded by a relatively rigid wall which is secreted by living cell (the protoplast) within. The plant cell wall differs in chemical composition from that of fungi:-
Plant cell wall contains cellulose.
Fungi cell wall contains chitin.
The wall formed during cell division of plants is called the primary wall which is later thickened to become a secondary wall.
Structure of cell wall
It is not continues as it contains tin pore called plasmodesmata which join one protoplast to another
It consists of cellulose fiber of microfibrils which form the frame work of the cell wall.
The cell wall matrix consists of polysaccharide of pectin, hemicelluloses and rigin.
The 60%-70% by mass of the cell wall is water which can move freely through free space in the cell wall.
The adjacent cell walls are separated by the middle lamella which is composed of sticky gel-like magnesium and calcium salts of pectin.
The cell wall has the great tensile strength and limited elasticity.
Functions of the Cell Wall,
It provides mechanical and skeletal support for individual cells and for the plant as a whole.
It allows the movement of water through it and along it due to presence of plasmodesmata
It prevents the cell from bursting when exposed to a dilute solution.
It acts as water proofing layer when impregnated with lignin
It acts as a food reverse such as seed cell wall store hemicelluloses
It reduce water loss since it contain a coating of waxy cutin
The presence of microfibrils in the cell wall limits and helps to control cell growth and shape.
It protects the internal delicate parts of the cell from damage.
The cell walls of root endodermal cells are impregnated with suberin that forms a banner to water movements.
It acts as a major pathway for the movement of water (apoplast).
The Plasma Membrane (Cell membrane)
Is an extremely thin structure surrounds the cell, separating its contents from the surroundings and controlling what enters and leaves the cell.
Structure of the cell membrane.
The membrane is made up of phospholipid bilayer with some proteins floating on the surface of phospholipids bilayer ( peripheral or extrinsic protein), other extend into it (intergral protein) and some extend completely across the membrane (transmembrane protein)
Phospholipid consists of polar heads made up by phosphate group and non polar tails made of fatty acid.
The ‘hydrophilic’ head is a polar molecule and have an affinity to water (hydrophilic i.e. water loving) and the ‘hydrophobic’ tail is non-polar and do not mix with water (hydrophobic i.e. water hating).
Some protein and lipids have short branching carbohydrate chain like antennae forming glycoprotein and glycolipids respectively
Cholesterols are embedded between the phospholipids giving the dynamic nature of the membrane.
Diagram: The structure of the plasma membrane
Plasma membrane Models
Different scientist explained the structure of the plasma membrane and they come out with different model of the structure of the membrane including;-
Daniel- Dauson Model of the Structure of the Membrane.
In 1940’s Daniel and Davson proposed that all the plasma membrane consist of lipid layer coated with protein molecules as continuous layer.
The two scientists proposed the structure of the cell membrane as bilayer of the phospholipids coated with protein molecule on both surface.
The model regard the cell membrane as being static with several pore and the protein layer is continuous
The model insisted on the existence of glycoproteins, glycolipids and cholesterol in the phospholipid bilayer.
The phospholipids have got two ends, the polar head molecule which posses great affinity to water (hydrophobic) and the non polar tail which does not have any affinity to water.
The Fluid Mosaic Model or Singer and Nicolson Model.
In 1972, J Singer and G. Nicholas put forward the “Fluid Mosaic Model” of membrane structure in which a mosaic protein molecules floats in a fluid lipid bilayer. They modified Daniel –Davson model and put forward the fluid mosaic model of membrane structure as follows;-
This model suggested that cell membrane is mainly composed of phospholipids, cholesterol, proteins, and carbohydrates.
This model proposed that membrane is made up of phospholipid bilayer coated by protein in both sides but the protein does not form a continuous layer as proposed by Daniel and Dauson.
Each phospholipid molecule has a head that is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail that repels water (hydrophobic: hydro = water; phobic = fearing).
The protein molecules are either partially (peripheral protein) or wholly embedded (integral protein) in the phospholipids bilayer.
The model suggests that the peripheral protein molecule float on the phospholipids bilayer as a fluid, hence fluid mosaic model.
Note; The fluid mosaic model states that membranes are composed of a phospholipids bilayer with various protein molecules floating around within it.
Function of the cell membrane.
It separates the contents of the cell from their external environment in animal cell.
Protect the internal organelles of the cell from damage.
It controls the exchange of material between the cell and its external environment. (i.e. it allows some material to pass through and retain other permanently).
It acts as receptor site for recognizing stimulus such as hormone, enzymes, and neurotransmitters coming into the cell.
It acts as the site for metabolic reactions such as energy production in mitochondria and also enzymes attached to the plasma membrane.
In some cell the membrane forms pinocytotic vesicles which aid in removing unwanted material from the cell.
Some membrane carries out phagocytosis where it capture foreign particle (protection against disease).
It contains glycoprotein which acts as cell identity markers, hence enables the cell to recognize other cells and to behave in an organized way.
Adaptation of the membrane to its function.
Presence of hydrophilic pores which aid the exchange of material between the cell and external environment.
Microvillus present in animals’ cell membrane offer a large surface area for absorption
Ability to form pinocytotic and phagocytotic vessels ensures the ability of protecting the cell against disease causing agents.
Presence of glycoprotein molecule which act as receptor site to detect the foreign materials.
Protein present in the membrane gives the cell its identification.
The fluid nature of the membrane makes easy for materials to pass through it.
Membrane Components and their function
Phospholipids
It gives the basic structural support of the membrane.
Glycolipids
Involved in cell- cell recognition
Acts as receptor site for chemical signals such as hormones and enzymes
Glycoproteins
Act as cell receptor sites, where hormones and drugs bid.
They are involved in cell signaling in the immune system.
Proteins
Gives the structural support to the membrane
Assist the transport of materials across the membrane
Give specificity of the cell.
Cholesterol
Increase the flexibility of the cell membrane
Makes the membrane more stable and
Prevents the membrane from solidifying when your body temperature is low.
Transport across the membrane.
Substances need in a cell or unwanted substances pass through the membrane to enter or leave the cell. They do so in a number of ways where some of these processes require no energy while others require energy.
Methods of transport across the membrane
There are two processes through which different materials move in and out across the membrane which are:-
Active transport which includes phagocytosis, pinocytosis and exocytosis.
Passive transport which includes diffusion and osmosis,
Active Transport
This is a movement of molecules across cell membranes against concentration gradient. When a substance is transported from a low concentration to a high concentration that is uphill against the concentration gradient, energy has to be used.
Types of Active transport across the membrane
There are two active process involving the bulk transport of material across the membrane which are;-
Endocytosis
Exocytosis
Endocytosis
Is the movement of materials into the cell across the membrane occurring by an unfolding or extension of the cell surface membrane to form a vacuole. The vacuole is the fluid filled membrane bound sac
Types of Endocytosis
There are two type of endocytosis which are
Phagocytosis ( cell eating) and
Pinocytosis (cell drinking)
Phagocytosis
Is the transport of material in the cell in the sold form. Phagocytosis is sometimes called “cell eating” because it takes in solid materials. The specializing cells in this process are called phagocyte or phagocytic cells.
For example the destruction of bacteria by white blood cells by the process of phagocytosis . Once within the cell, enzymes produced by the lysosomes of the cell destroy the bacteria, as shown below
Diagram: Phagocytosis
Pinocytosis (cell drinking)
Is the transport of material in the cell in the liquid form. The cell specializing in this process is called pinocytes.
Note; the vesicles formed are very small known as micropinocytotic vesicles, in this case the process is known as micropinocytosis. E.g. Human egg cells obtain the food /nutrient from the surrounding follicle cells by pinocytosis process.
Exocytosis
Is the movement of materials out of the cell through the cell membrane. Therefore substances formed in the cell are moved through the plasma membrane into the fluid outside the cell (or extra-cellular fluid) by the process known as exocytosis. It occurs in all cells but is most important in secretory cells such as cells that produce digestive enzymes and nerve cells.
2. Passive Transport
Passive transport is a movement of molecules or ions and other substances across cell membranes along or down the concentration gradient.
The main kinds of passive transport are
Diffusion
Is the net movement of material from the area of their high concentration to the area of low concentration down the concentration gradient. The concentration gradient is the difference of concentration between the two areas.
Types of diffusion
Simple diffusion
Is the passive movement of solute from the region of high concentration to the region of low concentration until the concentration of the solute is uniform throughout.
A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as O2 and CO2.
Facilitated diffusion
This is the transport of substances across a membrane by trans-membrane protein molecules that are embedded within the membrane. The transport proteins tend to be specific for one molecule, so substances can only cross a membrane if it contains the appropriate protein.
Osmosis
Is the movement of water molecules from the region of high water potential to the region of low water potential through a partially permeable membrane.
Or Osmosis is the passage of water molecules from a region of their high concentration to a region of their low concentration through a semi- permeable membrane.
Water Potential is a measure of the water molecule potential for movement in a solution. It is measured in units of pressure (Pa, or usually kPa), and the rule is that water always moves by osmosis from less negative to more negative water potential. 100% pure water has water potential = 0, which is the highest possible water potential, so all solutions have water potential < 0 (i.e. a negative number), and you cannot get water potential > 0.
A cell with large negative water potential will draw in water from less negative potential, but this depends on other factors, such as solute potential (ψs) and pressure potential (ψp).
Thus; ψw= ψs +ψp
Cells and Osmosis.
The osmotic concentration (or OP) of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:
Isotonic solution a solution of equal OP (or concentration) to a cell
Hypertonic solution a solution of higher OP (or concentration) than a cell
Hypotonic solution a solution of lower OP (or concentration) than a cell
The effects of these solutions on cells are shown in this diagram:
Effects of osmosis in a living cell
What happens if a plant cell is placed in hypotonic solution such as pure water?
Water will tend to move from the solution into the cell due to osmosis and the cell becomes turgid or swell. But plant cell does not burst due to the presence of cell wall, which exert inward force or turgor pressure.
What will happens to the animal cell when is placed in hypotonic solution?
Water will enter the cell and cause it to expand until it burst, unlike the plant cell due to lack of cell wall.
Explain what happens and why when the animal cell and plant cell are placed into the hypertonic solution (solution with lower water potential).
The Cytoplasm
This is the jelly-like substance composed of mainly water and variety of cell organelles found between the cell membrane and nucleus. The cytoplasm makes up most of the "body" of a cell and is constantly streaming.
Functions of the cytoplasm
It is a site of metabolic activities such as glycolysis
Act as a store of vital chemicals
Site for synthesis of some biochemical molecules such as fats, proteins, carbohydrates, nucleotides and coenzymes.
It facilitates the intracellular distribution of nutrients and metabolites
It assists the exchange of materials between the organelles
Components of the Cytoplasm
The cytoplasm is mainly composed of the following;-
Cytosol or intracellular fluid
Cell inclusions,
Organelles and
Microfilaments and microtubules
a) Cytosol
This is a clear jelly-like fluid that composed mainly of water in which various molecules are dissolved or suspended. These molecules include proteins, fats and carbohydrates as well as sodium, potassium, calcium and chloride ions. Many of the reactions that take place in the cell occur in the cytosol.
b) Cell inclusions
These are large particles of proteins, fat, carbohydrates and melanin that have been produced by the cell. They are often large enough to be seen with the light microscope.
c) Organelles
These are structures with characteristic appearances and specific “jobs” in the cell. Most cannot be seen with the light microscope but only by the electron microscope. The main organelles in the cell are the nucleus, ribosome, endoplasmic reticulum, mitochondrion, chloroplast Golgi complex and lysosomes. The cell organelles in the cell work as organs in the body of an organism.
Types of cell organelles
There are two types of cell organelles
Membranous organelles
Non membranous organelles
Membranous Organelles
These are the organelles in the cell which are bound by unit membranes similar to the cell membrane. Following are the membrane-bound organelles, which can be recognized in a generalized eukaryotic cell.
Nucleus
Nucleus means “core” is the oval spherical organelle.
This is largest organelle, found in all eukaryotic cells only, except the mature phloem sieve tube element and red blood cell have no nucleus.
Structure of the nucleus.
It is surrounded double membranes known as nuclear envelopes with nuclear pores.
The nuclear pores which allow exchange of material between the nucleus and cytoplasm
The outer membrane is continuous with the rough endoplasmic reticulum.
The matrix of the nucleus is called nucleoplasm which contains chromatin and nucleolus.
The chromatin materials are coiled DNA bounded by protein called histones. There are two types of chromatin in the nucleus, these are:
Heterochromatin – Tightly coiled and continues to stain intense.
Euchromatin – The looser coiled and more scattered chromatin during the interphase.
The nucleoplasm contains dissolved phosphate, ribose sugar, protein, nucleotide and chromosomes.
Function of Nucleus
The nucleus controls the development and all activities of the cell hence is known as control centre.
It store genetic information hence determines the character of an individual
It carries the instructions for synthesis of proteins in the nuclear DNA.
It synthesis the ribosomal RNA by nucleolus.
The nuclear membrane is covered by ribosome which are involve in the protein synthesis
It is the source of information that govern the morphology of a cell
Note; the shape, size, position, and chemical composition of the nucleus vary from cell to cell but they perform the same functions which are
To control the cell activities and
To retain the organism’s hereditary material the chromosomes
Qn; Explain the structural adaptations of the nucleus to its function.
Mitochondria
Mitochondria (sing. mitochondrion) are oval or rod shaped organelles scattered throughout the cytoplasm. Mitochondria are called “power stations” of the cell because it is the site where energy is made by “burning” food molecules like glucose. This process is called cellular respiration.
Figure; Diagram of a Mitochondrion
Structures of Mitochondrion
This is a sausage-shaped organelle (8µm long), and is where aerobic respiration takes place in all eukaryotic cells.
Mitochondria are surrounded by a double membrane, the outer membrane is simple, while the inner membrane is highly folded into cristae (sing. crista), which give it a large surface area.
Cristae increase the surface area for respiration since it provided abundant space for enzymes attachment such as ATPase.
The space enclosed by the inner membrane is called the matrix, and contains small circular strands of DNA, 70s ribosomes, food granules and phosphate granules.
Note; Distribution of mitochodria depend much on the location and function of the cell, a large number of mitochondria are found in active cells which are involve in high energy consumption such as brain, liver ,spinal cord, nerve cell, sperm cell and skeletal muscles cells.
Functions of mitochondrion
It is centre for aerobic respiration that is site for energy production
It is involve in protein synthesis due to the presence of ribosome inside of it
Mitochondrion is involved in storage of genetic material due since they posses DNA
The matrix of mitochondrion is the site of Krebs cycles and where fatty acids are oxides.
Adaptation of mitochondrion to its function
The inner membrane is folded inward to from cristae which increase the surface area for enzyme attachment.
They have cristae which are site for electron transport chain (ETC).
The permeability of the outer membrane allows some material to enter or leave the mitochondrion.
They have DNA for genetic information storage.
They posses ribosomes for protein synthesis.
Plastids
Plastids are ovoid or spherical shaped organelles found in plant cells and in certain unicellular organism like algae. They are surrounded by two membranes which form an envelope. There are three main types of plastids, these are:
Chloroplast
Chromoplast and
Leucoplast
The chloroplast
This is a plastid which found mainly in leaves and green part of the plant which contain chlorophyll and carotenoid pigments.
Chloroplast is surrounded by double membrane the outer membrane and inner membrane which forms the chloroplast envelope. Chloroplasts are where photosynthesis takes place, so are only found in photosynthetic organisms (plants and algae)
Structure of the chloroplast
This is oval shaped organelles of protoplasm which is surrounded of the double membrane, the outer and inner membrane.
It consists of many flattened fluid filled sacs called thylakoids which form stocks called grana.
Between one grana and the other there is a membrane called intergranal lamella.
The internal system membrane is suspended in an aqueous matrix called stroma which contains protein, circular DNA, starch grains and lipid globules.
The thylakoids contain chlorophyll which is responsible for absorbing energy from the sunlight and convert it to chemical energy
Function of chloroplast
It is the site of photosynthesis reaction ,producing sugar from water and carbon dioxide using solar energy
It captures and converted solar energy into chemical energy due presence of chlorophyll.
Production of ATP, NADPH2 and evolution of oxygen through the process of photolysis of water
Adaptation of chloroplast to its role
Presence of grana increase the surface area of photosynthesis reaction to take place
Presence of pores in the membrane ensures the exchange of the material between the chloroplast and the surrounding.
Presence of pores on the membrane facilitates the absorption of light and carbon dioxide gas.
Presence of enzyme such as RuBisCO and PEPC which facilitate photosynthesis reaction.
Symbiotic nature of chloroplast and Mitochondrion
This is explained by Endosymbiotic or Symbiogenesis theory. The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotes. It states that “several key organelles such as mitochondria and chloroplasts of eukaryotic cell originated as symbiosis between separate single-celled organisms”.
According to this theory, mitochondria and plastids (e.g. chloroplasts), and possibly other organelles, represent formerly free-living bacteria that were taken inside another cell as an endosymbiont, around 1.5 billion years ago.
Molecular and biochemical evidence suggest that the mitochondrion developed from proteobacteria and the chloroplast from cyanobacteria.
Evidence for Endosymbiotic Theory
This theory is supported by following observations:
New mitochondria and chloroplast are formed only through a process similar to binary fission.
Both mitochondria and chloroplast contain single circular DNA that is different from that of the cell nucleus and similar to that of bacteria.
They have 70s type ribosomes like those found in bacteria.
Mitochondria and chloroplasts DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis is similar to that for bacteria
Both mitochondria and chloroplasts have transport proteins called porins found in the outer membranes, similar to that found in bacterial cell membrane.
Mitochondria and chloroplasts are similar in size to bacteria, 1 to 10 microns.
Both mitochondria and chloroplasts have double phospholipid bilayers which have arisen by endocytosis and been engulfed and surrounded by the surface membrane.
Qn; What are the Similarities between mitochondrion and chloroplast and bacterial cell.
Chromoplasts
These are plastids containing mainly red, orange or yellow pigments which are commonly known as carotenoids. They are non-photosynthetic pigments which are found mainly in fruits and flowers. In the flower their bright color attracts insects and birds for pollination and seed dispersal.
Functions of chromoplast
They facilitates insect pollination
They insure seed dispersal
They indicates the ripe of fruits
Leucoplasts
These are colorless plastids which have no pigments. They are numerous in storage organs such as roots, seeds and young leaves where they store food. Examples of leucoplasts include:
Amyloplast store starch e.g. potato root.
Lipidoplast store oil and fat
Proteoplast stores protein e.g. beans seeds
Golgi apparatus
These bodies were named after the discovery by a scientist called Camillo Golgi (1898).
The Golgi apparati are bodies composed of membrane-bound flattened sacs called cisternae and are associated with vesicles called golgi vesicles.
They are manufactured by rough and smooth endoplasmic reticulum.
There is normally only one golgi apparatus in each animal cell but in plant cell are more than one, the collection of golgi bodies is known as dictyosome. Golgi apparati are most located in the secretory cells such, neurons and small in muscle cell.
The Golgi apparatus is particularly well developed in cells that produce secretions, e.g., pancreatic cells producing digestive enzymes and hormones.
Diagram A Golgi body
Functions of the Golgi apparatus
Used in modifying, sorting and packaging of proteins for secretion.
It transports proteins from the RER to the cell membrane for export.
It is involved in the transport of lipids around the cell.
They are concerned in the creation of lysosomes
They are used in the production of glycoproteins such as mucin which is a source of mucus.
Involved in the production of enzymes such as digestive enzymes of pancreas.
Endoplasmic Reticulum (E.R)
This is a network of membranes that form channels throughout the cytoplasm from the nucleus to the plasma membrane. The system of membrane form parallel flattened sacs called cisternae
Types of Endoplasmic reticulum (E.R)
Depending on the presence of ribosomes on the membrane of the tubules there are two types of E.R
The rough E.R this is the one whose the surface is covered with ribosome and
The smooth E.R this is the one whose surface is not covered with ribosome
Smooth Endoplasmic Reticulum (SER).
Series of membrane channels involved in synthesizing and transporting materials, mainly lipids, needed by the cell.
Rough Endoplasmic Reticulum (RER).
Similar to the SER, but studded with numerous ribosome, which give its rough appearance. The ribosome synthesize proteins, which are processed in the RER (e.g. by enzymatically modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi body.
Functions of E.R
It provide the large surface area for chemical reaction
RER concerned with the transportation of proteins which are synthesized by ribosome of its surface.
The smooth E.R is concerned with synthesizes of lipid from fatty acid and glycerol
The smooth E.R also makes steroids hormones such as corticosteroid, testosterone, progesterone and estrogen.
It collect and store synthesized material such as protein ,lipids and steroid
It provide skeleton structure of cell or maintain cellular shape such as smooth endoplasmic reticulum of the rod cell in the retina of the eye
Provide a path ways for transport of material in the cell.
Note; when there damage to a cell result in increase formation of RER in order to produce more protein necessary for the cell repair.
Diagram: Rough endoplasmic reticulum
Lysosomes
These are simple spherical sacs derived from the Golgi vesicles of the Golgi bodies. They are bounded by a single membrane and contain a mixture of digestive enzymes (hydrolytic enzymes) such as protease, nuclease and lipase which break down proteins, nucleic acids and lipids respectively.
The enzymes contained within lysosomes are synthesized on rough E.R and transported to the Golgi apparatus.
Golgi vesicles containing the processed enzymes later bud off to form the lysosomes.
Lysosomes are particularly abundant in animal cells, in plant cells the large central vacuoles may act as lysosome.
Functions of Lysosome
It digests the materials taken in by endocytosis such as bacteria since it contains digestive enzymes by the process known as autophagy.
They remove unwanted structure or damaged macromolecules within or outside the cell by autolysis.
They destruct the worn-out cells by releasing its contents and themselves are being digested.
The disappearance of the tail of the tadpole during metamorphosis of frog is due to autolysis by lysosomes.
They fuse with a feeding vacuole to digest its contents.
Definition of some terms.
Autolysis is the self digestion of a cell by releasing the contents of lysosome within the cell. For this reason, lysosomes sometimes called ‘suicide bags’ or ‘self breaking down’.
Autophagy is the process by which unwanted structures within the cell are engulfed and digested within lysosome.
How the lysosomes function
The hydrolytic enzyme of the lysosomes is released when unwanted structure in or out of the cell of the cell is to be digested and removed. However the lysosome burst and releases its contents.
Biological Effects of Lysosomes
They cause fertilization in which the sperm nucleus and the egg nucleus.
They cause meat deterioration
They cause a gradual return of the uterus to its normal size after birth
They digest foreign bodies such as bacteria and worm out organelles.
They facilitate the metamorphosis in amphibians and insect.
They assist the digestion of food store of the endosperm in the germinating seeds
They can change the normal cells into cancerous cells.
They cause disappearance of an unexercised muscles
Peroxisomes (Micro bodies).
These are spherical organelles bounded by a single membrane commonly found in eukaryotic cells.
They are believed to be derived from endoplasmic reticulum.
The peroxosimes containing the powerful enzymes known as oxidative enzymes such as Catalase enzyme
Eg: catalase which catalyses the decomposition of hydrogen peroxide to water and oxygen.
i.e H2O2 Catalase enzyme 2H2O + O2
Hydrogen peroxide as a byproduct of certain cell oxidation reaction is very toxic and therefore must be eliminated immediately.
Liver is an organ which deals with neutralization of toxic substances (detoxification function), it contains large number of peroxisomes. In plants peroxisomes are site of the glycolate cycle (photorespiration).
Roles of peroxisome
It release the catalase enzymes which calalyse the decomposition of hydrogen peroxide into water and oxygen.
The peroxisome of the leaves offer protection against any physiological attack in the leaves.
Non-membranous Organelles
These are organelles, which generally do not contain membranes, such as ribosomes, cytoskeletons, centrioles, cilia, and flagella.
Ribosomes
These are very small organelles made up of protein and ribonucleic acid (ribosomal R.N.A) from nucleoli.
These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis. Ribosomes are either found free in the cytoplasm, where they synthesize proteins for the cell's own use, or they are found attached to the rough endoplasmic reticulum, where they make proteins for export from the cell.
Ribosomes occur in both prokaryotic and eukaryotic cells. The ribosmes of prokaryotic cells are distinctly smaller (70’s ribosomes type) than those of eukaryotic cells (80’s ribosomes type).
Each ribosome consists of two units, small sub-unit and large sub-unit. When several ribsomes occur along a common strand of mRNA, the whole structure is known as Polyribosomes or Polysomes.
Diagram: structure of ribosome
Centrioles.
Centriole is a small cylindrical structure that is composed of groupings of microtubules arranged in a 9 + 3 pattern. The pattern is so named because a ring of nine microtubule "triplets" are arranged at right angles to one another.
Centrioles are found in animal cells only.
During cell division the centrosomes replicate and 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.
Diagram: structure of a centriole
Functions of Centrioles
It controls the separation of chromatids or chromosomes by a sliding motion.
It helps to organize the assembly of microtubules during cell division.
They are source of cilia and flagella such as centrioles called basal bodies.
Vacuole
A vacuole is fluid filled sac bounded by a single membrane. These are membrane-bound sacs containing water or dilute solutions of salts and other solutes.
Animal cell have small or temporary vacuoles such as food vacuole but plant cells have one large permanent vacuole that fills most of the cell.
Plant cell vacuoles are filled with cell sap surrounded by a membrane called tonoplast, and are very important in keeping the cell rigid, or turgid.
The cell sap is a watery fluid containing water, sugar, organic acids, mineral salts, pigments and toxic substances.
Functions of Vacuole
It maintains the hydrostatic pressure of the cell.
They are responsible for colors in flowers, fruits, buds and leaves since it contains pigments.
The vacuole sometimes contains hydrolytic enzymes and act as lysosomes break down complex molecules.
It stores nutrient and non-nutrient chemicals such as calcium oxalate, alkaloids and tannins which offer protection from consumption by herbivores.
Vacuole acts as a food storage organelle.
It provides structural support the plant cell.
Cytoskeleton
Cytoskeleton is a series of intercellular network of protein fibers extending throughout all eukaryotic cells. The cytoskeletons help a cell with shape, support, and movement.
Cytoskeleton has three main structural components:-
Microfilaments ,
Intermediate filaments and
Microtubules.
The cytoskeleton mediates movement by helping the cell move in its environment and mediating the movement of the cell's components. This is because each fiber has a corresponding motor protein that can move along the fibre carrying a cargo such as organelles, chromosomes or other cytoskeleton fibres. These motor proteins are responsible for such actions as:
Chromosome movement in mitosis,
Cytoplasm cleavage in cell division,
Cytoplasm streaming in plant cells,
Cilia and flagella movements,
Cell crawling and
Muscle contraction in animals.
Roles of cytoskeletons
They are used for cell structural support
Used in transportation of materials in a cell and motility of a cell.
They gives the cell its shape,
They are used for holding all the organelles in position.
Cilium and Flagellum.
Flagella and cilia are organelles that project from the surface of cells but are connected to a basal body just below the plasma membrane.
They are made up of 2 central singlet microtubules and 9 pairs of peripheral microtubules. This is called 9 + 2 arrangement of microtubules.
Flagella in prokaryotic cells do not have the 9+2 arrangement.
Differences between Cilia and Flagellum
Flagella are long structures, and cilia are short structures.
One cell normally contains one flagellum or 2 flagella, but a large number of cilia are present in a cell.
Both cilia and flagella are surrounded by a unit membrane.
Functions
They are used for movement of the cell or unicellular organisms
They contain enzymes that produce energy to move a cell, e.g. sperm or paramecium.
They are used for feeding, e.g. feeding current generated by paramecium in its oral groove.
They are used to sense the environment, e.g: sensory hair cells.
They used to remove mucus that has trapped dust particles in respiratory track.
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