CYTOLOGY: Cell Structure And Functions - Biology Form 5 & 6

CYTOLOGY: Cell Structure And Functions - Biology Form 5 & 6

The five structures are also known as ultra structure and are obtained by two techniques.

1. Physiological or metabolic activities take place within a cell. Viruses though are not cells, have life within their hosts.

2. The new cells arise from pre-existing cells by cell division. In this postulate the theory does not specify about the origin of the first cell.

3. All living things must have cells. This postulate is challenged by the existence of viruses, where when they are inside the body of their host, viruses act as living things even though they don’t have cellular organization.

4. Electronic microscope.

5. Cell fractionation.

A cell is usually a tiny, three dimensional sac of many organelles which are suspended within an aqueous medium (the cytoplasm) containing or contained (bounded) by a cell membrane.

In the case of plants, a cell wall is bounded by a cellulose cell wall.

The bulk of these structures (organelles) of the cells is referred to as a cytoplasm.

Cytocil is the fluid part of the cytoplasm.

PROKARYOTIC CELLS.

They are extremely small for example bacteria all range from 0.5 – 10 micrometers.

They appeared about 350 million years ago.

Cells of prokaryotes lack the true nuclei that are their genetic material (DNA) are not enclosed by the nuclear membrane and lies freely in the cytoplasm.

EUKARYOTIC CELLS

The cells of eukaryotic have three basic parts

1. The plasma membrane.

2. The cytoplasm.

3. The nucleus.

Plasma membrane.

This is also called the cell surface membrane as plasma membrane or plasma lemma which separates the contents of the cells from the external environment, controlling the exchange of materials.

In animal cells it is an outermost layer where as in plant cells it is beneath the cell wall. E.g. neurillema in neurons.

Muscle cells – sacrolemma.

STRUCTURE OF THE CELL MEMBRANE

There are two models suggested by different scientist to try to describe the cell membranes. These are;

i. Daniel-Davson model (1935)

ii. Fluid mosaic model (1972)

Daniel-Davson model  

Diagram

 

According to Daniel and Davson, the membrane is structurally composed of two chemical substances that form their own layer.

1. Protein layer made up of molecules. The layer is continuous and lacks pores.

2. Phospholipids (at least two layers of phospholipids) oriented with their polar (hydrophilic ends near the surface and their non polar (hydrophobic) hydrocarbon chains in the interior of the membrane as far as possible from the surrounding water.

According to the model, the membrane is structurally rigid static and non dynamic. 

Strength of the model.

1. The model suggests that the membrane is composed of proteins and lipids.

2. Ampliphetic (double) nature of phospholipids such as phospholipids molecule has a polar head (hydrophilic) and a non polar tail (hydrophobic).

WEAKNESS OF THE MODEL

1. The model suggests that the protein layer is continuous. Researches done by scientists show that the protein layer is in-continuous.

2. The membrane is static is a wrong concept since the membrane is a dynamic ever changing structure.

3. Lack of pores in protein layers.

The protein molecules in a membrane have pores for passage of materials.

4. The model does not indicate the presence of a carbohydrate.

THE FLUID MOSAIC MODEL.

The model was put forward by singer and Nicolson 1972 in order to modify the Daniel and Davson model.

According to the fluid mosaic model, the membrane is an ever-changing structure in which the mosaic protein floats on the lipid bilayer acting as a fluid.

Proteins in this model do not form a continuous layer covering both sides of the membrane as proposed by Daniel and Davson model.

According to this model, the membrane has 3 constituents.

• Lipids (45%)
• Proteins (45%)
• Carbohydrates (10%)

1. LIPIDS.

There are two types of lipids.

a. Glycolipids;

These are lipids associated with short carbohydrates chain.

ROLES OF GLYCOLIPIDS

Cell to cell recognition.

Act as receptors for chemical stimuli.

 

b. Phospholipids;

These are lipids associated with phosphates. They form 2 layers i.e. phospholipids bilayer. Each phospholipid consists of a polar head (hydrophilic) and a non polar tail (hydrophobic).

Act as a fluid and move about rapidly in their own layer. Since phospholipids are constantly in motion, the membrane is described as being fluidly.

ROLES OF PHOSPHOLIPIDS

1. Form the basic structure of the membrane.

2. Determine the fluidity of the membrane.

3. Allow the passage of fat soluble substances.

NB: cholesterol is a type of steroid located in between phospholipids keeping them fluidly.

ROLES OF CHOLESTEROL

1. Disturb the close package of phospholipids keeping them fluids.

2. Increase the flexibility of the membranes by allowing relative movements of the bilayers without actual displacement because it acts as an unsaturated fatty acid lubricating bilayer.

2. PROTEINS

These exist as globular in the membrane, i.e. they never form a continuous layer.

Within protein molecules or between adjacent there are poles. These may either be hydrophobic or hydrophilic.

Since the phospholipids are always in constant motion (fluid) proteins float in it forming a fluid mosaic model. The proteins are organized in a particular pattern known as mosaic.

There are protein molecules that extend/ transverse both layers of membranes. Other proteins are partially embedded in the membrane. These are called intrinsic proteins.

Some proteins float freely inside the membrane, hence they are called peripheral or extrinsic proteins.

TYPES AND ROLES OF PROTEINS.

1. Carrier proteins or channel proteins.

These are involved in the selective transportation of polar molecules. i.e. ions across the membrane e.g. movement of glucose to the cell, chlorine ions. (Cl-)

2. Enzymes

Catalyze different metabolic reactions.

3. Receptor molecule.

Some act as receptors for chemical stimuli example hormones.

4. Antigen.

Identity markers. These are glycoprotein. They have different shapes in every kind of a cell. 

They have specific side chains thus are recognized by other cells and behave in an organized manner.

5. Energy transfer.

In some physiological processes such as photosynthesis and respiration, some proteins are involved in energy transfer (special form of membrane found in chloroplasts and mitochondria).

3. CARBOHYDRATES

These branches to the outside of the membrane as an antennae or feelers.

There are two types;

1. Glycoprotein ( carbohydrate chain – plus protein)
2. Glycolipids ( carbohydrate chain plus lipid)

They form a layer of glycocalyx

   

 ROLES

1. Cell to cell recognition (in making tissues since same cells combine so similar cells will have similar glycolipids/ glycoprotein).
2. To receive chemical stimuli.

STRENGTH OF FLUID MOSAIC MODEL.

It realizes the presence of phospholipids bilayer and protein layer.

The presence of polar head (hydrophilic) and non polar tail (hydrophobic) in the phospholipids.

It shows that the membrane is not static.

It shows the presence of carbohydrates.

It shows that the protein layer is not continuous.

It indicates the presence of pores in the membrane passage of materials.

Diagram

FUNCTIONS OF CELL MEMBRANES.

It protects the cytoplasm contents of the cells.

It allows passage of materials in and out of the cells since it has pores.

In some membranes e.g. those of the intestine cells, there are microvilli which increase the surface area for absorption of materials.

Acts as receptor sites for chemical stimuli such as hormones.

In nerve cells, the membrane is over lined with a fatty sheath (myelin sheath) 

which prevents the spreading of local currents to other neurons.

It aids cell to cell recognition when membranes of two cells come together.

VARIOUS WAYS BY WHICH MATERIALS PASS THROUGH THE MEMBRANES.

1. Permeability

The plasma membrane is a thin elastic membrane around the cell which usually allows the movement of small ions and molecules of various substances through it. This nature of plasma membrane is termed as permeability.

2. Osmosis

The plasma membrane is permeable to water molecules. To and fro movement of water molecules through the plasma membrane occurs due to the difference in concentration of the solutes on its either side. The process by which the water molecules pass through a membrane from region of higher water concentration to a region of lower water concentration is termed as osmosis.

3. Diffusion or passive transport.

The diffusion of a certain solute or substance takes place through the plasma membrane depends on the concentration and electrochemical gradient.

4. Active transport.

When molecules or ions move through the plasma membrane from low concentration to higher concentration, they require energy for such movement.

The energy is provided by ATP which is produced by the mitochondria.

Through the pores of plasma membrane some chemicals such as urea and glycerol could pass. It has been shown that large molecules of certain proteins also penetrate the cell.

5. Endocytosis and exocytosis.

The plasma membrane particles actively in the ingestion of certain large sized foreign or food substances.

The process by which the foreign substances are taken and digested is known as endocytosis.

In the process of exocytosis, the cells which have secretory functions such as pancreatic cells pass out their enzyme secretions outside the cell.

According to the nature of the food of foreign substance, endocytosis may be classified into two types;

6. Pinocytosis

When the ingestion of food materials in bulk takes place by the cell through the process known as pinocytosis.

7. Phagocytosis

Sometimes the large sized solid food or foreign particles are taken in by the cell through the plasma membrane. The process of ingestion of large sized solid substances by the cell is known as phagocytosis.

Question: what is the significance of a fluid mosaic model in the plasma membrane?

Ans:

• It explains easily the known physical and chemical properties of the membrane.
• It is the starting point to understanding the fix of the cell.
• All membranes of the cell plus the tonoplast and those of the organelles have the fluid mosaic construction.

NB: this point provides the clues about the distribution of cell membrane in the cell and its organelles.

 NOTE:

Where

R = rate of transport of material.

A = cross section surface area.

CYTOPLASM

This is the part of a cell, which is filled with fluid in the protoplasm. This part of the cell is the ground substance of the cell known as the hyaloplasm, where the cell organelles are suspended. Cytosil is the soluble part of the cytoplasm.

Cytoplasm is distinguished into the following structures

1. Cytoplasm matrix

The space between plasma membrane and nucleus is followed by a morphous, translucent, homogenous liquid known as cytoplasm matrix and hyaloplasm.

The cytoplasm matrix consists of various inorganic compounds e.g. carbohydrates, lipids, proteins, nucleon proteins, nucleic acids (RNA and DNA) and variety of enzymes.

The peripheral layer of a cytoplasm matrix is relatively non-glandular viscous and known as endoplasm.

2. Cytoplasm inclusion

The cytoplasm matrix contains many refractive granules of various sizes; these granules in the animal cells are known as cytoplasm inclusion.

The cytoplasm inclusion includes oil drops, yolk granules, pigments, secretory granules and glycogen granules.

Such granules in plant cells are known as plastids. The most common plastids are the chloroplasts (containing pigment chlorophyll), the leucoplastids (white color plastids) ,omyplastids ( the plastids that store starch) and lipoplastids ( which contain fats).

NB: plastids like cytoplasmic inclusion having only storage functions but also perform various important synthesis and metabolic activities such as the production of food materials due to the presence of chloroplasts.

ANIMAL CELL STRUCTURES

Diagram of the animal cells under light and electron microscope.

 

 

DIAGRAM OF ANIMAL CELL UNDER ELECTRON MICROSCOPE

ANIMAL CELL STRUCTURES

Characteristics;

1. Have irregular shape.
2. Have centrioles.
3. Have lysosomes.
4. Lack cell walls.
5. Lack plastids.
6. Store carbohydrates in the form of glycogen e.g. phagocytotic vacuoles, pinocytotic vacuoles, autophagic vacuoles and etc.
7. Cytokinesis occurs by furrowing i.e. periphery – centres direction of constriction of cell membrane.

STRUCTURE OF THE PLANT CELL

A plant cell is incased in a tough and rigid cellulose cell wall.

Beneath the cell wall is the cell surface membrane which surrounds the cytoplasm.

The latter contains organelles; the prominent being vacuole plastids e.g. chloroplasts and nucleus.

-Since a greater part of the cell is occupied by the vacuole, then the cytoplasm and nucleus are squeezed by the vacuole to the periphery.

-When viewed under light microscope; only a few structures are seen under high magnification power, even finer details are seen.

Diagram

 

Diagram of a plant cell under light microscope

CHARACTERISTICS OF PLANT CELLS

1. It has a fixed shape.
2. It has a cell wall made up of cellulose.
3. It has large permanent vacuole,
4. It has plastids; chloroplasts, chromoplast and leucoplasts.
5. Stores carbohydrates in the form of starch.
6. Lack lysosomes.
7. Lack centrioles.
8. Cell division; cytokinesis follows cento-periphery direction.

Similarities between a plant and an animal cell:

Both Have;

1. Plasma membrane
2. Distinct nucleus
3. Ribosome
4. Endoplasmic reticulum
5. Cytoplasm
6. Golgi apparatus

Qn What is an organelle?

An organelle is a distinct part of a cell which has a particular structure and function e.g. Mitochondria, chloroplast, ER etc.

CELL WALL

Cell wall is the structure that occurs externally to the cell.

Organisms with cell wall include.

1. Bacteria – have cell wall made up of murein and peptidoglycogen.
2. Fungi – has cell wall made up of chitin.
3. Algae and plant have cell wall made up of cellulose.

Plant cells cell walls.

It is the structure external to the cell; it isn’t an organelle although it is a product of various cell organelle e.g. microtubules and Golgi apparatus.

CHEMICAL COMPOSITION.

It is made up of cellulose (mainly fibres) forming amorphous matrix of the cellulose that surrounds the entire cell.

Such fibre is made up of several hundred microfibrils which form the network of cell wall.

In addition to cellulose plant cell wall consists of pectron and hemicellulose which contribute to mechanical strength of the organism.

Pectron

These are polysaccharides of galactose and galactronic acid. Pectron may combine with Ca²⁺ or Mg²⁺ to form calcium pectate or magnesium pectrate, which are important components of the first layer of cell wall to be laid down on middle lamella.

Hemicellulose

Hemicellulose is the mixture of many compounds, but the chief ones are sugar e.g. glucose and sugar acid residue.

Hemicelluloses which form hydrogen bounds with cellulose fibres in the cell matrix. The cell wall is usually modified by deposition of other substances such as alginic acid and calcium carbonate in the case of algae.

Functions of cell wall.

1. Mechanical support and skeletal support of individual cell and plants as well. This is through lignifications.
2. To prevent cell from bursting in hypotonic solution.
3. Control cell growth and shape. Orientation of cellulose microfibrils limits and helps to control cell growth and shape because of the cells ability to stretch is determined by their arrangements.
4. Movement of water and material salts.

The system of interconnected cell walls (apoplast) is a major pathway of the movement of water and dissolved mineral salts.

The cell walls are held together by middle lamellae, they also posses minute pores through which structures called plasmodesmata form living connections between cells and allows the protoplast to be linked in a system called symplast.

1. Reduction of water loss and reduced risk of infection (due to its waxy cuticle).
2. Transportation of materials. The walls of xylem vessels and sieve tubes are adopted for long transportation of materials through the cells.
3. Barrier to water movement.
4. The cell walls of root endodermal cells are impregnated with suberin that forms a barrier to water movement.
5. Some cell walls are modified as food reserves as in the storage hemicelluloses in some seeds.
6. Transport of materials by active transport.

The cell wall of transfer cells develops an increased surface area and this increases the efficiency and transfer materials by active transport.

 

CELL ORGANELLES OR ORGANOIDS.

Besides the cellular inclusion and plastids, the cytoplasm matrix contains many large sized structures known as cell organelles or organoids which perform various important synthesis, transportation, support and reproduction.

These organelles are the endoplasmic reticulum, ribosome, Golgi complex, liposomes, mitochondria, plastids, centrioles, cilia etc.

Functions of cytoplasm

1. It provides medium for chemical reaction to take place like protein synthesis, lipids synthesis and etc.
2. It stores useful materials such as amino acids, proteins, starch, carbohydrates, lipids, O₂ etc.
3. It stores waste materials such as C0₂ and nitrogen waste etc.
4. It controls the absorption of materials across the membrane due to its concentration gradient.

CELL ORGANELLES

1. ENDOPLASMIC RETICULUM

Is the cytoplasm matrix, is transverse by a vast reticulum or network at interconnecting tubules and vesicles which is known as endoplasmic reticulum or ER.

The endoplasmic is having a single vast and interconnected cavity which remains bounded by a single membrane. The membrane of endoplasmic reticulum is supposed to be originated in pushings of plasma membrane in the hyloplasm (matrix) because chemically it consists of a lipoproteinous structure like plasma membrane.

The membrane of the endoplasmic reticulum may be either smooth when they do not have attached ribosome and rough when they have the attached ribosome.

The membranes of endoplasmic reticulum are found to be continuous with the nuclear membrane and plasma membrane.

FUNCTIONS OF ENDOPLASMIC RETICULUM

1. Transport of materials from exterior to the nucleus or to cytoplasm organelles such as Golgi complex.
2. It provides mechanical support to the cytoplasm matrix.
3. Functions as a cytoplasm framework.

Surfaces for some of the biological activities of the cell catalyst its complex folding provide an enormous surface for such activities.

1. Synthesis and transfer of lipids.( smooth endoplasmic reticulum)
2. In the liver the smooth endoplasmic reticulum detoxifies many poisons and drugs.
3. The rough endoplasmic reticulum transports proteins synthesized in the ribosome of the rough endoplasmic reticulum.
4. Formation of Golgi bodies as they are modified endoplasmic reticulum.
5. Routes for movement of materials from the nucleus to the cytoplasm.

 2. GOLGI APPARATUS/ DICTYLOSOMES

This cell organelle is also known as the Golgi body, Golgi complex or sityasome.

 

It is the apparatus which consists of membranous sacs called cisternae and a system of small vesicle (called Golgi vesicles or dictysome vesicles) and vacuoles of various sizes.

The membranes of Golgi complex are of lipoproteins and these are supposed to be originated from the membrane of endoplasmic reticulum.

FUNCTIONS

1. Produce secretions

There are many Golgi apparatus in;

• Cells of salivary gland
• Cells of root cap
• Cells of endocrine glands i.e. pancreas

2. Modification of materials.

The combination of carbohydrates and proteins to form glycoprotein takes place in them. Many materials such as mucin are glycoprotein. It takes place in the cistern.

Carbohydrate chain + lipids = glycolipids

3. Production of carbohydrates example cellulose produced in plants after division. Thus this separates one cell from another.

4. Transport of lipids (storage and transport of proteins and lipids) after digestion, the fatty acids and glycerol are formed. In the endoplasmic reticulum fatty acids and glycerol unite to form lipids  (triglycerides). 

The latter are passed to the Golgi apparatus where it transports them to the plasma membrane as lymphatic system and going to the lymphatic system.

5. Formation of lysosomes.

6. Synthesis of various types of carbohydrates from simple sugars.

7. It activates the mitochondria to produce ATP.

8. It forms the acrosome of the sperms.

3. LYSOSOMES.

These are spherical single membrane bound organelles containing digestive enzymes.

-lipase

-carbohydrases

– Nucleases

The enzymes are synthesized in ribosome RER transported to the Golgi apparatus for modification. The Golgi vesicles are detached from the Golgi apparatus and remain in the cytoplasm as lysosomes because they contain digestive enzymes.

FUNCTIONS

1. Functions as storage vesicle for many powerful digestive (hydrocytic) enzymes.

2. Acts as digestive system of the cell enabling it to process some of the bulk materials taken in by phagocytosis or pinocytosis. Digests parts of the cell such as worn out organelles and also to digest the stored food contents of chloroplast A and B in extracellular digestion.

3. Play role in some developmental process e.g. remolding of bones and fractures.

NB: in plant cells, the large contrast vacuole may act as lysosomes although bodies similar to lysosomes of an animal cell sometimes seen in the cytoplasm of a plant cell.

4. RIBOSOMES.

Structurally it has two sub-units, i.e. small subunit and large subunit.

Each of the two subunits is composed of rRNA (ribosomal RNA) and proteins.

It is present in both eukaryotic and prokaryotic cells. The sizes can be determined by the sedimentation when centrifuging showing the 80’s and 70’s ribosome.

-80’s ribosome are present in R.E (rough endoplasmic) reticulum of eukaryotic cells.

-70’s ribosomes are present in prokaryotes as well as mitochondria and chloroplasts of eukaryotic cells.

FUNCTIONS OF RIBOSOMES

1. They provide large surface area for protein synthesis.

2. They are binding sites of the RNA.

ADAPTATIONS OF RIBOSOMES.

The ribosomes are the sites for protein synthesis. it has the following characteristics.

1. Presence of enzymes capable of catalyzing the synthesis of peptide bonds.
2. Presence of ribosomal RNA (rRNA) that attract other types of RNA i.e. mRNA and tRNA towards the ribosome’s.

5.  VACUOLES

A vacuole is a fluid filled sac which is bound by a single membrane.

In animal cells, there are relatively small and temporary vacuoles such as phagocytotic, pinocytotic, autophagic vacuoles in plant cells; the vacuole is large and occupies a greater proportion of the cytoplasm.

The membrane bounding the vacuole is the tonoplast and the fluid inside is the cell sap or vacuole sap.

The cell sap is a mixture of many substances; concentrates solutions of sugar, salt, organic acids, gases such as C0₂ and oxygen, pigments and waste products of metabolism.

It also contains enzymes similar to those of lysosomes.

ROLES OF CELL VACUOLES

1. They are involved in primary plant growth. It is a result of turgor pressure generated inside the vacuoles as a result of entry of water. This causes cell expansion as the tonoplast is pressed against the cell wall.

2. The pigment contained in the cell sap is responsible for flower color and therefore play a key role to pollination.

3. They contain enzymes similar to those of lysosomes when plant cell dies. The tonoplast looses the differential permeability and enzymes escape causing autolysis.

4. Vacuole acts as a temporary store of waste products such as crystals of waste calcium oxalate, toxins and metabolic waste products of plants.

5. The vacuoles sometimes functions as food reserves e.g. sucrose mineral salts and insulin are stored in vacuoles.

6. In prokaryotes it serves for buoyancy.

6. MITOCHONDRIA

Structure of mitochondria

It is a sausage shaped or an oval shaped organelle surrounded by a double membrane (mitochondrial envelope). The envelope consists of the outer and inner membrane.

Between the two membranes there is a space, the intermembranal space.

The outer membrane is smooth while the inner membrane is coiled to form t=surface area for attachment of membranes.

The ground substance of the mitochondrion is called matrix. This contains

1. Several enzymes responsible for Krebs cycle.
2. Circular DNA that resembles that of prokaryotic cells. It is for self replication of mitochondria.
3. 70s ribosome like those of prokaryotic cells. These are for protein synthesis e.g. enzymes

  

Diagram of mitochondrion

      

                                                                                                                       

Functions of mitochondrion

The main function of mitochondrion is to yield energy during respiration.

About 98% of energy is synthesized e.g. one molecules of glucose yield 38 ATP. Out of 38ATP 36 is synthesized in the mitochondrion by the reactions of Krebs cycle and electron transport chain. Thus it is called power house or POWER station or power plant of the cell.

Adaptations of the mitochondrion to energy productio

Presence of outer membrane and inner membrane to allow entry and exit of materials.

The inner membrane is coiled to increase the surface area for attachment of enzymes responsible for electron transfer.

Presence of matrix which is as granular and gives enough space for reaction to take place (Krebs cycle reaction) also matrix contains Krebs cycle enzymes.

Presence of circular DNA for replication of the mitochondrion.

Have 70s ribosome’s for synthesis of proteins.

Presence of phosphate for production of ATP.

Presence of Oxysome and water accompany aerobic respiration.

NB: the inner folded to form partitions called cristae which enables different types of metabolic activities to take place. This phenomenon is called compartmentalization hence enables multi-enzymes systems to operate.

ENDOSYMBIOTIC THEORY

(Evolution of mitochondria)

The mitochondria were originally independent prokaryotic bacteria like organisms which entered hosts cells and develop mutual relationship (symbiosis).

 MITOCHONDRIA AS PROKARYOTIC CELL

Posses its own DNA and is able of self replication / reproduction.

Have a circular like bacteria DNA.

It is sensitive to different antibiotics such as chlorophyll and streptomycin which inhibit mitochondrial activities.

It contains ribosomes similar to those of bacteria.

7. PLASTIDS

These are organelles with double membrane, located in plant cells and algae

Types

1. Chromoplasts
2. Leucoplasts
3. Chloroplasts

1. CHROMOPLASTS

(Chromo – color / pigment)

These are types of plastids bearing pigments i.e. yellow, red, orange, purple pigments.

Found in

1. Flowers
2. Fruits
3. Seeds
4. Leaves
5. Roots of carrots.

2. LEUCOPLAST (embryos and germ cells)

Leuco- colour / white.

These are colour plastids found mainly in storage organs. There are various types of leucoplasts;

1. Amyloplasts- contain starch
2. Lipoplasts – stores lipids
3. Proteoplasts- stores proteins

Structure of chloroplasts

The chloroplast

-the chloroplast is an oval shaped green in color due to presence of chlorophyll.

– It has two membranes an outer and an inner membrane which constitutes the double membrane or chloroplast envelope.

-Between the membranes there is the inter membrane space.

– The ground substance of the chloroplast is the stroma.

– The latter has a system of parallel running membranes called thylakoids.

-the interval between one grannum and the other is called intergranal lamellae.

– The stroma contains circular DNA and fewer small 70’s ribosomes and starch granules.

Functions of chloroplasts

1. It is the site of photosynthesis.

This is the process whereby green plants manufacture food from CO₂ and water in the presence of light energy, it stores starch temporarily.

The thylakoids have chlorophyll pigment for trapping sunlight energy.

It has grana and thylakoids to hold the chlorophyll in proper position for maximum absorption of light energy.

Stroma contains enzymes for dark reactions of photosynthesis.

Presence of phosphate which acts as a source of phosphate during phosphorylation.

Ribosomes and circular DNA for synthesis of proteins such as enzymes

Endosymbiotic nature of chloroplasts and mitochondria.

The chloroplast and the mitochondria are endosymbiotic structures within a cell. They are capable of leading life within a cell because;

They have double membrane which allows passage of materials in and out of their inside.

They have their own hereditary materials i.e. circular DNA. They are capable of self replicating.

They have ribosomes (70’s) thus synthesize proteins. E.g. enzymes.

Have matrix or stroma, the ground substance where various reactions take place.

STROMA; various photosynthetic membrane are found where light reactions take place and dark reactions in the aqueous part.

MATRIX: Krebs cycle of respiration.

1. They have their own enzyme system.

Therefore chloroplasts and mitochondria are said to be cells within cells.

The endosymbiotic nature of chloroplasts and mitochondria can be described as serial endosymbiotic theory (SET).

SERIAL ENDOSYMBIOTIC THEORY.

This theory accounts for the evolution of eukaryotic cells from prokaryotic cells.

Evidence / similarities of organelle and prokaryotic cells

• Double membrane as cell membrane.
• Circular DNA.
• 70’s ribosomes.
• System of enzymes.

SERIAL ENDOSYMBIOTIC THEORY.

It was suggested that mitochondrion, chloroplasts are descendants of ancient prokaryotic organisms.

-Eukaryotic cells arose from invasion of one large cell by other prokaryotic cells.

The SET states that;

“All eukaryotic cells contain genetic material (DNA) ribosomes that resemble those of prokaryotic cells”.

-It suggests that prokaryotic heterotropes ingested other mitochondrion like prokaryotic and roughly at the same time began forming an organized nucleus.

Subsequently, non motile cells established a symbiotic relationship with yet another prokaryote in the form of spirochetes or spiroplasma bacterium, attached to the outside of the cell. Such as bacterium has a function like flagellum.

Eventually a photosynthetic prokaryote engulfed by this regardless as a primitive plant cell.

QNS

1. Chloroplasts, mitochondria and bacteria have features in common. Enumerate the features to reveal the truth of this statement.
2. Where in the body would you expect to find large number of mitochondria? Give reasons.
3. If mitochondria were to perform the function of the function of the chloroplast, what modification would it require.

8. MICROBODIES OR PEROXISOMES

These are small spherical bodies with 0.5 – 1.5 micrometers in diameter. The ground substance of a micro body contains important enzymes especially catalyze or peroxidase.

These enzymes catalyse the hydrolysis of hydrogen peroxide in water and oxygen.

These peroxisomes are found in liver, potatoes, pea seeds and bean seeds.

Diagram

FUNCTIONS OF PEROXISOMES

1. To break down the poisonous hydrogen peroxide to water and oxygen in the presence of peroxidase enzyme/ catalase.

2. In plants special peroxisomes called glycoxisomes are centre’s for glycoxylate cycle i.e. conversion of fats into carbohydrates especially during germination.

3.The leaf of peroxisomes are centers of photorespiration, especially in C₃ plants e.g. beach plants, potato plant, tomato, coffee in cold areas.

CYTOSKELETON

This is a complex network of fibrous protein structure that exists in cytoplasm of eukaryotic cell and anchor proteins or organelles such as nucleus to their fixed location.

The structures which constitute cytoskeleton include;

1. Microfilament( actin filaments)
2. Intermediate filaments
3. Microtubules

MICROFILAMENTS(ACTIN FILAMENTS)

These are thread like structures arranged in sheets or bundles first beneath the cell surface membrane.

Diagram

-Chemically they contain actin and myosin.

-Each fibre is composed of two chains of protein loosely twisted about one another in helical manner. These proteins molecules can be assembled and dis-assembled.

FUNCTIONS

• Interactions of these fibres with myosin help in muscle contraction.
• Determine the shape of cell’s skeleton.
• Responsible for movement of materials within the cells.
• Cleavage of animal cells is brought about by the constriction of a ring of microfilaments after nuclear division, cytokinesis.

2. INTERMEDIATE FILAMENTS.

 

These are structures intermediate between microtubule and microfilament (rope like microtubule of polypeptides)

Skin cells for example form intermediate filaments from proteins called KERATIN. When the skin dies the intermediate filament of the cytoskeleton persists.

Hair and nails are formed this way.

FUNCTION

1. Provide cells shape
2. Act as intercellular tendons preventing excessive stretching of cells.

MICROTUBULES

Microtubules are tubular structures made up of helizelly arranged globular subunit called tubulin.

-They are about 25 nm in diameter. Each has a chain of proteins wrapped round and round in a tight spiral. Large microtubules are found in cilia, flagella, centrioles (formation of spindle- fibres microtubules).

Functions

1. They bring about movement of chromosomes during metaphase in nuclear division.
2. Since they are tubular, they transport materials from one part of the cytoplasm to another, i.e. they are cytoconductors.
3. In cilia and flagella, they help in rhythmical beating up movement.
4. They determine the shape of the cell. (Skeletal support).

9. CILLIA AND FLAGELLA.

The cells of many unicellular organisms and ciliated epithelium of multi-cellular organisms consists of some hair like cytoplasm projections outside the surface of the cell.

-These are known as cilia or flagella and they help in locomotion of the cells. The cilia and flagella are made up of proteins adenosine triphosphate (ATP).

-In prokaryotic cells, cilia and flagella (If they have structure lacking 9+2 arrangement of microtubules and arise from basal bodies).

-In eukaryotic cilia and flagella are complex. They have the 9+2 arrangement of microtubule and arise from basal bodies.

10. CENTRIOLES.

Centrioles are present in animal cells only.

-They are two placed at right angle to each other.

-A number of rays called ultra rays usually surround the centrosomes.

Each centriole is composed of nine paired thin threads and is in the form of cylinder.

They aid in cell division.

11. PINOCYTOTIC VESSICLE

These are organelle formed as a result of in folding of plasma membranes as it takes large particles of food from outside the cell.

The process is called pinocytosis.

Eventually pinch off and form very small vacuole (vesicle).

FUNCTIONS

Transport large particles into the cell.

12. NUCLEUS.

-Nucleus is the functional unit of a cell.

It contains materials which control different activities within the cell; the genetic materials.

STRUCTURE OF THE NUCLEUS.

The nucleus has a membrane called nuclear membrane envelope.

Then nuclear membrane has some pores which allow some materials to pass in and out of nucleoplasm to allow communication on with cytoplasm called nuclear pores.

-Nuclear pores are made up of non-membrane materials forming nuclear pores.

-Nuclear envelope is semi permeable membrane allowing some materials to pass and others not to pass.

-The space inside the nucleus is filled by fluid materials which are called nucleoplasm. These are semisolid granules ground substance or matrix.

Within the nucleoplasm there are two components;

1. Nucleolus
2. Chromatin
3. Matrix (aqueous)

Chromatin threads

Chromatin threads are grainy network of strands that undergo cooling into rod-like structures called chromatin.

Chemically chromatin and therefore chromosomes contains DNA (deoxyribose nucleic acids) and much protein and some RNA (ribonucleic acids) and few minerals.

Nucleolus

These are small dark regions where different RNA type examples ribosomal RNA is produced and RNA joins the protein to form the subunit of ribosomes.

-It synthesizes the ribosomes protein and is used in controlling the cell division.

Functions of nucleolus

1. Controls all metabolic activities of the cells
2. It regulates cell division.
3. Concerned with transmission of hereditary traits from parent to offspring.
4. Synthesizes and stores proteins.

PROKARYOTIC  CELL

A WELL LABELLED DIAGRAM OF A BACTERIAL CELL.

PROKARYOTIC CELL e.g. bacteria, cyano bacteria.	EUKARYOTIC CELL e.g. protoctista, green plants, animal and fungi.
Usually extremely small cells.	Usually large cells about 10-100 micrometer
2. Nucleus absent, naked circular DNA	Distinct nuclear region DNA helical shaped enclosed in a protein coat.
3. No nucleus.	Nucleus present
4. Few organelles and non are surrounded by an  envelope (double membrane).	Many organelles envelope(bound) organelles ( i.e. double membrane bound organelles)
5. Internal membrane if present usually associated with respiration or photosynthesis.	Great diversity of internal membrane organelle e.g. Golgi apparatus, lysosomes, ER.
6.Flagella are simple lacking arrangement of microtubule.	Complex flagella with ( 9+2) arrangement of microtubule.
7. Have mesosome for respiration.	Use mitochondria for respiration
8. Some are nitrogen fixing.	No ability to fix nitrogen.
9. 70’s ribosomes.	80’s ribosomes

Similarities between prokaryotic and eukaryotic cells.

Both have;

1. Structure for movement (cilia and flagella)
2. Cell wall.
3. Cell membrane.
4. Ribosome’s.
5. Genetic material.(DNA)
6. Storage of food organelles.

QUESTIONS

1. a. Give the principle constituent of the cell membrane.

b. Draw a fully labeled diagram to illustrate the arrangement of these constituents and others in the fluid mosaic model of the cell wall membrane.

c. why is the model described as being fluidy?

d. Give two functions of the cell membrane.

2. Describe the role of the following membranous organelles; lysosomes, endoplasmatic reticulum, ribosome’s and Golgi apparatus.