THE CONCEPT OF CYTOLOGY.
Cytology is the study of cells, their structures, functions, characteristics and adaptations.
THE
CELL THEORY
The bodies of all living things are made up of cells.
Robert Hooke (1665) was the first person to discover a cell
from a plant cork. The cells looked like boxes. Other people who studied the
structure of cells are Lamark (1809), Detrochet (1824) and Turpin (1826).
Schleiden (1838) studied the plant cells and emphasized
that the cells are organisms and entire animals and plants are aggregations of
these organisms arranged according to the definite laws.
In 1839 Schwann, a German botanist stated that ” we have
seen that all organisms are composed of essentially like parts namely of
cells”.
IMPORTANCE
OF CYTOLOGY
Cytology has been very important discipline in the research
diagnosis and treatment of human diseases. Most of health problems people
encounter involve the cell disturbances.
The study examines cell interaction. Studying how cells
interact or relate to other cells or environments the cytologists can predict
problems or examine the dangers to the cell and identity type of infections.
THE
MAIN IDEAS OF THE CELL THEORY
1. All
organisms are made up of cells.
2. The
new cells are derived from the pre-existing cells by the process of cell
division (mitotic and meiotic division).
3. All
chemical reactions/metabolic activities in the bodies of the organisms take
place within the cells.
4. The
cells contain hereditary materials which are passed from one generation to
another.
5. Given
a suitable condition, a cell is capable of independent existence.
CHALLENGES
OF THE CELL THEORY
• Hereditary
materials are also found in viruses, mitochondria and chloroplasts, all of
which are not viruses.
STRUCTURE
OF CELLS AND FUNCTIONS
The
five structures are also known as ultra structure and are obtained by two
techniques. Physiological
or metabolic activities take place within a cell. Viruses though are not cells,
have life within their hosts.
• 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.
• 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. Electronic
microscope. 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
1
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.
1. It
realizes the presence of phospholipids bilayer and protein layer.
2. The
presence of polar head (hydrophilic) and non polar tail (hydrophobic) in the
phospholipids.
3. It
shows that the membrane is not static.
4. It
shows the presence of carbohydrates.
5. It
shows that the protein layer is not continuous.
6. It
indicates the presence of pores in the membrane passage of materials.
Diagram
2
FUNCTIONS
OF CELL MEMBRANES.
1. It
protects the cytoplasm contents of the cells.
2. It
allows passage of materials in and out of the cells since it has pores.
3. In
some membranes e.g. those of the intestine cells, there are microvilli which
increase the surface area for absorption of materials.
4. Acts
as receptor sites for chemical stimuli such as hormones.
5. In
nerve cells, the membrane is over lined with a fatty sheath (myelin sheath)
which prevents the spreading of local currents to other neurons.
6. 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;
1. Pinocytosis
When the ingestion of food materials
in bulk takes place by the cell through the process known as pinocytosis.
2. 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.
o 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:
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 3
DIAGRAM
OF ANIMAL CELL UNDER ELECTRON MICROSCOPE
DIAGRAM 4
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
5
Diagram
of a plant cell under light microscope
DIAGRAM 6
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; cytokineses follows centro-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
7. 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.
8. Bacteria
- have cell wall made up of murein and peptidoglycogen.
9. Fungi
– has cell wall made up of chitin.
10. 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.
5. Reduction
of water loss and reduced risk of infection (due to its waxy cuticle).
6. Transportation
of materials. The walls of xylem vessels and sieve tubes are adopted for long
transportation of materials through the cells.
7. Barrier
to water movement.
The cell walls of root endodermal cells are impregnated
with suberin that forms a barrier to water movement.
8. Some
cell walls are modified as food reserves as in the storage hemicelluloses in
some seeds.
9. 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.
4. Synthesis
and transfer of lipids.( smooth endoplasmic reticulum)
5. In
the liver the smooth endoplasmic reticulum detoxifies many poisons and drugs.
6. The
rough endoplasmic reticulum transports proteins synthesized in the ribosome of
the rough endoplasmic reticulum.
7. Formation
of Golgi bodies as they are modified endoplasmic reticulum.
8. Routes
for movement of materials from the nucleus to the cytoplasm.
DIAGRAM 7
2. GOLGI APPARATUS/
DICTYLOSOMES This cell organelle is also known as the Golgi body, Golgi
complex or sityasome.
DIAGRAM 8
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
1. 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 9
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
1. Presence
of outer membrane and inner membrane to allow entry and exit of materials.
2. The
inner membrane is coiled to increase the surface area for attachment of enzymes
responsible for electron transfer.
3. 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.
4. Presence
of circular DNA for replication of the mitochondrion.
5. Have
70s ribosome’s for synthesis of proteins.
6. Presence
of phosphate for production of ATP.
7. 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 multienzymes 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
1. Posses
its own DNA and is able of self replication / reproduction.
2. Have
a circular like bacteria DNA.
3. It
is sensitive to different antibiotics such as chlorophyll and streptomycin
which inhibit mitochondrial activities.
4. 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.
2. The
thylakoids have chlorophyll pigment for trapping sunlight energy.
3. It
has grana and thylakoids to hold the chlorophyll in proper position for maximum
absorption of light energy.
4. Stroma
contains enzymes for dark reactions of photosynthesis.
5. Presence
of phosphate which acts as a source of phosphate during phosphorylation.
6. 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;
1. They
have double membrane which allows passage of materials in and out of their
inside.
2. They
have their own hereditary materials i.e. circular DNA. They are capable of self
replicating.
3. They
have ribosomes (70’s) thus synthesize proteins. E.g. enzymes.
4. 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.
5. 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 10
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
1. MICROFILAMENTS (ACTIN FILAMENTS)
These are thread like structures arranged in sheets or
bundles first beneath the cell surface membrane.
Diagram 11
-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.
1. 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.
3. 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
1. A
WELL LABELLED DIAGRAM OF A BACTERIAL CELL.
DIAGRAM
12
|
PROKARYOTIC CELL e.g. bacteria, cyano
bacteria.
|
EUKARYOTIC CELL e.g. protoctista, green
plants, animal and fungi.
|
|
1. 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 Complex flagella with ( 9+2) arrangement of of microtubule.
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.
CELL DIFFERENTIATION
This is the specialization of a cell in terms of both
structure and functions. Ability of a cell to perform single function is called
cell specialization. Cells work in interdependence with each other such that
such that group of cells must be coordinated so that they carry out their
activities efficiently such coordination is called integration.
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