Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
Ombiology4u Form 4
Cell biology or cytology is the study of the cellular structure and function.
 All cells share at least four components in common:
1. Plasma membrane (Cell membrane)
2. Cytoplasm
3. Genetic material (DNA)
4. Ribosomes
What is cell?
Cell is the smallest structural and functional unit of an organism.
The Cell Theory
 The cell theory states that;
1) Living organisms consist of one or more cells. (All living things are
made up of cells).
2) Cells are the basic structure and function of living organisms. (Cells are
the basic units of life).
3) All cells arise from pre-existing cells.
The cell was discovered
and named by an English
scientist Robert Hooke,
in 1665.
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Plant and Animal Cells
o Animal and plant cells have the below similarities in common:
1. Cell membrane (Plasma membrane)
5. Golgi apparatus
2. Cytoplasm
6. Mitochondria
3. Nucleus
7. Ribosomes
4. Endoplasmic reticulum (ER)
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
The Nucleus
Ombiology4u Form 4
Control Center of the Cell
o Nucleus is a double-membrane bounded organelle found in eukaryotic cells,
except mature red blood cells in mammals.
o In eukaryotic cells, most of the genetic material (DNA – deoxyribonucleic acid) is
housed within the nucleus.
o The nucleus is known as the control center of the cell as it directs the activities of
the cell.
Functions of the Nucleus
1234-
It controls the activities of the cell (control center of the cell).
It stores the genetic material. (it carries the hereditary information).
It controls cell reproduction (regulates cell division).
It regulates the information needed to synthesize proteins.
Structure of the Nucleus
o The nucleus is surrounded by a double membrane called nuclear membrane
(nuclear envelope), which separates the nucleus from the cytoplasm.
o The nucleus contains a semi-fluid called nucleoplasm in which both genetic
material and nucleolus are suspended in it.
 The nucleus consists of four main structures, which are:
(1) Nuclear membrane (nuclear envelope)
(2) Nuclear pores
(3) Nucleolus
(4) Chromatin
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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Parts and Functions of the Nucleus
Structure
Nuclear
envelope
(also called
nuclear
membrane)


Description
Function
The nuclear envelope is a
double membrane which
 The nuclear envelope
encloses the nucleus of
separates the nuclear material
eukaryotic cells.
from rest of the cell’s contents.
The nuclear membrane is full of  Protects the nucleus.
small holes called nuclear pores.
Nuclear pores

Nuclear pores are small holes
on the nuclear membrane.
Nucleolus

(Site of ribosome
assembly)

Chromatin

Nucleolus (Nucleoli) is spherical
Makes ribosomes
structure in the nucleus.
Nucleolus is made up of RNA
and proteins
When the cell is not dividing, the
nucleus contains thin thread –
 Provides instructions for the cell
like structure called chromatin,
activities.
made up of DNA and protein.
 Packages DNA into a smaller
During cell division, chromatin
volume to fit into the cell.
condenses into chromosomes.
(The heredity
material)

The nuclear pores allow materials
to pass between the nucleus and
cytoplasm.
Cytoplasm
o
Cytoplasm is a semi-fluid (jelly-like substance) in the cell, between the cell
membrane and nucleus.
o The cytoplasm is composed of 70% to 90% of water, and also contains enzymes,
salts, organelles and different organic molecules.
o The liquid part of the cytoplasm is called cytosol.
o The living substance of the cell is called protoplasm.
Form 4 Past Exam Review Question – 2017
1. The part of the living substance other than the
nucleus is called;
A. Protoplasm B. Cytoplasm
C. Endoplasm
D. Ectoplasm
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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Functions of the Cytoplasm
i) Site of most chemical reactions in the cell. (Site of cellular reactions)
ii) Gives support and maintains the shape of the cell.
iii) Holds the organelles of the cell.
Cell Membrane
o
Cell membrane (plasma membrane) is the outer layer of animal cells and second
layer of plant cells.
o
The cell membrane is a double layer of phospholipids (lipid bilayer) with proteins
and carbohydrates.
o
The plasma membrane (cell membrane) acts as a selective barrier between the
cell and its surrounding.
Plasma membrane regulates what gets in and out of the cell.
The plasma membrane is called selectively permeable membrane (semipermeable membrane) because it allows the passage of some substances and
stops others.
o
o
Functions of the Plasma Membrane
(i) It regulates movement of substances into and out of the cell. (It regulates
what gets in and out of the cell).
(ii) It isolates the cytoplasm from the external environment.
(iii) It gives shape to the animal cells.
(iv) It allows interaction with other cells.
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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Endoplasmic Reticulum
o Endoplasmic reticulum (ER) is a network of parallel and interconnected
membranous tubes in the cytoplasm.
o It is present in eukaryotic cells but absent in prokaryotic cells and RBCs in
mammals.
o The two types of endoplasmic reticulum are;
1. Rough endoplasmic reticulum (Rough ER)
2. Smooth endoplasmic reticulum (Smooth ER)
Types of Endoplasmic Description
reticulum (ER)
1. Rough
endoplasmic  Ribosomes attach on the
reticulum (RER)
surface of Rough ER.
 Attached on nuclear membrane
of the nucleus.
 Found in protein forming cells
2. Smooth
endoplasmic  Has no ribosome on its surface.
reticulum (SER)
 May
attach
the
nuclear
membrane of the nucleus or
found in the cytoplasm.
 Found in lipid forming cells.
Function
Transport
proteins
made
by
the
ribosomes.
-
Synthesis lipids
Makes
steroid
hormones
Functions of the Endoplasmic Reticulum (ER)
1.
2.
3.
4.
5.
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Transports materials in the cell, e.g. proteins.
Synthesizes lipids.
Keeps the cell’s shape.
Breaks down foreign chemical substances.
Storage of synthesized substances.
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Golgi Apparatus
o Golgi apparatus is also known as Golgi body or Golgi complex.
o Golgi apparatus was discovered in the year 1898 by an Italian biologist Camillo
Golgi.
o Golgi apparatus is found in the cytoplasm of all
eukaryotic cells except RBCs ad also absent in
prokaryotic cells.
o Golgi apparatus is a specialized flattened membranous
sacs derived from the ER.
o Golgi apparatus collects, processes, sort molecules and
packages materials into vesicles (secretary vesicles).
o Secretary vesicles transport substances to other parts of the cell or discharge
(remove) substances to exterior or outside of the cell.
Functions of the Golgi body
(a)
(b)
(c)
(d)
(e)
It separates (sorts), modifies and packages substances into vesicles.
It creates and produces Lysosomes.
It is the major site of carbohydrate synthesis.
It produces enzymes.
It synthesizes cell wall components for plants, e.g. cellulose.
Mitochondria
o Mitochondria (singular = mitochondrion) are rod-shaped and double membrane
bounded organelles found in the cytoplasm of most eukaryotic cells.
o Mitochondrion is surrounded by two membranes (outer and inner membranes).
o The outer membrane of mitochondria is smooth.
o The inner membrane of the mitochondria is folded
which forms cristae and filled with a fluid called
matrix.
o Mitochondria contains DNA (deoxyribonucleic acid).
o Mitochondria are the site for aerobic respiration.
o Mitochondria contain enzymes responsible for
synthesis of energy for the cell.
o Mitochondrion is the site of energy production in the
cell.
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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o Mitochondria generate most of the energy of the cells and store it in form of
adenosine triphosphate (ATP) which is used as a source of chemical energy.
o ATP (adenosine triphosphate) is a short term energy carrier molecule.
Functions of the Mitochondria
a) Site of energy production (power house of the cell // site of cellular respiration).
b) Formation of ATP molecule.
Ribosomes
o
o
o
o
Ribosomes are small organelles found in both eukaryotic and prokaryotic cells.
Ribosomes are made up of rRNA and proteins.
Ribosomes are not surrounded by a membrane.
Ribosomes are rounded organelles found in the cytoplasm which assemble the
amino acids transferred by tRNA to make proteins.
o There are two types of ribosomes:
1. Free ribosomes - they are found freely in the cytoplasm
2. Attached or fixed ribosomes – they are attached on the surface of Rough
endoplasmic reticulum.
Function of Ribosomes
1. Site for protein synthesis in the cell (make proteins in the cell).
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Lysosomes
o Lysosomes are small spherical bags surrounded by a single membrane and
more abundant in animal cells than in plant cells.
o Lysosomes contain digestive enzymes and are known as digestive bags,
suicide sacs or cellular keepers.
Functions of the Lysosomes
(A) They digest proteins, fats and carbohydrates in the cell.
(B) They digest worn out cells and organelles.
(C) Lysosomes of white blood cells (phagocytes) digest or break down
bacteria.
(D) They break down unwanted materials in the cell.
(E) The head of the sperm contains Lysosomes which digest the female
egg layers.
Centrioles
o Centrioles are pair of small organelles made up of microtubules which lie close
together at right angle to each other.
o Centrioles are only found in animal cells, not in plant cells.
o Centrioles are hollow and cylindrical structures made up of nine triplets of
microtubules arranged in a ring.
o Centrioles are found in spherical body called centrosome, found in the
cytoplasm near the nucleus.
Functions of Centrioles
1. Centrioles play an important role in cell division.
2. Centrioles form spindle fibers which separate chromosomes during cell division.
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Cytoskeleton
o Cytoskeleton is an internal framework of the cells, made up of protein filaments
and found in the cytoplasm of most cells.
o There are three kinds of cytoskeleton, microtubules, microfilaments and
intermediate filaments.
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Functions of Cytoskeleton
1)
2)
3)
4)
5)
It gives mechanical support to the cell (supports the cell).
It provides anchorage for organelles inside the cell.
It provides cell motility (cell movement).
It plays an important role in cell division.
It helps muscle cell contraction.
Cilia and Flagella
o Cilia and flagella are extensions of the plasma membrane.
o Each cilium and flagellum contains a ring of nine fused pairs of microtubules,
with an unfused pair of microtubules in the center of the ring forming 9+2
arrangement of microtubules.
o The flagella (singular = flagellum) are long, hair-like structures that extend from
the plasma membrane, example, sperm cells and bacteria.
o Cilia (singular = cilium) are short, hair-like structures are present on the entire
surface of the plasma membrane.
Functions of Cilia
(A) Cilia help movement of some single-celled organisms.
(B) Cilia move mucus or trapped particles out of the respiratory tract.
(C) Cilia lining the fallopian tubes (oviduct) move the fertilized egg toward the
uterus (womb).
Functions of Flagella
A) Flagella help movement of the unicellular organisms. (Used in motility for
single-celled organisms).
B) Sperm cells use flagella to move.
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Form 4 Biology Notes – Cell Biology – Topic: Cell Structure & Function –
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Plant Cell
o Plant cells differ from animal cells; they contain the below cellular structures
which are not present in animal cells;
1. Cell wall
2. Large permanent vacuole
3. Chloroplast
Cell Wall
o Cell wall is tough, rigid layer which surrounds the cell membrane of plant cells.
o Animal cells have no cell wall.
o The cell wall is made up of a carbohydrate called cellulose, which makes plant
cell tough.
o The cell wall is a fully permeable membrane, which allows water and other
substances to pass through it freely, while the cell membrane is selectively
permeable membrane by allowing the passage of some substances and stops
others.
o Plant cell walls have microscopic channels called plasmodesmata, which allows
cell to cell communication.
Functions of the Cell Wall
(a) It gives plant cells a definite shape. (Protects and keeps the shape of the cell).
(b) It prevents the cell from bursting (rupturing).
(c) It gives protection, support and strengthening.
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Vacuole
o Vacuole is a membrane-bounded fluid filled sac found within the cytoplasm of
plant cells.
o Mature plant cells have large permanent vacuole.
o Animal cells may have small vacuole, but not large central vacuole.
o Vacuole is surrounded by a single membrane called tonoplast (vacuolar
membrane).
o Tonoplast separates the contents in the vacuole from the cytoplasm.
o The fluid or solution filled in the vacuole is called cell sap.
o Cell sap in the vacuole stores water, sugar, salts and waste materials.
Functions of the Vacuole
a- It provides plant cell turgidity. (It helps to regulate the osmotic properties of the
cell).
b- Support and storage (it stores the cell sap).
c- Acts as a depository of waste materials.
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Chloroplast
o Chloroplasts are double membrane bounded organelles found in plant cells, but
not in animal cells.
o The chloroplast is surrounded by a double membrane (outer and inner
membrane) called chloroplast envelope.
o Chloroplasts are found in the mesophyll cells (palisade and spongy cells) of the
leaves.
o Chloroplast contains its own DNA (deoxyribonucleic acid).
o Chloroplast contains a green pigmented substance called chlorophyll that
traps solar energy (sunlight energy) for making food by photosynthesis.
o Chlorophyll is a pigmented molecule which traps sunlight energy and also
makes leaves green in color.
.
Structure
of the Chloroplast

The chloroplast structure consists of the
following parts:
1. Double membrane called chloroplast envelope
2. Stroma
3. Thylakoids
4. Grana (singular: granum)
5. Lamellae
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Structures of the Chloroplast
Structures of
the Chloroplast
Description


Stroma


Thylakoids
and
Grana



Stroma is a semi-fluid filled in the
chloroplast.
Stroma of the chloroplast contains
ribosomes, DNA, starch, enzymes
required for the photosynthesis in the
day time (light-dependent reaction of
photosynthesis).
Stroma also contains the enzymes
required for the synthesis of proteins
and carbohydrates.
Thylakoids are membranous sacs
found in the stroma of the chloroplast.
Thylakoids contain chlorophyll which
absorbs sunlight for photosynthesis.
Thylakoids are arranged into stacks
called grana (singular: granum)
Grana (singular” granum) are stacks of
thylakoids found the stroma of the
chloroplast.
Function
Site of lightdependent reaction of
photosynthesis
Site of lightdependent reaction of
photosynthesis.

Lamellae are thin membrane tubules
that connect thylakoid stacks or grana.
Lamellae
(stroma lamellae)

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It helps maximum
absorption of
sunlight to increase
the rate of
photosynthesis.
It connects and
separates thylakoid
stacks.
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