CELL STRUCTURE AND ORGANISATION
THE CONCEPT
OF CELL
The short history about discovery of a
cell
- The cell was first named and
discovered by an English biologist called Robert Hooke in 1665 when he was
observing a piece of cork under the microscope he had made.
- Robert Hooke noticed that the cork
was made up by many box- like units which were packed together to form a
structure like a honeycomb. He called these units cells.
- Hooke observed many other living
materials and noted that they were also made up of cells.
MEANING OF CELL
Cell: Is the basic unit of life.
OR
Cell: is a structural and functional
unit of life in living organisms.
All living things are made up of one
or more cells.
Unicellular organisms
Are organisms which are made up of one
cell.
- Example: bacteria and protozoans
Multicellular organisms
Are organisms which are made up of
many cells.
- Example: animals, plants and fungi
The characteristics of the cell
Characteristics of the cell include
the following:
(i) Cells are
microscopic.
(ii) Cells are
capable of growing and developing
(iii) Cells can reproduce and increase
in number
(iv) Cells can
move e.g. white blood cell and unicellular organisms such as bacteria
(v) Cells are
capable of detecting changes in the environment and respond to the changes.
The cell theory
The cell theories were proposed by two
scientists namely Schleiden, a Belgian botanist, in 1838 and Schwann, German
Zoologist, in 1839.
The following are observations noted
down by the two biologists after careful study of cells
(i) New cells only come from
pre-existing cells.
(ii) Cells contain structures called
organelles.
(iii) All living things are composed
of one or more cells.
(iv) Cells
carry hereditary information which is passed from one generation to another.
(v) There is a
relationship between structure and function of a cell.
(vi) Cells are
the building blocks of living things and life exists only in cells.
TYPES OF CELLS
There are two main types of cells,
namely:
(i) Prokaryotic
cells
(ii) Eukaryotic
cells
PROKARYOTIC CELLS
Are cells with no membrane-bound
nucleus.
Example: Bacteria cell
Prokaryotic organisms: are
single-celled organisms
- Prokaryotic organisms are also
called prokaryotes Example: bacteria and blue-green bacteria Characteristics of
prokaryotic cells.
(i) They have
no true nuclei.
(ii) They lack
membrane- bound organelles such as mitochondria
(iii) They are smaller in size than
eukaryotic cells.
(iv) They have few cell organelles
THE DIAGRAM OF PROKARYOTIC CELL
EUKARYOTIC CELLS
Are cells whose nuclei are bounded by
nuclear membrane.
- Example: plant cells, animal cells,
fungi cells, protoctists cells Eukaryotic organisms: are organisms which
contain eukaryotic cells
- Eukaryotic organisms are also called
eukaryotes
- Example: protoctists, fungi, plants
and animal cells
Characteristics of eukaryotic cells
(i) They have
true nuclei
(ii) They have
a membrane-bound nucleus called nuclear envelope.
(iii) They are
bigger and more complex than prokaryotic cells
(iv) They
contain specialized membrane-bound organelles such as mitochondria
(v) They have many organelles
THE DIAGRAM OF EUKARYOTIC CELL
Types of eukaryotic cells
There are two main types of eukaryotic
cells, namely:
(i) Animal
cells
(ii) Plant
cells
Other types of eukaryotic cells
include:
-Fungi cells
- Protoctists cells such as amoeba
cells, euglena cells
DIFFERENCES BETWEEN PROKARYOTIC AND
EUKARYOTIC CELLS
|
PROKARYOTIC CELL |
EUKARYOTIC CELL |
|
It lacks true nucleus |
It has true nucleus |
|
It lacks membrane-bound organelles |
It has membrane-bound organelles |
|
It is smaller in size |
It is larger in size |
|
It has few cell organelles |
It has many cell organelles |
|
It is found in bacteria and blue-green bacteria |
It is found in animals, plants, fungi and protoctists |
ANIMAL CELLS
Are cells which found in animals
Characteristics of animal cell
(i) It has
irregular shape
(ii) It lacks
cell wall
(iii) It is nucleus located at the
centre
(iv) It stores
food in form glycogen
(v) It lacks
chloroplasts
ORGANELLE
Is a specialized cellular structure or
part that perform a specific function.
Example of organelles
- Nucleus, mitochondria, ribosomes,
chloroplasts Parts/organelles of animal cell
Animal cell has the following
organelles:
(i) Nucleus
(ii) Cytoplasm
(iii) Mitochondria
(iv) Cell
membrane
(v) Ribosome
(vi) Golgi
bodies
A diagram of animal cell as seen under
a light microscope
PLANT CELLS
Are cells which found in plants
Characteristics of plant cell
(i) It has cell
wall
(ii) It has
chloroplast
(iii) It has a
large permanent vacuole
(iv) It stores
food in form of starch
(v) It is large
in size Parts/organelles of a plant cell Plant cell has the following
organelles:
(i) Nucleus
(ii) Cellulose
cell wall
(iii) Cell
membrane
(iv) Mitochondria
(v) Cytoplasm
(vi) Ribosome
(vii) Golgi
bodies
(viii) Cell sap
vacuole
Parts/organelles of a plant cell
Plant cell has the following
organelles:
(i) Nucleus
(ii) Cellulose cell wall
(iii) Cell membrane
(iv) Mitochondria
(v) Cytoplasm
(vi) Ribosome
(vii) Golgi bodies
(viii) Cell sap vacuole
A diagram of plant cell as seen under
a light microscope
SIMILARITIES AND DIFFERENCES OF PLANT
AND ANIMAL CELLS
Similarities between animal cell and
plant cell
(i) Both have
cell membranes
(ii) Both
contain cytoplasm
(iii) Both have nucleus
(iv) Both have
mitochondria
(v) Both have
Golgi bodies
(vi) Both have
ribosomes
Differences between plant cell and
animal cells
|
Plant cell |
Animal cell |
|
(i) Has definite (regular) shape |
Has no definite shape |
|
(ii) Has chloroplasts |
Has no chloroplasts |
|
(iii)Has large permanent vacuole |
Has small temporary vacuole |
|
(iv) Has cell wall |
Has no cell wall |
|
(v) The nucleus is located periphery |
The nucleus is centrally located |
|
(vi) Centrioles usually absent |
Centrioles present |
Note:
Animal cell and plant cell have common three main structures or organelles,
namely:
(i) Nucleus
(ii) Cell
membrane
(iii) cytoplasm
FUNCTIONS OF DIFFERENT PARTS OF PLANT AND ANIMAL CELLS
1. NUCLEUS
Is a dark spherical body which is surrounded
by double membrane called a nuclear membrane
- Nucleus is the largest organelle of
the cell
- The nucleus and cytoplasm form the
protoplasm.
- The nucleus is made of a viscous
fluid called nucleoplasm.
Functions of nucleus
The following are the functions of the
nucleus:
(i) Nucleus
controls all activities of the cell.
(ii) Determines
the cell's size, shape and function. (iii)Determines hereditary characteristics
of a cell.
(iv) The
nucleus contains chromosomes which carry genetic material.
(v) It contains
nucleolus which manufactures ribosomes.
(vi) Determines
the chemical processes that take place in cell.
2. CYTOPLASM
Is a transparent jelly-like fluid in
which chemical reactions take place.
- It contains up to 90% water and
dissolved substances such as carbondioxide, oxygen, proteins, sugars and
minerals.
- The movement of cytoplasm is known
as cytoplasmic stream
Functions of cytoplasm
(i) It is a
place where chemical reactions take place
(ii) It is a
site where cell organelles such as nucleus, vacuoles are suspended. (It is a
medium in which cell organelles are found)
(iii)It store useful substance such as
starch, glycogen, fat droplets, proteins, sugars and minerals.
3. CELL MEMBRANE
Is a semi-permeable membrane made of
protein and oil.
- Cell membrane is also known as
plasma membrane or plasmalemma.
- It has pore or microscopic holes to
allow in and out movement of materials of the cell.
- The cell membrane is the living part
of the cell.
Functions of cell membrane
(i) Cell
membrane controls the movement of materials in and out of the cell.
(ii) Cell
membrane encloses the contents of the cell.
(iii) It allows cell to communicate
with another cells.
Question: Why Cell membrane is said to be selective permeable
membrane?
Answer: Cell membrane is said to be selective permeable membrane
because it allows selective movement of substances into and out of the cell.
4. CELL WALL
Is a rigid outer layer that surrounds
the cell membrane in plants, fungal, algae and bacterial cells.
- Animal cell has no cell wall
- The cell wall in plant cell is made
up of cellulose.
- The cell wall is non-living part of
the cell.
Functions of the cell wall
(i) It gives
the cell its shape.
(ii) It is
freely permeable to all kinds of molecules.
(iii) It supports and protects the
cell.
(iv) It
provides mechanical support for plant body in herbaceous plants.
(v) It prevents
the cell from bursting.
(vi) It allows
gases, water and substances to go through.
Differences between cell wall and cell
membrane
|
Cell wall |
Cell membrane |
|
It is a non–living structure of a cell |
It is a living structure of a cell |
|
It is made up of cellulose |
It is made up of lipoprotein |
|
It is freely permeable |
It is selectively permeable |
5. VACUOLE
Is a fluid-filled space surrounded by
a membrane called tonoplast.
- Animal cells have small and
temporary vacuoles. In animal cells, vacuoles are small droplets of fluid in
the cytoplasm variable in size and position
- Plants cells have a large and
permanent vacuole that usually occupies the central part of the cell. Vacuole
in plant cell is made up of the fluid called sap hence the name cell sap
vacuole
- The cell sap may contain salts,
sugar and pigments dissolved in water.
- The outward pressure of the vacuole
on the cell wall makes the plant cells firm, giving strength and resilience to
the tissues.
Functions of vacuole
(i) It is responsible for food storage
and osmoregulation.
(ii) It excretes and secretes waste
from the cell.
(iii)It stores substances such as
sugar, minerals and wastes
(iv) It maintains the shape of a plant
cell
6. MITOCHONDRION
(plural: Mitochondria)
Is a sausage –shaped (oval- shaped)
organelle bounded by double membrane.
- The outer membrane is smooth but the
inner membrane is folded to form cristae.
- Mitochondria are known as "power houses of the cells" because they are used to produce energy for the cells.
Function of mitochondria
(i) To produce
energy for activities of the cell.
(ii) It is a
site where respiration process takes place
DIAGRAM OF MITOCHONDRION
7. CHLOROPLAST
Is an oval-shaped organelle that
contain the green pigment known as chlorophyll - Chloroplasts are found in plant cells
and algae cells.
- Each chloroplast is enclosed by a
double membrane, the outer and inner membranes
- They consist of layers of membranes
known as lamellae suspended in an aqueous matrix called stroma.
- Chloroplast is an organelle in which
photosynthesis takes place, producing sugars from carbon dioxide and water
using light energy trapped by chlorophyll.
Functions of chloroplast
- It is a site where photosynthesis
process takes place.
DIAGRAM OF CHLOROPLAST
CHLOROPHYLL
Is a green pigment in chloroplasts of
green plants
- It is found in the grana of the
chloroplast
Function of chlorophyll
- Used to absorb sunlight needed for
photosynthesis.
8. RIBOSOMES
Are small spherical structures
attached to the endoplasmic reticulum
- Ribosomes are manufactured in the
nucleolus. Some ribosomes are found scattered in the cytoplasm and on the
surface of the nuclear membrane.
Function of ribosome
- It is a site where protein synthesis
takes place.
9. ENDOPLASMIC
RETICULUM
Is a network of interconnected
channels running throughout the cytoplasm.
Function of endoplasmic reticulum
- Serves as a network of pathways
through which materials are transported from one part of the cell to another
Types of endoplasmic reticulum
There are two types of endoplasmic
reticulum, namely:
(a) Rough
endoplasmic reticulum
(b) Smooth
endoplasmic reticulum
(a) Rough
endoplasmic reticulum
Is the endoplasmic reticulum which is
covered with ribosomes Function of rough endoplasmic reticulum
- Transports protein synthesized in
the ribosomes
Diagram of rough endoplasmic reticulum
(b) Smooth
endoplasmic reticulum
Is the endoplasmic reticulum with no
ribosomes Function of smooth endoplasmic reticulum
- Transports lipids within the cell
Diagram of smooth endoplasmic
reticulum
10. GOLGI
BODIES
Are stacks of flattened,
membrane-bound sacs.
- Golgi bodies are also called Golgi
apparatus
Functions of Golgi bodies
(i) They form
lysosomes
(ii) They
produce materials for making plant cell walls.
(iii) They replenish the cell membrane
(iv) They process
many secretory substances such as mucus
(v) They add
carbohydrates component to the protein packaging of the finished products
before leaving the cells.
(vi) They
produce a substance, which makes the cuticle of insects
Diagram of Golgi body
THE CELL DIFFERENTIATION
Cell differentiation
Is the process by which cells are
specialized to perform a particular function OR
Is the process in which a cell changes
from simple to complex form and become more specialized to perform a specific
function.
- Cell differentiation is also called
division of labour.
Importance of cell differentiation
(i) It leads to
division of labour
(ii) It helps
the body to carry out all life processes at the same time (iii)It makes the
cell to perform a particular function.
SPECIALIZED ANIMAL CELLS
The following are common examples of
specialized animal cells:-
1. WHITE BLOOD CELLS
These cells can change their shape in
order to engulf and destroy harmful microorganisms. -
White blood cell is a cell which moves and feeds like amoeba
- Some white blood cells contain
digestive enzymes which destroy the microorganisms
Function of white blood cell
- Used in body defence against
infections
Diagram of white blood cell
2. RED BLOOD CELLS
These cells lack nuclei and are
biconcave in shape
- Lack of nucleus in the red blood
cells provides a large surface area for transporting oxygen in the body
Function of red blood cell
- Used to transport oxygen from the
lungs to various body parts
Adaptation of red blood cell to its function
(i) It lacks
nucleus at maturity to increase the surface area for transporting oxygen in the
body.
(ii) It has
hemoglobin which carries oxygen to different parts of the body.
Diagram of red blood cell
4. SPERM CELL
This cell fertilize the female egg in
order for reproduction to take place
- Sperm cell is a motile cell
Function of sperm cell
- Used in sexual reproduction to
fertilize the egg
Adaptation of the sperm cell to its
function
(i) It has a
long tail that enables it to swim to the egg
(ii) It has
numerous mitochondria for energy supply needed for swimming
(iii) It has a pointed head with
acrosome which enables it to penetrate the egg.
Diagram of sperm cell
5. NERVE CELL
Is a longest cell in the human body
- It is thread-like structure
Function of nerve cell
- It carries and conducts nerve
impulses from one part of the body to another
Adaptation of nerve cell to its
function
(i) It is long
to ensure the impulse is transmitted to a long distance of the body.
(ii) It has
numerous mitochondria for energy supply
Diagram of nerve cell
6. Egg cell
This cell is bigger in size with a lot
of cytoplasm.
- The outer layer of this cell can
produce chemicals to prevent any more sperms from entering the egg after
fertilization.
- It is also called ovum
Function of egg cell
(i) It fuses
with the sperm cell to form a zygote
(ii) It
develops into embryo after fertilization
(iii) It carries the maternal genetic
material
Adaptations of egg cell to its
function
(i) The outer
layer of this cell can produce chemicals to prevent any more sperms from
entering the egg after fertilization.
(ii) It is
large in size with a lot of cytoplasm to carry genetic materials and enough
food reserve for developing zygote
DIAGRAM OF EGG CELL
7. MUSCLE CELL
Is a cell with strong contractile
ability
à Muscle cells contract and relax to bring about movement
Function of muscle cell
- Brings about movement by contracting
and relaxing
Adaptation of muscle cell to perform
its function
(i) It has
numerous mitochondria for energy supply
(ii) It is
multinucleated to increase coordination
DIAGRAM OF MUSCLE CELL
SPECIALIZED PLANT CELLS
The following are common example of
specialized plant cells;-
(I) ROOT HAIR CELLS
These cells absorb water and mineral
salts from the soil
- They are elongated and have thin
membrane to increase the surface area for absorption of water and nutrients
from the soil.
Function of root hair cell
- Absorbs water and mineral salts from
the soil
Adaptation of root hair cell to
perform its function
(i) It is elongated to increase the
surface area for absorption of water and nutrients from the soil.
(ii)It has thin membrane to increase
the surface area for absorption of water and nutrients from the soil
(iii) It has root hairs to increase
the surface area for absorption of water and mineral salts from the soil
Diagram of root hair cell
(ii) GUARD CELL
Is a cell surrounded by stomata
- The inner walls of guard cell are
thicker than the outer walls
- Stomata are tiny pores found on the
leaf used for gaseous exchange and water loss
- When the guard cells expand, the
stoma opens and when they contract the stoma closes
Function of guard cell
- Controls opening and closing of
stomata to allow gaseous exchange
DIAGRAM OF GUARD CELL
(iii) PALISADE CELLS
Are plant cells that contain a lot of
chloroplasts
- Palisade cells are found at the top
of the plant leaves.
Function of palisade cells
- Used to carry out photosynthesis
Adaptation of palisade cells to
perform its function
(i) They have
large number of chloroplasts for carrying out photosynthesis
(ii) They
contain chlorophyll to trap sunlight energy during photosynthesis
Diagram of palisade cell
FORMATION OF TISSUE, ORGAN AND BODY
SYSTEM
- Cells in multicellular organism can
be organized into tissues
- Tissues can be organized into organs
- Organs can be organized into organ
system (body system)
- Organ system (body system) can be
organized into organism
The following below is the
organization showing the correct order from simplest to the most complex
Organelles >
Cells>Tissues>Organs>Organ system>
Organism
TISSUE
Is a group of cells that perform the
same function
Example of tissues
|
Animal tissues |
Plant tissues |
|
- Bones - Muscles - Blood - Nerve |
- Xylem - Phloem - Epidermis - Cork |
Note:
Xylem is a plant tissue used to
transport water and mineral salts from the roots to the upper plant parts.
Phloem is a plant tissue used to
transport manufactured food from the leaves to the other plant parts.
ORGAN
Is a group of different tissues that
perform a certain function.
Example of organs
|
Animal organs |
Plant organs |
|
- Heart -Eye -Brain -Liver -Kidney |
- Stem - Roots - Flowers - Leaves - Fruits |
Note: The following are types of animal organs:
(i) Respiratory organs e.g. lungs,
trachea
(ii) Digestive organs e.g. mouth,
stomach
(iii) Reproductive organs e.g. testes,
ovaries, vagina, penis
(iv) Sense
organs e.g. eyes, Nose, Tongue, ears and skin
(v) Circulatory
organs e.g. Heart
(vi) Excretory
organs e.g. Kidneys, Liver, Skin
ORGAN SYSTEM
Is a group of specialized organs that
work together to perform a certain function.
Examples of systems
ORGANISM
Is a combination of several systems
working together
Example of organisms
- Human being
- Cow
- Frog
- All green plants e.g. maize plant,
bean plant
Revision questions
1. (a) Name
three main parts of any cell
(b) Mention one
function of each of the three parts
(c) Give one
example of each of the following in the human body
(i) The longest
cell
(ii) A motile
cell
(iii) A cell
with strong contractile ability
(iv) A cell
which moves and feeds like amoeba
2. Figure 3
represents four different cell types A, B, C and D
(a) Identify
cells A, B, C and D
(b) Each of the
above cell is specialized in performing a specific function
(i) State the
function(s) of each cell
(ii) For each
cell, list the adaptations which enable it to perform its function
(iii) Name a cell which is required by
cell B in order for it to perform its function
(iv) Explain how cell B and the cell
you have named in 7 b (iii) accomplish their function
3. Name the
parts of the cell described in each of the following statements.
(i) The
semi-permeable membrane that encloses the cytoplasm of a cell
(ii) A large
cellular organelle that contains hereditary information
(iii) The site of respiration in the
cell
(iv) A
jelly-like mixture consisting mostly of water, located between the cell
membrane and the nucleus
(v) A part that
protects, supports and gives shape to plant cells
4. (a)
Distinguish between:
(i) Prokaryotic
cell and eukaryotic cell
(ii) An organ
and tissue
(b) Draw a
diagram of an animal cell and label the parts which perform the following
functions
(i) Site for
energy production
(ii) Controls
all the functions (activities) of the cell
(iii) Where organelles are suspended
(iv) Allows passage of some substances
in and out of the cell
5. (a) Using examples of plant and
animal cells, explain the meaning of cell differentiation
(b) Give three reasons, explain why
cell differentiation is very important?
(c) Describe
the functions of any three specialized cells in plants
6. Study the
figure below and answer the following questions
(a) Label the
parts shown by letters A to E
(b) State the functions of the parts labelled A to E
7. (a) What are the differences between cell wall and cell membrane?
(b) Mention two
structures that are found in plant but not in animal cell
8. (a) Explain
what will if the following are removed from plant and animal cell
(i) Nucleus
(ii) Cell wall
(iii)Chloroplast
(iv) Mitochondria
(b) The
following terms are arranged in alphabetical order. Rearrange them into an
appropriate biological sequence, starting with lowest level:
Cell 
Organ Organism System Tissue
(c) Draw and
label the following:
(i) An animal
cell as seen under the light microscope
(ii) A
prokaryotic cell
9. (a)What are
the similarities and differences between cat cell and onion cell.
(b) State the
function(s) performed by the following organelles in plant and animal cells
(i) Rough
endoplasmic reticulum
(ii) Lysosome
(iii) Vacuole
(iv) Cytoplasm
(v) Chloroplast
(c) Mention
four characteristics of a cell.
