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Cell Structure and Functions

 SS Academy Rajapuram

Cell Structure and Functions



There is a large variety of organism around us. They have different shapes and sizes. Their food habits and habitats are also different. Inspite of great variation all the living organism have a basic similarity among them :they are made up of tiny units called cells.

Cells are the structural and functional units of life. Cork is a part of the bark of a tree. Cork is a dead plant material.

In 1665,An English Scientist named Robert Hook used a microscope to investigate the structure of a thin slice of cork. He observed that cork had tiny compartment in it. Robert Hooke thought of these compartments as small rooms and called them cells.

When better microscopes were made, scientists investigated pieces of living plants and found that like cork, they also had a cell structure. The living plant cell were found to contain a number of tiny structures.

The cell theory states that the basic unit of structure and function of all living organism is the cell. All cells arises from pre-existing cells by cell division. The cells were not studied or observed for thousands of years because most of them are extremely small and cannot be seen with naked eyes.

Cells are the basic units of life. All the living things are made up of cells. They are the building blocks of plants and animals.

The simplest organism like amoeba consist of only one cell but a complex organism like man is made up of trillions of cells.

A cell is the smallest unit of life which has a definite structure and performs a specific function. Most of the cells are very, very small which cannot be seen with naked eyes. They can be seen only with the help of microscope.

The cells are of 2 main types: Animal cells and plant cells.



Nucleus is a large, spherical organelle present in all the cells.

(1) In animal cells, nucleus lies in the centre of the cell whereas in plant cells the nucleus may be on the periphery of the cell.

(2) Nucleus is the largest organelle in a cell.

(3) It can be stained and seen easily with the help of a microscope.

(4) Nucleus is separated from the cytoplasm by a membrane called nuclear membrane.

(5) The transmission of characteristics from the parents to the offsprings is called inheritance. Nucleus contains thread like structures called chromosomes

(6) Chromosomes contain genes. The function of chromosomes is to transfer the characteristics from the parents to the offspring through the genes. The nucleus also contains a tiny round structure called nucleolus.

(7) The nucleus containing chromosomes and nucleolus is bound by a membrane called nuclear membrane. It has tiny pores for the exchange of materials with cytoplasm. Red blood cells, however, do not have a nucleus.

(8) Gene is a unit of inheritance in living organism(which is transferred from a parent to offspring during reproduction and determines some characteristics feature of the offspring)

(9) Genes control the transfer of heredity characteristics from parents to offspring. This means that our parents pass on some of their characteristics features to us through their genes.

(10) It is the genes on the chromosomes which decide all sort of things, the colour of our eyes, the look of our hair, the shape of our nose, our complexion, our appearance etc.

(11) The different combinations of genes from both the parents results in different characteristics in the offspring.

(12) All the living matter in a cell is called protoplasm. It is a liquid substance which is present inside the cell membrane. It includes cytoplasm, nucleus and other organelles. Most of the protoplasm is made up of compounds of only 4 elements :Carbon, hydrogen, oxygen. Some of the compounds present in protoplasm are water, carbohydrates, proteins, fats, nucleic acid and mineral salts.



Mitochondria are the tiny rod shaped organelles which are found in all cells.

They provide energy for all activities of the cell. This energy is produced by the process of respiration in which food is broken down by oxygen. Mitochondria use glucose and oxygen to produce energy.

Cell wall

The plant cell wall is made of a tough material called cellulose. Cell wall gives shape and support to the plant cell.

The plant cells have a thick cell wall around them. Cell wall give plants most of their strength. Plants need protection against high wind speed, variations in temperature and atmospheric moisture. The tough cell wall present in plant cells provide protection to plants.

Cell wall is not there in animal cell. The cell wall is a non-living part of the plant cell.


They are green coloured organelles present in the cytoplasm of plant cells.


The process of food making by plants  known as photosynthesis takes place in chloroplasts. The green colour of chloroplast is due to the presence of a green pigment called chlorophyll in them. Chlorophyll can absorb sunlight energy.

In chloroplast, carbon dioxide and water combine in the presence of sunlight energy to produce food such as glucose. They are found only in those plant cells which carry out photosynthesis.

The organelles containing pigments present in the cytoplasm of plant cells are called plastids.The plastids containing green pigment(chlorophyll) are known as chloroplasts.

The green coloured plastids present in the cells of leaves provide green colour to the leaves. The red colour of tomatoes is due to the presence of plastids having red pigments in the cytoplasm of its cells.The different colour of flowers are due to the presence of plastids containing pigments of different colour.



It is a space in the cytoplasm of a cell which is enclosed by a membrane and usually contains substances dissolved in water.

All the plant cell have a large vacuole. The vacuole is filled with a liquid called cell sap, which contains dissolved sugar and salts.

(1) The pressure of liquid pushes on the outer parts of the plant cell keeping the plant cell firm.

(2)The function of vacuole in a cell is to store various substances including waste products of the cell.

(3) Most of the animal cells do not have vacuole. Some of the animal cell have vacuole but they are much smaller than those in plant cells.

Cell membrane and cytoplasm

Cell membrane

Every cell is covered by a thin sheet of skin which is called cell membrane or plasma membrane.

1)The cytoplasm and nucleus are enclosed within the cell membrane.It encloses the living part of the cell called protoplasm.

2)Cell membrane protects the cell and also gives shape to the cell.

3)The cell membrane has tiny pores in it.The cell membrane controls the movement of substances into the cell and out of the cell.

4)The dissolved substances such as food and oxygen can enter into the cell whereas the waste products such as carbon dioxide can go out from the cell whereas the waste products such as carbon dioxide can go out from the cell membrane through the pores of the cell membrane.

5)The cell membrane separates the cells from one another and also from surrounding medium.


It is a transparent jelly like material which fills the cell between nucleus and cell membrane.

1)New substances are built from materials taken into the cell, and energy is released and stored.

2)It has many tiny structures in it.The various structures present in the cytoplasm of a cell are called organelles.

3)Cytoplasm of all cells contain other organelles such as mitochondria, golgi bodies , nucleus and ribosomes.

4)The cytoplasm and nucleus taken together make up protoplasm.

Structure of Animal cell

1)A cell consist of a jelly like material enclosed in a thin membrane.The jelly like material which fills the cell is called cytoplasm.The function of cytoplasm is to carry out all the activities of the life processes.

Animal cell

2)The thin outer covering of the cell is called cell membrane.

3)There is a large floating body usually in the centre of a cell which is called nucleus.The function of nucleus is to control all the activities of the cell.

4)The nucleus contain a tiny round structure called nucleolus and a fibrous material called chromatin.It also has a nuclear membrane on the outside.Chromatin form chromosomes during cell division.These chromosomes transfer characteristics from the parents to the next generation.

5)The cytoplasm and nucleus taken together form the protoplasm of the cell.It is the living material of a cell.

6)There are a number of small bodies called mitochondria in a cell.The function of mitochondria is to carry out respiration for releasing energy from food.Since mitochondria produce energy from food, they are kind of power house of the cell.

7)The tiny air spaces in the cytoplasm of an animal cell are called vacuoles. The function of a vacuole in an animal cell is to hold air , water and particles of food.

8)The cell membrane is a thin sheet of skin all around the cell.The main function of cell membrane is to control the passage of materials which go into the cell or go out from the cell.It also protects the cell and gives shape to the cell.

Structure of Plant cell

Plant cell

1)A cell consist of a jelly like material enclosed in a thin membrane.The jelly like material which fills the cell is called cytoplasm.The function of cytoplasm is to carry out all the activities of the life processes.

2)There is a large floating nucleus in the cytoplasm.The nucleus contain a tiny round structure called nucleolus and a fibrous material called chromatin.It is surrounded by nuclear membrane.

3)There are a number of small bodies called mitochondria in a cell.The function of mitochondria is to carry out respiration for releasing energy from food.

4)All plant cell have thick cell wall all around the cell membrane.This cell wall is made up of cellulose.The cell wall protects the plant cell, gives it a fixed shape and makes it rigid.

5)The plant cell contain green coloured plastids called chloroplasts.They are green coloured bodies in the cytoplasm of the plant cell.They make food in green plants by the process of photosynthesis.Plant cell may contain plastids of different colours.

6)The plant cells have very large vacuoles.These vacuoles are filled with cell sap.It is a solution of sugars and mineral salts.

Staining of plant and animal specimen is done to identify the different components of a cell easily through the microscope.

For Ex:Dilute iodine solution, methylene blue, safranin.

Prokaryotic cells

Prokaryotic cell

The cell having nuclear material without a nuclear membrane around it, are called prokaryotic cells.

The organism made of prokaryotic cells are called prokaryotes.

1)The nucleus of prokaryotic cell is not well organised.

2)There is no nuclear membrane around the nuclear material in the prokaryotic cells.

3)The nuclear material is in direct contact with the cytoplasm.

4)All the prokaryotes are simple, unicellular organism.

5)They were probably the first living things to evolve on the earth.

For Ex: Bacteria, blur green algae are prokaryotes.

Eukaryotic Cells

Eukaryotic cell

The cells having nuclear material enclosed by a nuclear membrane are called eukaryotic cells.

The organism whose cells possess a nucleus bound by a nuclear membrane are called eukaryotes.

1)They have proper, well organised nucleus.

2)The nuclear membrane in eukaryotic cell is not in direct contact with cytoplasm, it is separated from cytoplasm by the nuclear membrane.

For Ex: Amoeba, cheek cell, onion peel cell , plants, animals, protozoa, fungi etc.

The various organisms differ in the number of cells which make up their body; they differ in the shapes of cells in their body ; also they differ in the size of cells in their body.

Variety in number of cell

Different organism have different number of cells in their bodies.The number of cells varies from organism to organism.Some are made up of just one cell while other are made up of large number of cells.

The organism which are made up of only one cell are called unicellular organism or single-celled organism.

For Ex:Amoeba, paramecium, Euglena, Chlamydomonas etc.

A unicellular organism perform all necessary life functions with the help of just one cell.All basic function such as taking of food, digestion, respiration, movement, response to environment, excretion , reproduction etc are performed by single cell.

The organism which are made up of only many cell are called multicellular organism.

A multicellular organism is made up of millions, billions, trillions of cell joined together.Most of the plants and animals are multicellular.

In multi cellular organism different cells perform different functions.

The number of cell being less in small organism does not in any way affect the functioning of the small organism.Even an organism made of million, billions or trillions of cells begin its life as a single cell called fertilised egg cell.The fertilised egg cell divides and multiplies due to which the number of cell increases as the development of the organism proceeds.

Variety in shape of cell

There are many types of cells in the bodies of multicellular bodies.These cell differ in shapes.

The shape of a nerve cell  in nerve cell in animals is different from the shape of a muscle cell.

A nerve cell is long and branched whereas a muscle cell is pointed at both ends and has a spindle shape.The different shape of cells are related to their function which they have to perform in the body of an animal or plant.

Nerve cell receives and transmit messages between brain and other body parts whereas muscle cells bring about movement in body parts.Nerve cells and muscle cells are actually specialised animal cells because they perform functions in the bodies of animals.

Cells are different in shapes and sizes so they can perform different functions.Each one of these cells are adapted to perform some specific functions.

The human body is made up of about 20 different types of cells and each type of cell perform different function.

Some of the example of animal cell are:Nerve cell or neuron, muscle cell, Epithelial cell, Red blood cell, white blood cell, bone cell etc.

Nerve cell

The nerve cell are long and have projections so that they can make contacts with many other nerve cells and carry messages over long distances.Since nerve cells receive and transfer messages they help to control and coordinate the working of different parts of a body.

Muscle cell

They are specially adapted for movement.The muscle cell can contract and relax.when the muscle are contract, they shorten in length.So, the contraction of muscle cells moves the body part.When these contracted muscle cell relax, they expand so that the body part comes back to its original position.Muscle cell bring about movement of body parts by contracting and relaxing.

Epithelial cell

They are rectangular in shape.They form a thin layer over the body parts and protect the cells below them from injury.

Red blood cells

They are spherical in shape.RBC carry oxygen around the body.

White blood cells

They are irregular in shape.They eat up or kill bacteria which enter the blood and save us from many diseases.

Some of the plant cells are:Epidermal cell , Xylem cells, phloem cells, photosynthetic cells

Epidermal cells form a layer around the plant organs and protects the cell below from injury.

Xylem cells are the tube like plant having thick and strong walls which carry water and mineral salts from the roots of the plant to its leaves.

Phloem cells are also tube like plant cells having thin walls which carry the food made by leaves to all parts of the plant.

The photosynthetic cells of plant contain chlorophyll and prepare food by photosynthesis.

The mesophyll cells of leaf are the photosynthetic plant cells.These cells in the leaf of a plant are specially adapted for making food by photosynthesis.

Amoeba cell

The shape of an amoeba cell is irregular.

1)The amoeba cell keeps on changing its shape continuously.The shape of amoeba changes because amoeba can make its cytoplasm flow in any direction it wants to.

2)The amoeba cell has finger like projections of varying lengths protruding out of its body which are called pseudopodia.

3)Amoeba can produce pseudopodia on any side by pushing the cytoplasm in that side.The pseudopodia appear and disappear when amoeba moves or feed.Amoeba moves very slowly with the help of pseudopodia.

4)Amoeba also uses pseudopodia to catch the food particles from the water in which it lives.

Amoeba is a single cell which can change its shape.

A white blood cell present in human blood is a single cell which can also change its shape.The difference between amoeba cell and white blood cell is that amoeba cell is a full fledged organism capable of independence existence, white blood is merely a cell of human blood which is not a full fledged organism and hence cannot exist independently.

Variety in size of cell

The cells are of many different sizes.The cells in living organism may be as small as a millionth of a metre or it may be as large as a few centimetres.

Most of the cells are extremely small in size and hence cannot be seen by naked eyes.Most of the cells found in living organism have to be enlarged by using a microscope before they can be seen by us.

The bacteria cells have a length of 0.1 to 0.5 micrometer.Bacteria mycoplasma is the smallest cell having the size of only 0.1 micrometer.

Even the long cells are so thin that they can be seen only with a microscope.The muscle cell in animals are a few centimetres long and the nerve cells are more than a metre long, but they are are so thin that they can only be seen with the help a microscope.

Some of the cells are big and can be seen easily with naked eyes.The bird’s egg are very large cells.Each egg of the bird is a single cell.

The hen’s egg is a single cell.The hen’s egg is quite big.We can see the single cell called hen’s egg with naked eye.

The biggest cell is the ostrich egg.The ostrich egg can be as much as 17 centimetres long.

In Human body, some blood cells are the smallest and the nerve cells are the longest.The ostrich egg is actually 17 cm * 13 cm.The size of cells has no relation with the size of the body of an animal.


Cell is the smallest unit of life which has a definite structure and perform a specific function.There are many different shapes and sizes.Most of the cells are specialised to perform different function.They are called specialised cells.

Muscle cells are specialised to contract and relax so that they can bring about movement in body parts.

Photosynthetic cells are specialised to carry out photosynthesis and make food.There are many types of specialised cells in animals and plants which perform different functions.


The group of similar cells which work together to perform a particular function is called tissue.

A multicellular organism is made up of millions of cells.The cells in multicellular organism do not work as single cells, they work in groups of similar cells.

Muscle tissue is a group of muscle cells joined together which is specialised to contract and relax so as to move body parts.

Photosynthetic tissue is a group of photosynthetic cells joined together which is specialised to do photosynthesis and make food.There are many types of tissue as there are cells.


An organ is a collection of different tissues which can work together to perform a particular function in the body of an organism.

The different tissue combine together to form organs.These organs perform different tasks for the animals or the plants.

The multicellular organism are made up of many different organs which do different jobs for the organism.

Some of the organs which are found in the bodies of animals are heart, stomach, lungs, brain, kidney, eyes, mouth, hands etc.

The function of heart is to pump blood around the body.

The function of stomach is to digest food.

The function of brain is to control all the parts of a body.

Lungs are the organs of breathing.The function of lungs is to take in oxygen and give out carbon dioxide.

The various organs found in the body of a plant are roots, stem, leaves, flower, fruits.

Roots absorb water and dissolved mineral salts from the soil.

Stem carries water and mineral from the roots to the leaves, and the prepared food from the leaves to other parts of the plants.It holds branches and leaves.

The leaves prepare food for the plant by the process of photosynthesis.

The flowers are the reproductive organs which lead to the formation of fruits and seed.The fruit protects the seeds.

Organ system

A group of interconnected organs which work together to do a big job for the organism is called organ system.

All the multicellular animals and plants have many organ system in their bodies to carry out various life processes.

For Ex:Digestive system, circulatory system, muscular system, skeletal system, excretory system etc.

The function of digestive system is to break down the food into simple substance which can be absorbed by the body.The main organs of the digestive system are:Mouth, food pipe, stomach, small intestine, large intestine, rectum, anus.Each organ of digestive system perform a different function such as ingestion, digestion, absorption, assimilation and egestion.

The plants have 2 main organ system:Root system and shoot system.The root system consist of various types of roots whereas shoot system is made up of organs like stem, leaves, flowers, fruits.


An organism is an animal or plant which can exist on its own.

An organism is made up of many different organ system which work together to perform all the functions necessary for maintaining .

For Ex:Human being is an organism.It is made up of many different organ system.All the organ system work together to make man a living organism.

A cat, dog, neem tree, mango tree, rose plant are all organism which are made of many different organ systems working together.