Cell as a basic unit of life;

Introduction -

• Cell is the structural and functional unit of life. It is the basic unit of life.
• It is discovered by Robert Hook in 1831 in cork slice with the help of primitive microscope.
• Leeuwenhoek (1674), discovered the free living cells in pond water with the improved microscope
• Robert Brown discovered the nucleus in the cell in 1831.
• Purkinje coined the term ‘protoplasm’ for the fluid substance of the cell in 1839.

The cell theory-

• The theory that all the plants and animals are composed of cells and the cell is the basic unit of life,
was presented by two biologists, Schleiden and Schwann.

• The cell theory was further expanded by Virchow by suggesting that all cells arise from preexisting

prokaryotic and eukaryotic cells

→ Types of organisms

• On the basis of no. of cells, organisms are of two types:
(i) Unicellular Organism
(ii) Multicellular Organism

(i) Unicellular Organism: These organisms are single celled which perform all the functions.
Example: Amoeba, paramecium, bacteria.
(ii) Multicellular Organism: Many cells grouped together to perform different function in the body
and also form various body parts. Example: fungi, plants, animals.

• The shape and size of cell are different according to the kind of function they perform. There is
division of labour in cells.
• Each cell has certain kind of cell organelles to perform different type of function like mitochondria
for respiration.

→ Types of cells
• There are two types of cells:
(i) Prokaryotes
(ii) Eukaryotes

Eukaryotic Cell — Definition & Examples - Expii

multicellular organisms

Multicellular Organism: Many cells grouped together to perform different function in the body
and also form various body parts. Example: fungi, plants, animals.

cell membrane and cell wall

Plasma membrane or Cell membrane

• This is the outermost covering of the cell that separates the contents of the cell from its external
• The plasma membrane allows or permits the entry and exit of some materials in and out of the
• It also prevents movement of some other materials. The cell membrane is called selectively
permeable membrane.
• It is made up of lipid and protein.

→ Properties of Plasma membrane

• It is flexible (made up of organic molecules called lipids and proteins).
• Its flexibility enables cell to engulf in food and other from the external environment. This process
called endocytosis. Amoeba acquire food through this process.

→ Functions of Plasma membrane

• It permits the entry and exit of some materials in and out of the cell.
• It prevents movement of some other materials not required for the cell as it acts like selectively
permeable membrane.

Cell Wall

• Cell wall is another rigid outer covering in addition to the plasma membrane found in plant cell.
The cell wall lies outside the plasma membrane.
• The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance which
provides structural strength to plants.

→ Function of Cell Wall

• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external
media without bursting.
• In such media the cells tend to take up water by osmosis. The cell swells, building up pressure
against the cell wall. The wall exerts an equal pressure against the swollen cell.
• Because of cell wall, cells can withstand much greater changes in the surrounding medium than
animal cells.


• When a living plant cell loses water through osmosis there is shrinkage or contraction of the
contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

cell organelles

Difference between Animal cell and Plant cell

PMF IAS, Author at PMF IAS - Page 25 of 36Calaméo - cells

Difference Between Plant and Animal Cell Division | Characteristics,  Phases, Comparison

chloroplast, mitochondria, vacuoles, endoplasmic reticulum, Golgi apparatus


• The cytoplasm is the fluid content inside the plasma membrane.
• It also contains many specialised cell organelles. Each of these organelles performs a specific
function for the cell.

→ Function of Cytoplasm

• It helps in exchange of material between cell organelles.
• It act as store of vital chemicals such as amino acid, glucose, vitamins and iron etc.
• It is the site of certain metabolic pathways such as glycolysis.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets.
• It looks like long tubules or round or oblong bags (vesicles).
• The ER membrane is similar in structure to the plasma membrane. It is also made up of lipid and

→ Types of Endoplasmic Reticulum

(i) Rough endoplasmic reticulum (RER)
(ii) Smooth endoplasmic reticulum (SER)

→ Functions of Endoplasmic Reticulum

• RER looks rough under a microscope because it has particles called ribosomes attached to its
surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture.
The manufactured proteins are then sent to various places in the cell depending on need, using the
• The SER helps in the manufacture of fat molecules, or lipids, important for cell function.
• Some of these proteins and lipids help in building the cell membrane. This process is known as
membrane biogenesis.
• Some other proteins and lipids function as enzymes and hormones.
• Although the ER varies greatly in appearance in different cells, it always forms a network system.
• One function of the ER is to serve as channels for the transport of materials (especially proteins)
between various regions of the cytoplasm or between the cytoplasm and the nucleus.
• The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical
activities of the cell.
• In the liver cells of the group of animals called vertebrates, SER plays a crucial role in detoxifying
many poisons and drugs.

Golgi Apparatus

• The Golgi apparatus consists of a system of membrane-bound vesicles arranged approximately
parallel to each other in stacks called cisterns.
• These membranes often have connections with the membranes of ER and therefore constitute
another portion of a complex cellular membrane system.

→ Function of Golgi Body

• The material synthesised near the ER is packaged and dispatched to various targets inside and
outside the cell through the Golgi apparatus.
• Its functions include the storage, modification and packaging of products in vesicles. In some
cases, complex sugars may be made from simple sugars in the Golgi apparatus.
• The Golgi apparatus is also involved in the formation of lysosomes.


• Lysosomes are a kind of waste disposal system of the cell.
• It helps to keep the cell clean by digesting any foreign material as well as worn-out cell organelle
• Lysosomes have membrane-bounded structure whose sacs are filled with digestive enzymes.

→ Functions of Lysosomes

• Lysosomes break foreign materials entering the cell, such as bacteria or food as well as old
organelles into small pieces.
• They contain powerful digestive enzymes which are made in RER which is capable of breaking
down all organic material made in RER.
• During the disturbance in cellular metabolism such as when the cell gets damaged, lysosomes
may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the
‘suicide bags’ of a cell.


• Mitochondria are known as the powerhouses of the cell.

→ Structure of mitochondria

• Mitochondria have two membrane coverings.
• The outer membrane is very porous while the inner membrane is deeply folded.
• These folds create a large surface area for ATP-generating chemical reactions.

→ Functions of mitochondria

• The energy required for various chemical activities needed for life is released by mitochondria in
the form of ATP (Adenosine triphopshate) molecules.
• ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new
chemical compounds and for mechanical work.
• Mitochondria have their own DNA and ribosomes. Therefore, mitochondria are able to make some
of their own proteins.



• Plastids are present only in plant cells.
• There are three types of plastids:
(i) Chromoplasts (coloured plastids).
(ii) Leucoplasts (white or colourless plastids).
(iii) Chloroplasts (contains the pigment chlorophyll).

→ Structure of Plastids

• The internal organisation of the plastids consists of numerous membrane layers embedded in a
material called the stroma.
• Plastids also have their own DNA and ribosomes like mitochondria and similar to its structure.

→ Function of Plastids

• Chloroplasts are important for photosynthesis in plants.
• Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.
• Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules
are stored.


• Vacuoles are storage sacs for solid or liquid contents.
• They are small sized in animal cells while plant cells have very large vacuoles.

→ Function of vacuoles

• The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
• Many important substance in the life of the plant cell are stored in vacuoles which include amino
acids, sugars, various organic acids and some proteins.
• In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba
has consumed.
• In some unicellular organisms, specialised vacuoles also play important roles in expelling excess

nucleus, chromosomes - basic structure, number


Function of Nucleus Class 9 - CBSE Class Notes Online - Classnotes123

• It is called the brain of the cell as it controls all the activities of cell.
→ Composition of Nucleus

• The nucleus has a double layered covering called nuclear membrane.
• The nuclear membrane has pores which allow the transfer of material from inside the nucleus to
the cytoplasm.
• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell
is about to divide.
→ Functions of chromosomes

• Chromosomes contain information for inheritance of features from parents to next generation in
the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and
• DNA molecules contain the information necessary for constructing and organising cells.
• Functional segments of DNA are called genes.
• In non-dividing cell, this DNA is present as part of chromatin material.
• Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is
about to divide, the chromatin material gets organised into chromosomes and perform cell division

→ Functions of Nucleus

• The nucleus plays a central role in cellular reproduction. It is the process by which a single cell
divides and forms two new cells.
• It also plays a crucial part, along with the environment, in determining the way the cell will develop
and what form it will exhibit at maturity, by directing the chemical activities of the cell.


• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the
absence of a nuclear membrane.
• Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

1. About Cell and Cell Theory

The Fundamental Unit of Life

Introduction of cell

The introduction to cell began back in the year 1655 when a revolutionary observation was made by an English scientist Robert Hooke. 
In all the living beings, cells are the basic structural units. We can compare the presence of cells in our body to the bricks in a building. All the bricks are assembled to make a building. Similarly, all the cells are assembled to make the body of an organism. 
Thus, it is the basic structural and functional unit of life and all the organisms are made up of cells. The subcellular structures of the cell comprise of the plasma membrane, organelles and in some cases a nucleus as well. As for the size of the cell, it is variable and maybe anything from 1 to 100 micrometre.
Robert Hooke was examining a dried section of the cork tree using a crude light microscope. In this analysis, he observed multiple small chambers which he named the cells. 
The first theory was proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838. This theory was formalized in the year 1858 by the German researcher Rudolf Virchow by suggesting that all cells arise from pre-existing cells.


Cell Theory

•    The cell is the basic functional and structural unit of life. All the living organisms are composed of cells.
•    All cells are formed by the division of the already existing cells which in terms of biology means reproduction.       Every cell of our body comprises of genetic material which is passed down during the process.
•    All the basic physiological and chemical functions i.e. the growth, repair, movement, communication, immunity and digestions are performed inside the cells.
•    All the activities of the cell depend mainly on the activities of the subcellular structures that lie within the cell. These subcellular structures comprise of the plasma membrane, organelles and if present, the nucleus.

Types of Cells

Broadly, there are two key types of cells i.e., the Prokaryotic Cell and the Eukaryotic Cell. The difference between the two is defined mainly by the presence or the absence of the nuclear membrane.

  1. Prokaryotic Cell

 If a cell has a nuclear material without a nuclear membrane, then it is known as the prokaryotic cell, where ‘pro’ stands for primitive and ‘karyon’ stands for the nucleus. Some of the organisms that have prokaryotic cells include bacteria and the blue-green algae.

  1. Eukaryotic Cell

If a cell has a nuclear material with a nuclear membrane, then it is known as the Eukaryotic Cell, where ‘EU’ stands for true and ‘karyon’ stands for the nucleus. All the living organisms except bacteria and blue-green algae have Eukaryotic Cells.

1. About Cell and Cell Theory


The Fundamental Unit of Life

About Cell and Cell Theory

Introduction of cell

As you all know the body of an individual is of complex type. If we study about the detailed structure of the human body or of any lower organism, we will come across many things that makeup the body. But at the smallest level, our body is made up of tiny units called cells. Let us learn about cell in detail.

Cell is often referred as “structural and functional unit of living organisms”. The reason is that the cell is the smallest unit of body and directly or indirectly, it is responsible for functions also. If we study about the hierarchy then we will notice that the level of organization is :

That means that the smallest unit of body is cell.

Discovery of cell

The cell was discovered by Robert Hooke in a thin slice of cork in 1665. Cork is actually a dead part of tree that is bark of tree .He discovered it with the help of instrument called microscope. This microscope is an instrument that enables us to see those things that can’t be otherwise seen with the help of naked eye. It has the property to magnify the objects. He discovered with the help of instrument that is microscope which was discovered by Anton von Leeuwenhoek.

When he saw the internal structure of cork with the help of microscope it looked like compartments .So, earlier these cells were named as compartments but later on it was replaced by term cell. It was concluded that all the cells have three basic characteristics. –

  • They contain plasma membrane: – made up of lipid & proteins
  • They contain genes: – Genetic material in form of DNA or RNA containing genes is present.
  • They contain metabolic machinery: – Cytoplasm is present which contain organelles like mitochondria etc.

These all arts like plasma membrane, cytoplasm, RNA, DNA etc will be taken in to consideration step wise step. Let us first learn more about cell.

Cell theory

To know more about cell, few scientists sum up their views about cell as cell theory. It was given by volunteers: Sheldon & Schwann. According to this theory: –

  • Cell is the structural unit of life.
  • Cell is functional unit of life.
  • All cells arise from pre-existing cells. Every organism starts life as a single cell. (It was given by R. Virchow)

Types of cells – Prokaryotic and Eukaryotic cells

There are two main types of divisions of cells depending upon their basic and important difference in their structures. The types are:

  • Prokaryotic (primitive cell)
  • Eukaryotic (advanced type)

Prokaryotic and eukaryotic cell: The basic difference between the two is: In prokaryotic, the nuclear material lies naked in cytoplasm that is no special membrane is present that separates it from the cytoplasm. It is present as such in cytoplasm.

 In eukaryotic, the nuclear material is well protected and enclosed by special membrane that is nuclear membrane .We can say in it the nucleus is present. Let us some up more difference in them.

Cell shape and Cell size

Now, let’s talk about the cell shape and size. It is seen that cells can be of different shapes and sizes and even their number that make up a body can be different in different living forms as it depends upon the function the cell is going to perform. Accordingly the structures, sizes, shapes are assigned to cells . The shape of a cell may change or fixed.

Shape of cells

  • The shape can be fixed or can be variable.
  • The cells with Variable shape are WBC (white blood cell) and Amoeba.

Please do remember that “the shape of the cell depends upon the function it performs”.

Different Shapes that the cell can have: –

  • Spherical => eggs of many animals
  • Spindle shaped => smooth muscle fiber
  • Elongated => nerve cell
  • Branched => cells of skin
  • Discoid => RBC.


Size of cell

Sizes also show a lot of variations. We have cell that is too tiny like PPLO to largest cell in the form of an ostrich egg

In Human body the smallest cell is: – RBC and longest /largest is Nerve cell .Nerve cell is designed accordingly  to function it performs. As we cleared above also shapes are in accordance to function. Like, nerve cell help in transmission of impulse. So, it has to be long and branched.

Cell number

The number of cells also varies from species to species. There can be organism made up of one cell and there can also be organism like us whose body is made up of many cells. Depending upon the number of cells present, cells are of two types:

  • Unicellular
  • Multicellular

Unicellular organisms – are single celled body (example Amoeba).

Multicellular organisms – are multi-celled body that is many cells (example Man)

Structure of cell

If we look at the structure of cell we have already seen that all cells have three things in common:

  • Cell membrane
  • Nuclear material
  • Metabolic machinery.

 Let us study about cell membrane first:

2. Cell Membrane and Cell Wall

Cell Size

The cell size is variable. In the living organisms, the cell size may be as small as a millionth of a metre or may be as large as a few centimetres. Usually, all the cells are microscopic in size and aren’t visible to the naked eye. Thus, they need to be enlarged by a microscope for seeing.

Cell Shape

  • Usually, the cells are round, elongated or spherical.
  • There are also some cells which are long and pointed on both the ends. Such cells exhibit spindle shape.
  • In some cases, the cells are very long.
  • Some may be branched like the neuron or the nerve cell. The nerve cell transfers and receives messages.

It, therefore, helps in coordinating and controlling the working of the different parts of the body. The components of the cell are enclosed in a membrane. This membrane provides shape to the cells of animals and plants. There is a cell wall surrounding them. The cell wall is an additional covering over the cell membrane in the plant cells. It offers rigidity and shape to the cells.

Depending on the purpose, the cell takes on a number of forms. Cells come in a variety of forms, including:

Skin cells

  1. These skin cells have a flat surface.
  2. They are a protective body cover with a flat form that allows them to cover a large area.

Muscle cells

            i. Muscle cells are long, thin, and elongated.
            ii. Muscle cells expand and contract, and their long and thin structure aids in this process.

Nerve cells

            i. These nerve cells have a lengthy fibrous structure.
            ii. Long, thin extensions (axons and dendrites) connect these nerve cells to other nerve cells.
            iii. The extensions aid the neuron in swiftly transmitting chemical and electrical information throughout                    the body.
            iv. Nerve cells can contain fibres that are over a metre long.
             v. Messages are transmitted down these fibres from one nerve cell to the next.

 Blood cells

            i. White blood cells are disc-shaped but have the ability to alter form.
            ii. WBC in the blood alters their form to kill bacteria or any pathogens that are disease-causing                                organisms.                                                                                                                                                           
iii. The structure of red blood cells (erythrocytes) allows them to pass through capillaries with ease.

Plant cells

            i. Depending on their purpose, plant cells might be rectangular, circular, oval, or elongated. Plant cells                   on the outside of the stem give the plant its strength.
            ii. For support, these cells have strong cell walls. Food is stored in specific cells in plants, and these                       cells are bigger than other cells.

Cell Wall

           The cell wall is the outer covering of a cell, present adjacent to the cell membrane, which is also called the plasma membrane. The cell wall is normally present in all plant cells, fungi, bacteria, algae, and some archaea. Animal cells are not regular in their shape and this is mainly due to the lack of cell walls in their cells. Cell wall composition usually varies along with organisms.

The Function of the Cell Wall

There are various functions of cell wall like:

•    The cell wall of the plants provides definite shape, strength, and rigidity.
•    The cell wall also protects against mechanical stress and physical shocks.
•    Cell wall helps to control cell expansion due to the intake of water.
•    The cell wall also helps in preventing water loss from the cell.
•    The cell wall is responsible for transporting substances between and across the cell.
•    The cell wall acts as a barrier between the interior cellular components and the external environment.

Cell Membrane

•    The cell membrane is also called the plasma membrane. 
•    Cell membrane is found in all cells and separates the interior of the cell from the outside environment.
•     The cell membrane also consists of a lipid bilayer that is semipermeable. 
•    The cell membrane is also responsible for the transportation of materials entering and exiting the cell.
•     The thin membrane surrounds every living cell, fixing the cell from the environment around it. 
•    The cell membranes are the cell’s constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acid, carbohydrates, and substances involved in cellular metabolism. 
•    The surrounding of the cell membrane is a water-based environment, containing ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb to live and grow. 

Functions of the Cell Membrane

•    There are various functions of cell membrane-like:
•    The cell membrane protects the integrity of the interior cell.
•    The cell membrane provides support and maintains the shape of the cell.
•    The cell membrane helps in regulating cell growth through the balance of endocytosis and exocytosis.
•    It also plays an important role in cell signalling and communication.
•    The cell membrane acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.




2. Cell Membrane and Cell Wall

Cell Membrane and Cell Wall

Cell membrane and cell wall

This gives a covering to all cells and the characteristics that it possess are listed below :

  • It is present in both plant & animal cell
  • It is an outer covering of cell
  • It is living, thin and delicate membrane.
  • It is semi-permeable – “allows selective particle to pass through.”
  • It is made up of lipids & proteins.

It allows only selected substances to move through it .The movement of substances takes place by activities like: –

  • Diffusion – “movement of molecules from higher concentration to lower concentration”.  It is mostly seen in case of Gases.   For example: Cell respire i.e. intake of O2 & giving out CO2 take place through diffusion.
  • Osmosis: “movement of solvent molecules from higher cone. to lower conc. across the semi-permeable membrane.

Functions of plasma membrane are as follows

a) It provides support to cell

b) It allow exchange of materials.

c) It provides shape to the cell.

In plant cells outer to cell membrane another layer or covering is present called as cell wall. It is present in all plant cells .It is somehow different from cell membrane in many aspects as it is listed below .

Cell wall

  • It is outer to cell membrane
  • It is dead
  • It is present only in plant cell
  • It is made up of cellulose (in fungus it is made of chitin)
  • It is freely permeable

Functions of cell wall are as follows –

  • It   provides mechanical support to cell.
  • It allows materials to exchange.
  • It maintains the shape of cells.

Now, if we go into detail, we can say that cell consist of cell membrane and it encloses called a living material protoplasm. Protoplasm is defined as living material of cell. The term protoplasm was coined by: J.E Purkinjee. It is further divided into two parts:

  • Nucleus (discovered by Robert Brown)
  • Cytoplasm


Like we have brain, the cell has a nucleus. All the activities are controlled by nucleus. It is located in the centre of the cell. It consists of different components:

1. Nuclear membrane

2. Nucleoplasm

3. Nuclear material

4. Nucleolus

1. Nuclear membrane: -It is the membrane that separates the nucleus from the cytoplasm. The membrane has pores in it which allow only selected exchange of materials. It is therefore called semi permeable in nature.

2. Nucleoplasm: It is the liquid in the nucleus that is cytoplasm of nucleus. It is similar to cytoplasm outer to nucleus except that it lacks organelles.

3. Nucleolus: It is a small oval body rich in proteins and RNA, RNA is ribonucleic acid and it is the genetic material. It is site for ribosome formation. That is why it is often termed as factory for ribosome.

4. Nuclear material: It contains thread like structures which further condense at the time of cell division to ribbon like structures called chromosomes. These chromosomes are made up of DNA and proteins. There are segments of DNA that are responsible for characters in a body or transfer of characters from parents to offspring and are called genes. They act as hereditary vehicles.

Functions of nucleus are as follows

  • It regulates the cell cycle.
  • It controls metabolic activities of cell.
  • It contains genes that help in transmission of characters from parents to offspring .


It is the liquid material of cell. Its soluble part is called cytosol. It contains organelles. These organelles perform necessary functions for the cell like some organelles help in cleaning the cell, some help in synthesizing certain substances for the cell, some perform catabolic function like breakdown of food, etc. Let us study them in detail:

3. Cell Organelles and their Functions


Cytoplasm Structure

  • Cytoplasm is a jelly-like substance found between cell membrane and nucleus.
  •  All the cell organelles are embedded in the cytoplasm. It is composed of water, organic and inorganic compounds. 
  • Cytoplasm is one of the essential components of the cell that is present in both plant and animal cells. 
  • Cytoplasm functions by controlling all the metabolic activities taking place within the cell and most of the chemical reactions are carried within it.


Endoplasmic reticulum

Endoplasmic reticulum is a network of tiny tubular structures scattered in the cytoplasm. It divides the intracellular space into two distinct compartments, one is luminal (inside ER) and another is extra luminal (outside ER). They function as the transport system of a cell, involved in transporting materials throughout the cell.

Endoplasmic reticulum is divided into two types based on presence and absence of ribosomes.

  • Rough endoplasmic reticulum: The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). It is involved in the protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
  • Smooth endoplasmic reticulum: The endoplasmic reticulum with the absence of ribosomes on its surface is called smooth endoplasmic reticulum (SER). They are responsible for lipid syntheses. In animal cells, lipid like steroid hormones is synthesized in SER. They are also responsible for detoxifying the cell.



  • Mitochondrion is a double membrane bound cell organelle that are not normally visible under the microscope.
  • This cylindrical or sausage shaped structure is also called the power house of the cell as they are the sites for aerobic respiration of the cell and produce cellular energy in the form of ATP. 
  • The outer membrane and the inner membrane divide its lumen into two compartments i.e., inner compartment called matrix possessing single circular DNA molecule and a few RNA molecules. forming a number of infoldings called cristae and the outer membrane forming the continuous limiting boundary of the organelle.
  • Depending on the shape, size and physiological activity of the cell, the number of mitochondria per cell are variable. 
  • The structure has a diameter of 0.2 to 1µm (average 0.5 µm) and length 1.0 to 4.1 µm.


Plastids are large and membrane bound organelles that are found in all plants and in euglenoids. They contain some specific pigments that are responsible for imparting specific colours to the plants. On the basis of the type of pigment, plastids can be classified into three types.

  • Chloroplasts: Chloroplasts are double membrane bound cell organelles that contain chlorophyll and carotenoid pigments that are responsible for trapping light energy and this energy is used for the purpose of photosynthesis.


The inner membrane surrounds a space called stroma. Chlorophyll containing structures called thylakoids are arranged as piles of coins and each pile is called a granum. Thylakoids are connected by flat membranous tubules known as stromal lamella.

  • Chromoplasts: These include carotenoid, fat soluble pigments like carotene, xanthophyll etc. providing characteristic colours like yellow, orange, red, etc. to the plants.
  • Leucoplasts: These types of plastids store nutrients and are colourless. They include amyloplasts storing carbohydrates, leucoplasts storing proteins, and leucoplasts storing oils and fats.

The Golgi Apparatus

  • It is responsible for sorting, modifying, and shipping off the cellular products that come from the rough endoplasmic reticulum (ER), just like a post office.
  • The Golgi apparatus is similar to stacked flattened discs, almost like stacks of oddly shaped pancakes.
  • The Golgi apparatus has two distinct sides with each having a different role. 


  • An organelle that contains enzymes that break down and digest inessential cellular components, such as a damaged organelle is called the lysosome.
  • Protein products packaged by the Golgi such as digestive enzymes are meant to stay inside the cell in order to break down certain materials.
  • The enzyme-containing vesicles are released by the Golgi, and they may form new lysosomes or fuse with existing lysosomes.


  • Vacuoles are generally characterized as storage bubbles of unpredictable shapes which are found in cells.
  • They are liquid-filled organelles enclosed by a film.
  • The vacuole stores the food or a spread of nutrients that a cell might need to survive.
  • In addition, it also stores waste products.
  • The by products are at last tossed out by vacuoles. Thus, the remainder of the cell is shielded from contamination.
  • The animal and plant cells have different sizes and numbers of vacuoles. Compared to animals, plant cells have larger vacuoles.

3. Cell Organelles and their Functions

Cell Organelles and their Functions

Cell organelles and their functions

There are many types of organelles that help the cell in performing different functions. The different organelles are :

  • Endoplasmic reticulum
  • Ribosome
  • Golgi bodies
  • Lysosomes
  • Mitochondria
  • Plastids
  • Vacuole
  • Centrosome

Let us study them in detail:

1. Endoplasmic reticulum

It is an irregular network of membranes enclosing fluid in its cavity .they are of two types that is:

  • Rough endoplasmic reticulum(RER)
  • Smooth endoplasmic reticulum(SER)

The basic difference between the two is that:

  • RER – It has ribosomes attached to it.
  • SER : – No ribosome  are attached

Functions of endoplasmic reticulum are as follows –

  • SER helps in synthesizing fats etc.
  • Endoplasmic reticulum forms skeleton framework of cell.
  • Rough endoplasmic reticulum helps in transportation of proteins.
  • Smooth endoplasmic reticulum helps in detoxification.
  • Oxidative enzymes of lysosomes are produced by Rough endoplasmic reticulum.
  • Endoplasmic reticulum helps in formation of membrane of cell .That is :

RER – SER – Golgi body – sec. vesicles – p membrane

2. Ribosomes

They are present in plant as well as in animal cell. They are spherical particles found free or attached to Rough Endoplasmic Reticulum. They contain their own genetic material that is RNA & proteins .Their function is protein synthesis. Due to the reason they are called as factory of proteins.

3. Golgi bodies

This organelle is present in plant and animal cells both. They  are membrane bound fluid filled vesicles and flattened membranes stacked over one another called  cisternae. In plant cells  it is called by a special name that is  Dictyosome. Function: They secrete, packages and dispatch the cellular secretions and also helps in the formation of cell membrane.

4. Lysosomes

They are present in plant as well as in animal cell. They are membrane bound vesicle containing powerful digestive enzymes. These enzymes can carry out the catabolic function

Function: Its functions are:

  • They can destroy foreign material.
  • They can remove worn out parts of the cell.

It acts as suicidal bags as they can even eat up their own cells if in case it gets damaged.

5. Mitochondria

They are rod shaped structures having a double membrane.

  • Its outer membrane is – porous
  • Its inner membrane is folded to form cristae & contain small bodies called oxysomes.
  • It has a cavity that is filled with matrix (fluid) containing DNA and ribosomes.
  • Its function is to produce energy in the form of ATP (Adenosine triphosphate) and is also designated as called as Power House of cell (due to production of energy).

6. Plastids

These are only found in plant cells. They are of three types: –

  • Leucoplast – It is colorless plastid.
  • Chromoplast – It is colored plastid.
  • Chloroplast –It is green colored plastid.

As you all know that plants have the ability to synthesize their own food. They do so with the   help of pigment chlorophyll. This chlorophyll is present in organelle chloroplast.

Chloroplasts are double membrane organelles containing liquid stoma and stacked structures called lamellae. When lamellae are stacked over one another they give rise to grana. Grana contain green pigment chlorophyll. This chlorophyll actually helps in capturing solar energy and converting it to chemical energy of food.

Functions of plastids are as follows –

  • Chloroplast helps in photosynthesis
  • Leucoplast – stores food, fats etc.
  • Chromoplast impart colour to flowers.

7. Vacuole

They are fluid filled membrane bound spaces. They are prominent and large in plants but in animal cell they are either absent or if present they are small and temporary. In animal cell it helps in  storing substances. In plant cell vacuole are large & permanent. Outer membrane is called as tonoplast and is filled with fluid called cell sap Functions of vacuole are as follows

  • It provides turgidity & rigidity to cell.
  • It stores substances.
  • It maintains the osmotic pressure of cell.

8. Centrosome

It is found only in animal cell. It consists of two granules called centrioles. The function of centrioles is to help in spindle formation during cell division.

Transportation of substances

As we have seen that substances move in and out from the cell. These exchange mainly occur by two processes:

  • Diffusion
  • Osmosis

4. Diffusion and Osmosis


Diffusion is the net movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration. Net movement means there are more molecules moving in one direction than in the opposite direction.

Example: Opening a bottle of perfume in a room will result in the gradual diffusion of the perfume from the region of higher concentration (the bottle) out into the room. Diffusion will continue until the perfume has a more or less uniform concentration throughout the bottle and room.


It can be defined as the movement of water molecules from a higher water concentration area to the area of less water concentration through a semipermeable membrane. For example, water in the roots of plants is transported through osmosis.

Difference between diffusion and osmosis:



It happens only in the liquid state.

It occurs in all states of matter i.e., solids, liquids or gases.

It should be movement of only water or solvent through semipermeable membrane from lower concentration to higher concentration.

Any type of substance that moves from higher concentration area to lower concentration area.

It is applied only for the solvent part of the solution.

Diffusion is applied to all states of matter.


It requires semipermeable membrane.

This phenomenon does not require semipermeable membrane.

Osmotic Solutions

There are three different types of solutions:

Isotonic Solution
Hypertonic Solution
Hypotonic Solution

An isotonic solution is one that has the same concentration of solutes both inside and outside the cell.

A hypertonic solution is one that has a higher solute concentration outside the cell than inside.

A hypotonic solution is one that has a higher solute concentration inside the cell than outside.


Types of Osmosis

Osmosis is of two types:

Endosmosis– When a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes de plasmolysis. This is known as endosmosis.
Exosmosis– When a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis. This is known as exosmosis.






4. Diffusion and Osmosis

Diffusion and Osmosis

Diffusion and Osmosis

Diffusion: It is the movement of any substance from higher concentration to lower concentration. Have you noticed the fragrance of your favorite food at your home when it surprisingly cooked for you by your mom. This is due to the process diffusion. When it is being cooked in kitchen its fragrant  molecules from kitchen start moving out into the kitchen’s surroundings and with time get dispersed in the air. The diffusion is seen in solids, liquids and gases but the rate of diffusion is faster in gases. This is because gas particles are energetic as they possess high kinetic energy. The movement of gases in and out from the cell occurs by diffusion

Osmosis: It is the movement of pure liquid from  higher concentration to lower concentration across the semipermeable membrane . For example: if you take two flasks and fill one with low sugar so lution and other flask with highly concentrated sugar solution. Both the beakers are separated by a semipermeable membrane. Then we will notice that the water will move from the beaker where it is more to the side where it is less. The membrane allows only water to pass through it not sugar as it is semi-permeable that is selectively permeable.

Types of solutions

We can make three different types of solutions that is :

1. Hypotonic solution – The solution that has higher water concentration.

2. Hypertonic solution- The solution that has less water concentration.

3. Isotonic solution-The solution that has the same concentration of water as in cell.

Now let’s do an activity in which we will be placing a cell say red blood cell in three different solutions. Let’s see what happens:

Cells placed in different types of solutions

This is because when it is placed in a hypotonic solution, the water from solution moves into cell (endo-osmosis) due to  which cell starts swelling up .The fully swollen cell in which the protoplasm almost touches the cell membrane is called Turgid cell.

Have you seen the red blood cell kept in hypertonic solution looks flaccid This is because when it is placed in a hypertonic solution, the water from cell moves into solution (exo-osmosis) occurs due to which cell starts shrinking up .The cell whose protoplasm almost gets shrunk is called flaccid cell.

Have you seen the cell in isotonic solution It remains as such. Nothing happens. The reason being the concentration of cell and a solution is the same.

Q. Have you noticed that sometimes when you forget to water plants especially in summers they droop, dry and ultimately they are about to die .

A. This is because the water from the plant is moving out due to transpiration. So, when there is no water left in them they start to droop. This basically occurs due to exo-osmosis. But when it is watered again, the endo osmosis occurs and it again become turgid .this is called plasmolysis (shrinkage) and deplasmolysis (again becoming healthier).