Cell Introduction

The cell is regarded as a fundamental structural and functional unit of

life because the basic structure of all the living organisms is made up

of cells. Hence cell is known as the structural unit of life. Let us learn

about cell study and cell theory.

Landmarks in Cell Study

● Discovery of Cell: Cell was discovered by an English scientist

Robert Hook in the year 1665. He published this information in

his book ‘Micrographia’. He observed that dead cells in a thin

cork slice in a self-made microscope. And this was the

beginning of cell study and discovery.

● Why were they named cells? Because the cells looked like the

small rooms of a Buddhist monastery which were called

cellullae. Hence the name cell was given.

● Anton Von Leeuwenhoek designed an improved microscope

and observed first free-living cells (Bacteria) in the year.

● Robert Brown A Scottish Botanist discovered nucleus in the

year 1831.

https://www.toppr.com/guides/biology/cell-the-unit-of-life/introduction-to-cell-and-cell-theory/https://www.toppr.com/guides/science/the-living-organisms-and-their-surroundings/living-organism/https://www.toppr.com/guides/physics/nuclei/atomic-mass-and-composition-of-nucleus/

● A French Zoologist Dujardin discovered the semifluid living

material inside a cell and named it Sarcode.

● Purkinje in the year 1840 renamed it as ‘Protoplasm – The First

Substance’.

● A cell can be viewed as an enclosed vessel, within which

innumerable chemical reactions take place simultaneously.

These reactions are under very precise control so that they

contribute to the life and formation of new cells.

Cell Theory

A German botanist M.J. Schleiden in the year 1838 found out that all

plants are made of cells. A German Zoologist T. Schwann in the year

1839 also said that all the animals are made up of cells. They

commonly presented their findings as Cell Theory.

Postulates of Cell Theory are

● All living organisms are composed of cells.

● All cells are basically alike in structure and functions.

https://www.toppr.com/guides/science/sorting-materials-into-groups/properties-of-materials/https://www.toppr.com/guides/chemistry/chemical-reactions-and-equations/chemical-reactions-and-equations/https://www.toppr.com/guides/business-management-entrepreneurship/organizing/structure-of-organization/https://www.toppr.com/guides/maths/relations-and-functions/functions/

● The function of an organism as a whole is the result of the

activities and interactions of constituents of the cell.

Modern Cell Theory

In the year 1858 scientist Rudolf Virchow presented the idea ‘Omnis

Cellula e Cellula’, which means that all living cells arise from

pre-existing cells. After addition of this postulate in the cell theory, it

was called Modern Cell Theory.

Cell Facts

● Cells vary in shape and size.

● The nerve cell is the longest cell in our body. It can be about

1m long.

● Ostrich egg is the largest cell.

● Red Blood Corpuscles are the smallest cell in the human body

and female ovum is the largest cell in the human.

● Some organisms may be made up of a single cell (Unicellular

Organisms), eg; Amoeba. Others are multicellular.

https://www.toppr.com/guides/biology/cell-structure-and-function/cell-number-shape-and-size/https://www.toppr.com/guides/science/nutrition-in-animals/nutrition-in-amoeba/

● Proper division of labour is seen in multicellular organisms as

different organs perform different functions but in unicellular

organisms, the single cell performs all the functions.

Types of Cells

Cells are of two types depending on the presence or absence of a

well-defined nucleus.

● Prokaryotic cell

● Eukaryotic cell

S. No. Prokaryotic cell Eukaryotic cell

1 Pro-Primitive; Karyon- Nucleus Eu-True; Karyon- Nucleus

2 The nuclear membrane is absent. Content of nucleus present in the cytoplasm known as a nucleoid.

Nucleus distinct with the nuclear membrane.

3 Size: 1µm to 10 µm Size: 5 µm to 100 µm

4 Membrane-bound cell organelles are absent. Membrane-bound cell organelles like Golgi bodies, mitochondria etc are present.

5 Ribosome -70 S Ribosome – 80 S

6 A single circular chromosome is present. Many chromosomes are present which are linear.

7 A respiratory enzyme is present in mesosoma. Respiratory enzymes are formed in mitochondria.

Solved Questions for You

Question 1: Bacteria do not have chloroplast but some bacteria are

photoautotrophic in nature and perform photosynthesis. Which part of

bacterial cell performs this?

Answer: They have chlorophyll which helps in absorption of light for

photosynthesis.

Question 2: Which is smaller- Cell or an atom. Also, justify your

answer.

Answer: Cell study shows us that atoms are smaller than cells.

Thousands of atoms make a single cell. Atoms like carbon, oxygen,

hydrogen etc unite to form molecules like glucose, fat, amino acid etc.

these molecules further unite to form macromolecules like starch,

lipids and proteins. These molecules combine together to form cells.

Cell Organelle

You already know that all living organisms are made up of cells,

which are the tiniest units. But do you know that cells are made up of

even smaller objects? Each of these objects is called a cell organelle.

Do you want to know what these are and what their functions are?

Let’s learn about them below.

Cell Organelle

A small organ-like structure present inside the cell is called a cell

organelle. It has a particular structural makeup and performs a specific

function. Depending upon the presence or absence of membrane, cell

organelles can be classified into three categories, namely:

● Without membrane: Some cell organelles like ribosomes are

not bounded by any membrane. They are present in prokaryotic

as well as eukaryotic organisms.

● Single membrane-bound: Some organelles are bounded by a

single membrane. For example, vacuole, lysosome, Golgi

Apparatus, Endoplasmic Reticulum etc. They are present only

in a eukaryotic cell.

https://www.toppr.com/guides/science/the-living-organisms-and-their-surroundings/living-organism/https://www.toppr.com/guides/biology/cell-the-unit-of-life/introduction-to-cell-and-cell-theory/https://www.toppr.com/guides/science/sorting-materials-into-group/objects-around-us/https://www.toppr.com/guides/biology/structural-organisation-in-animals/organ-and-organ-system/https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/structure-of-cell/https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/cell-organelle/https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/cell-organelle/https://www.toppr.com/guides/biology/cell-the-unit-of-life/eukaryotic-cell/https://www.toppr.com/guides/biology/cell-the-unit-of-life/eukaryotic-cell/

● Double membrane-bound: Cell organelles like mitochondria

and chloroplast are double membrane-bound organelles. They

are present only in a eukaryotic cell.

Browse more Topics under The Fundamental Unit Of Life

● Cell- Introduction

● Structure of Cell

Learn more about Cell Division and Cell Cycle here.

Structure and Functions of Cells

Structure and functions of different cell inclusions are as follows-

Cell

Organelle Occurrence/ Characteristic &

Structure Function

https://www.toppr.com/guides/biology/photosynthesis-in-higher-plants/introduction-to-photosynthesis/https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/cell-introduction/https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/structure-of-cell/https://www.toppr.com/guides/biology/cell-cycle-and-cell-division/cell-cycle/

Cell

Membrane/

Plasma

Membrane

● Present in both plant cell and animal cell.

● Selectively Permeable: Allows the materials in

and out of the cell

according to the

requirement of the cell.

● Made up of bilipid layer and protein (Fluid Mosaic

Model)

● Encloses the contents of the

cell.

● Provides shape: animal cell.

● Allows transport: by Diffusion and

Osmosis.

https://www.toppr.com/guides/science/sorting-materials-into-groups/properties-of-materials/https://www.toppr.com/guides/biology/biomolecules/proteins/

Cell Wall

● Present only in a plant cell.

● Hard and rigid.

● Fully permeable.

● Made up of Cellulose in plant and peptidoglycan

in bacteria.

● Protection

● Gives shape and turgidity.

Cytoplasm

● Contains 80-90% water and many organic and

inorganic compounds.

● Colloidal, Viscous, Jelly like fluid inside the cell.

● Contains enzymes

responsible for

all the metabolic

activity taking

place inside the

cell.

Nucleus

(Director/

Brain of the

Cell)

● Covered by a double membranous nuclear

membrane in a

Eukaryotic Cell.

● Contains DNA, RNA, Protein, nucleolus, and

Chromatin network.

● Controls the activity of the

cell.

● Starts cell division.

● It has the chromosomes or

DNA which

controls the

hereditary

characters

Mitochondria

(The Power

House of

The Cell /

Storage

Batteries)

● Double membranous structure.

● Autonomous body as contains its own DNA.

● Self-duplicates

● The main seat of respiration.

● Stores energy in the form of ATP molecules.

https://www.toppr.com/guides/biology/the-molecular-basis-of-inheritance/the-dna/https://www.toppr.com/guides/science/respiration-in-organism/respiration-and-its-types/

Golgi Bodies

(Shipping

Department of

Cell)

● Discovered by Camillo Golgi in 1898.

● Originates from RER.

● Contains Sac like Cisternae and Vesicles.

● Has two faces – cis face

or receiving face and trans face or supplying

face.

● Modification, Packaging, and

transport of

materials

● Synthesis of lysosomes,

plasma

membrane

Endoplasmic

Reticulum

(Framework

of Cell)

● A network of membranes.

● RER bears ribosomes and appears rough

● SER does not have ribosomes

● Forms the skeletal

framework of the

cell.

● Transport of materials from

one cell to other.

● Provides a surface for the

synthesis of

material –

Proteins in RER

and Lipids in

SER.

● Formation of lysosomes, Golgi

bodies and vacuoles

● Membrane Biogenesis

● Detoxification of harmful

substances in the

liver.

Vacuole

● Arise from ER and GB

● Surrounded by tonoplast and filled with cell sap

● Store cell sap which may be

liquid or solid

food, toxic

byproduct.

● Provide rigidity and turgidity to

plant cell

Lysosomes

(Suicidal

bags of Cell,

natural

scavenger,

cellular

housekeeper

)

● Membrane-bound organelles

● Present in all animal cells and few plant cells

● Tiny circular single membrane-bound

structures filled with

digestive enzymes

● Intracellular digestion of food

in unicellular

organisms.

Ribosomes

(Protein

Factories)

● Without a membrane

● Consist of two subunits – 60S and 40S in eukaryote

both made up of RNA

● Synthesis of Proteins

https://www.toppr.com/guides/biology/cell-the-unit-of-life/eukaryotic-cell/

Plastids

● Double membrane-bound

Types-

1. Leucoplast –Colourless plastid;

2. Chromoplast –Coloured Plastid – blue, red, yellow

3. Chloroplast – Green plastid

● Autonomous self-duplicating body

● Chloroplast – Perform

Photosynthesis –

Helps in the

release of oxygen

● Chromoplast –

impart colour to flowers which

help in

pollination

● Leucoplast – Storage

● Amyloplast –Store starch

● Aleuroplast – Store Protein

● Elaioplast – Store fat

Learn more about Prokaryotic Cell here.

Solved Exercise

Question 1: Write the name of different plant parts in which

chromoplast, chloroplast and leucoplast are present.

Answer:

● Chloroplast are present in green parts of the plant like leaves,

the stem of cactus, raw mango fruit etc

● Chromoplast are present in colourful parts of the plant like

flowers, fruits etc

https://www.toppr.com/guides/biology/cell-the-unit-of-life/prokaryotic-cell/https://www.toppr.com/guides/biology/anatomy-of-flowering-plants/stem/

● Leucoplast is present in parts not exposed to sunlight like roots,

potato etc.

Question 2: Name the organelles which show the analogy written as

under

Answer:

a. Transporting channels of the cell—— Endoplasmic Reticulum

b. Powerhouse of the cell—— Mitochondria

c. Packaging and dispatching unit of the cell—— Golgi Body

d. Digestive bag of the cell—— Lysosomes

e. Storage sacs of the cell—— Vacuole

f. Kitchen of the cell—— Chloroplast

g. Control room of the cell—— Nucleus

https://www.toppr.com/guides/science/light/sunlight-white-or-coloured/

Structure of Cell

How small is a cell? If you keep the tip of a ballpoint pen on your

skin, that will cover around one thousand cells. In human, cells vary

between 10 µm and 100 µm (µm stands for micron, which is 1

millionth of a meter or 1 thousandth of a millimetre). Let us see the

structure of cells.

Cells

Different cells have different shapes and sizes depending on the

function they perform. For example, RBCs are circular in shape, nerve

cells are elongated, muscle cells are spindle-shaped, skin cells are flat

etc.

http://sciencewave.com

https://www.toppr.com/guides/biology/the-fundamental-unit-of-life/structure-of-cell/https://www.toppr.com/guides/biology/cell-structure-and-function/cell-number-shape-and-size/https://www.toppr.com/guides/maths/relations-and-functions/functions/

A cell is regarded as a fundamental unit of life because the basic

structure of all the living organisms is made up of a cell. Hence cell is

known as the structural unit of life. Cell performs certain functions

which are the characteristic of life and which are important for

maintenance of life. Hence cell is known as the functional unit of life.

A cell can be viewed as an enclosed vessel, within which innumerable

chemical reactions take place simultaneously. These reactions are

under very precise control so that they contribute to the life and

formation of new cells.

Structure of Cells

A generalized cell consists of a membrane-bound structure enclosing a

nucleus and cytoplasm with many small organelles floating in it. Plant

and animal cell have a few differences given in the figure below.

Differences Between Plant Cell and Animal Cell

Plant Cell Animal Cell

Cell wall is present. Cell wall is absent.

https://www.toppr.com/guides/chemistry/chemical-reactions-and-equations/chemical-reactions-and-equations/https://www.toppr.com/guides/reasoning-ability/images/grouping-of-figures/

Nucleus is peripheral. Nucleus is central.

Large central vacuole is present. Vacuoles may be small or absent.

Plastids are present. Plastids are absent.

Centriole is absent. Centriole is present.

Golgi bodies present in the form of units known

as dictyosomes. Golgi bodies well developed.

Subcellular components

1. Cell Membrane/ Plasma Membrane:

● Present in both cell structure of the prokaryotic cell and

eukaryotic cell

● The cell membrane, or plasma membrane, is a biological

membrane that surrounds the cytoplasm of a cell. In animals,

the plasma membrane is the outer boundary of the cell, while in

plants and prokaryotes it is usually covered by a cell wall.

● It is flexible and elastic

https://www.toppr.com/guides/science/nutrition-in-animals/introduction-to-nutrition/

● A cell membrane is called a selectively permeable membrane.

Why? This is because it allows the materials in and out of the

cell according to the requirement of the cell.

● Structure: It is made up of bilipid layer and protein (Fluid

Mosaic Model)

● Functions: Encloses the contents of a cell.

○ Provides shape

○ Allows transport – Transport is of two types-

Diffusion and osmosis

Comparison of Diffusion and Osmosis

Diffusion Osmosis

Particles of solid, liquid or gas move from its

higher concentration to lower concentration

Solvent particles move from its higher

concentration to its lower concentration through

a semipermeable membrane.

It does not require a semipermeable membrane Semipermeable membrane is required

https://www.toppr.com/guides/science/sorting-materials-into-groups/properties-of-materials/https://www.toppr.com/guides/biology/transport-in-plants/means-of-transport/

It occurs in liquid and gaseous medium It occurs only in liquid medium

What will happen when a cell is placed in solutions of different

concentrations?

● Hypotonic solution: A solution which contains more quantity of

solvent and less quantity of solute as compared to the cell is

called hypotonic solution. When a cell is placed in hypotonic

solution endosmosis takes place. An animal cell will swell and

burst. A plant cell will resist swelling and burst due to the

presence of hard and rigid cell wall. For example distilled

water

● Hypertonic Solution: A solution which contains less quantity of

solvent and more quantity of solute as compared to the cell is

called hypertonic solution. When a cell is placed in hypertonic

solution, exosmosis takes place. An animal cell will shrink. In a

plant cell, plasma membrane and cytoplasm will separate from

the cell wall. This is known as plasmolysis. When a

plasmolyzed cell is placed in a hypotonic solution, endosmosis

will occur and the cell will regain its structure. This is called

de-plasmolysis. For example, saturated salt solution.

https://www.toppr.com/guides/chemistry/solutions/types-of-solutions/

● Isotonic Solution: A solution which contains the same quantity

of solvent and solute as compared to the cell is called isotonic

solution. When a cell is placed in isotonic solution, no net

movement of particles takes place. Example Ringer’s solution

is isotonic as compared to human RBC.

2. Cell Wall

Next component of the structure of call is the cell wall. They are

present only in plant cell and they are hard and rigid.

● Fully permeable

● The cell wall of two adjacent cells are cemented together by

middle lamella made up of Calcium Pectate

● Made up of Cellulose in plant and peptidoglycan in bacteria

● breaks in the cell wall (Pits) have cytoplasmic connections

(Plasmodesmata) through which exchange of materials occur

● Function: Protection, gives shape and turgidity

3. Cytoplasm

● Colloidal, Viscous, Jelly like fluid inside cells.

https://www.toppr.com/guides/maths/the-fish-tale/size-and-quantity/

● Contains 80-90% water and many organic and inorganic

compounds.

● Cytoplasm contains enzymes which are responsible for all the

metabolic activity.

● Cytoplasm is responsible for giving a cell its shape.

● Various cell organelles are found floating in the cytoplasm.

4. Nucleus (Director/ Brain of the Cell)

● Covered by a double membranous nuclear membrane in a

Eukaryotic Cell.

● contains DNA, RNA, Protein, nucleolus, and Chromatin

network.

● Functions: Controls the activity of the cell.

● Starts cell division.

● It has the chromosomes which are made up of DNA which

controls the hereditary characters.

https://y12hb.files.wordpress.com

Solved Question for You

Q: How does the exchange of materials take place in unicellular

organisms?

Answer: In unicellular organisms, the process of diffusion helps in

exchange of materials. Respiratory gases like oxygen and carbon

dioxide diffuse through general body surface into the surrounding

water. Food is taken in through phagocytosis.