CBSE 9th Biology | Organization in the Living World-The Fundamental unit of life

Organization in the Living World-The Fundamental unit of life



The year 1665 is of great importance in the history of science because Robert Hooke discovered that all living things consist of separate units called cells. Hooke’s observations were based on his experiment with cork which comes from the bark of Quercus Suber tree. Cork under microscope resembled honeycomb like structure made up of several compartments. Robert Hooke called them ‘cells’. ‘Cell’ is a Latin word meaning ‘littlie room’.

Electron microscope was first built in 1931 by Ruska and Knoll. It uses a particle beam of electron to create a highly magnified image of the specimen. It has high magnifying and resolving power. Modern electron microscope can magnify objects up to 2 million times. Applications – Life sciences, Industry.




All living organisms are made up of cells. In case of unicellular or a cellular organisms a single cell constitutes a whole organism, eg. Amoeba, Chlamydomonas, Paramaecium, Bacteria. In multicultural organisms many cells group together in a single body and assume different functions, eg. Fungi, plants. Cell Theory given by Schleiden (1838) and Schwann (1839) suggested that all plants and animals are composed of cells and Virchow (1855) suggested that all cells arise from pre-existing cells Omni-cellula-e-cellula).

The. cells vary in shape and size according to the function they perform, eg. various cells of human body vary in structure according to their function.


Each living cell perform certain basic functions, characteristic of all living forms. Like multicultural organisms, there is division of labour within a cell also. Each cell has certain specific components called cell organelles which perform specific functions.


Microscopic study of a cell reveals following structures from outside to inside — Plasma membrane, Cell wall, Nucleus, Cytoplasm.

1. Plasma membrane or Cell membrane:

Its the outermost covering of the cell which separates the contents of the cell from its external environment. It allows entry and exit of only certain materials so it is also called selectively permeable membrane. C02 and 02 move across the membrane through diffusion.

Diffusion — It is the movement of a substance from a region of higher concentration to a region of lower concentration. C02 is cellular waste and accumulates in high concentration inside the cell. Its concentration outside the cell is low. So, C02 diffuses out of the cell. Similarly 02 enters the cell by diffusion when its concentration inside the cell decreases.

Osmosis — It is the movement of water through a selectively permeable membrane from a region of high water concentration to a region of low water concentration.

Fate of animal or plant cell in Hypertonic, Hypotonic and Isotonic solution:

1. Hypertonic solution — In this the concentration of solution is more than {hat of cell. Therefore, the cell will loose water by exosmosis and shrink.

2. Hypotonic solution — In this the concentration of solution is less than that of cell. Therefore, the cell will gain water by endosmosis and swell up.

3. Isotonic solution — In this the concentration of solution is same as that of cell. Therefore, there will be no movement of water, and cell size will remain same.

Plasma membrane is made up of lipids and proteins. Fluid Mosaic Model of Plasma membrane was proposed by Singer and Nicholson.


(i) It helps in exchange of gases and water in and out of the cell.

(ii) It separate contents of the cell from external environment.

(iii) It helps in engulfing food by endocytosis as in Amoeba.

2. Cell Wall

It is outermost covering which lies outside the plasma membrane. Cell wall is present only in plant cells, animal cells lack cell wall. Plant cell wall is composed of a complex substance cellulose which provides structural strength to plants. It is completely permeable.

Plasmolysis: The phenomenon of shrinking of protoplasm of a cell due to exosmosis when kept in

hypertonic solution is called plasmolysis.

Due to cell wall, plants, fungi and bacteria withstand much greater changes in the surrounding medium than animal cell.


(i) It provides rigidity and strength to the cell.

(ii) It helps to sustain during unfavorable conditions.

3. Nucleus

Nucleus is the darkly coloured, spherical or oval structure near the centre of a cell. It has a double layered covering called nuclear membrane which has several pores for the transfer of materials from inside to outside. Nucleus contains hereditary material called chromosomes. Chromosomes contain hereditary units, DNA or Deoxyribonucleic acid which are transferred from parents to the next generation. Functional segments of DNA are called “Genes”. Chromosomes are visible only during cell division.

Prokaryotes — Bacteria and some other organisms lack a well defined nucleus surrounded by nuclear membrane. Those are called prokaryotes. Such nucleus is called nucleoid. Membrane-bound cell organelles are absent in prokaryotes.

Eukaryotes — Organisms with a well defined nucleus surrounded by nuclear membrane are called

eukaryotes. Membrane-bound cell organelles are present in eukaryotes.


(i) It plays an important role in cellular reproduction.

(ii) It plays a crucial role in development of a cell by directing chemical activities.

(iii) It plays an important role in inheritance of characters from parents to the off springs.

clip_image004Prokaryotic cell


4. Cytoplasm

The lightly stained region of the cell is cytoplasm. It contains many cell organelles with specific functions. Cell organelles are membrane bound little structures which support the structure and function of a cell through chemical activities,

(i) Endoplasmic Reticulum (ER)

Its a large network of membrane – bound tubules. It is of two types-

(a) Rough endoplasmic reticulum (RER)-contains ribosomes attached to its surface. Ribosomes are site of protein synthesis.

(b) Smooth endoplasmic reticulum (SER) – contains no ribosomes. It helps in manufacture of fats and lipids. Proteins and lipids help in building cell membrane and the process is called “Membrane Biogenesis.”


(i) It act as a channel for transport of materials within cytoplasm or between cytoplasm and nucleus.

(ii) It provides surface for biochemical activities like protein and lipid synthesis.

(iii) In liver cells of vertebrates, SER detoxifies poisons and drugs.






Animal Cell

(ii) Golgi apparatus

Discovered by Camillo Gojgi. It consists of membrane bound vesicles arranged parallel to each other in stacks called cisterns, connected with ER.


(i) It helps in transport of substances synthesised near ER inside and outside the cell,

(ii) It helps in storage, modification and packaging of products in vesicles,

(iii) It is involved in the formation of lysosomes.

(iii) Lysosomes

These are membrane bound sacs filled with digestive enzymes. Digestive enzymes are capable of breaking down all organic material. Enzymes are made by RER. When cell gets damaged, lysosomes burst and enzymes digest their own cell. So, they are also called “suicide bags” of a cell.

Functions: It keeps the cell clean by digesting foreign materials and old worn-out cell organelles,

(iv) Mitochondria

These are double membrane bound organelles. The outer membrane is porous while the inner membrane is deeply folded providing large surface for ATP- generating chemical reactions. Energy rich molecules or energy currency of cell ATP (Adenosine triphosphate) are synthesized in mitochondria. So, they are also called “Power houses” of the cell. Mitochondria have its own DNA and ribosomes. Mitochondria is absent in bacteria.


(i) ATP synthesis

(ii) Protein synthesis.

Protoplasm = Cytoplasm + nucleus

Mitochondria was first discovered by Benda (1839).

Lysosomes was discovered by Christian de Duve.

(v) Plastids

They are present only in plant cells. They are two types – Chromoplasts (coloured plastids) and Leucoplasts (white plastids). Plastids that contain green colour pigment, chlorophyll are called chloroplasts. Chloroplasts help in photosynthesis in plants. Plastids are double membrane bound. Numerous membrane layers called grana lie embedded in matrix called stroma. They also have their own DNA and ribosomes.


(i) Chloroplasts play an important role in photosynthesis in plants.

(ii) Leucoplasts store starch, oil and protein granules.

(vi) Vacuoles

These are storage sacs for solid or liquid contents. They are small sized in animal cell and large sized in plant cells. They are filled with cell sap and store amino acids, sugars, organic acids and proteins.


(i) Vacuoles provide turgidity and rigidity to the cell.

(ii) They store some important chemical substances.

(iii) Contractile vacuoles help in expelling excess water and wastes in some animals.

Thus the various structures of a cell helps it to perform various functions like respiration nutrition, excretion etc. making it the functional unit of life.


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