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Chapter 5 Science class 9

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5. The Fundamental Unit of Life

| page 2 |

Notes 5. The Fundamental Unit of Life - page 2 | Class 9 Science - Toppers Study

Chapter -5


Cell wall is only found in plant cells, which is mainly made up of cellulose. It provides structural hardness to the plants due to cellulose.

Cellulose: It is a special kind of complex carbohydrate, which is only found in plants and it provides structural hardness to the plants. All herbivors digest cellulose easily while human intestine does not do so. This is so that human intestine is shorter than other herbivors animals.  

Plasmolysis: 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.

Type of solutions on the basis of concentration: 

(I) Hypotonic solution: If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.

The Consequence of this the cell is likely to swell up.

(II) Isotonic Solution: If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.

There is no overall movement of water in this solution, so the cell will stay the same size.

(III) Hypertonic Solution: If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.

The consequence of this solution water moves from inside to outside causes cell will shrink. 

The Absorption of water by plant's roots: Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.

Significance of diffusion in the life of cell: 

(i) Diffusion is important in exhange of gases and water in the life of a cell.

(ii) The diffusion also helps to cell to obtain nutrients from its environment. 

(iii) Different molecules move in and out of the cell through a type of transport that is diffusion. 

(iv) Absorption of water by plant roots also takes place by another similar process osmosis.

Plants Cells can easily withstand much greater changes inthe sorrounding medium than animal cells.

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 their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.


Metabolisms are biochemical proccesses which occur in all living organisms to maintain life. 

There are two type of metabolisms: 

(i) Anabolism : This is a group of creative chemical reaction in which the proccess uses energy produced during catabolism to synthesize or construct simpler molecules to complex molecules. This proccess also decides to distribute essential nutrients all parts of body even to the cells which causes new cells or tissue formation. 

(ii) Catabolism : This proccess breaks down complex carbonic molecules into simpler molecule to form energy during cellular respiration. 

Cell Organelles: 

1. 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.

There are two types of ER:

(I) Rough endoplasmic reticulum (RER): RER looks rough under a microscope because it has particles called ribosomes attached to its surface.

(a) The manufactured proteins are then sent to various places in the cell by RER. 


(c) Ribosomes are present in RER.  

(II) Smooth endoplasmic reticulum (SER): 

(a) SER helps in the manufacturing of fat moleciles and lipids. 

(b) Some of these proteins and lipids help in building the cell membrane.

(c) Ribosomes are not present in SER. 

Function of Endoplasmic Reticulum: 

(i)  It creates a network systems between cytoplasm and nucleus. 

(ii) It works as a pipeline for transportation of protien between cytoplasm ans nucleus. 

(iii) ER also works as cytoplasmic framework providing a surface for some cellular biochemical process. 

(iv) SER plays a crucial role in detoxifying many poisons and drugs in liver cells. 

(v) SER helps in the manufacturing of fat moleciles and lipids. 

Membrane biogenesis: Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis.

2. Golgi aparatus : 

The first Camillo Golgi discribed this cell orghanelle. This is a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.

Functions Of Golgi apparatus: 

(i) This forms another portion of a complex cellular membrane system to connect with ER. 

(ii) ER functions include the storage, modification and packaging of products in

(iii) Complex sugars may be made from simple sugars in the Golgi apparatus.

(iv) The Golgi apparatus is also involved in the formation of lysosomes. 

Black Reaction: Camillo Golgi carried out was a revolutionary method of staining individual nerve and cell structures. This method is referred to as the ‘black reaction’.This method uses a weak solution of silver nitrate and is particularly valuable in tracing the processes and most delicate ramifications of cells. 

3. Ribosome: 

Ribosomes are rounded structure which are mainly locate freely in cytoplasm or may be attach with ENdoplasmic reticulum. It is also called the protien factory of cell as they manufacture protien.

Functions of Ribosome:

(i) It is made up of RNA (Ribonucleic-acid).

(ii) It forms protien from amino-acid.

(iii) It provided surfaces for cellular biochemical activities.

4. Lysosome: 

Lysosomes are a kind of waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive
enzymes capable of breaking down all organic material.

The suicide bags:

During the disturbance in cellular metabolism, 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.

Functions of Lysosomes:  

(i) It cleans cells to digest cellular wastes.

(ii) Its highly strong enzymes digest such as bacteria or food, as well as old organelles end up in the lysosomes.

(iii) It removes dead and damaged cells from body. 

5. Mitochondriya:

Mitochondria have two membrane coverings instead of just one. The outer
membrane is very porous while the inner membrane is deeply folded. Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

The Powerhouses: 

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. Hence, ATP is a cellular energy that is manufactured and stored in mitochondria therefore Mytochondria is called powerhouses of the cell. 

Adenosine triphopshape (ATP): ATP is a cellular energy, which is used in varoius biochemical process to synthesise new molecules. 

Functions of mitochondria: 

(i) It provide energy to the cell in the form of ATP. 

(ii) There are many important enzymes for cellular respirations. 

(iii) It makes some of their own proteins.

(iv) The manufacturing and storage of cellular enrgy is also taken place by mitochondria.  

6. Plastids:  

Plastids are present only in plant cells. The internal organisation of the plastids
consists of numerous membrane layers embedded in a material called the stroma. Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

There are three types of plastid: 

(I) Chromoplasts (coloured plastids): Chlorophyll is not present in it and it does not take part in the process of photosynthesis. Its main work is to make beautiful to the plant. It mainly locates in fruits and petals of flowers.

(II) Leucoplasts ( White or Colouress plastids): Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are
stored. They locate in those parts of plant where the photosynthesis does not occur, because they have not green pigments chlorophyll.  

(III) Chloroplasts: Plastids containing the pigment chlorophyll are known as
chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

Chlorophyll: The green pigment which is mainly found in leaves of plants is known as chlorophyll. It takes part in the photosynthesis.

Functions of plastids:

(i) There are various colours in the various parts of plants due to various types of plastids.

(ii) The process of photosynthesis takes place in the presence of having green pigment plastid named chlorophyll.

(iii) Leucoplasts store starch, oils and protein granules in the form of stored products. 

7. Vacuoles: 

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large 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 (swollen) and rigidity (hardness) to the cell.

Functions of Vacuoles: 

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

(ii) They store many importants substances for plants like amino acids, sugars, various organic acids and some proteins. 

(iii) Specialised vacuoles also play important roles in expelling excess water and some wastes from the cell in unicellular organisms.



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Study Materials List:

Notes ⇒ Class 9th ⇒ Science
1. Matter in Our Surroundings
2. Is Matter around us Pure
3. Atoms and Molecules
4. Structure of The Atom
5. The Fundamental Unit of Life
6. Tissues
7. Diversity in Living Organisms
8. Motion
9. Force and Laws of Motion
10. Gravitation
11. Work and Energy
12. Sound
13. Why Do We Fall ill
14. Natural Resources
15. Improvement in Food Resources

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