CBSE Class 9 Science, The Fundamental Unit of Life: Chapter notes (Part-II)

May 23, 2017 14:35 IST

Class 9 Science Chapter Notes, The Fundamental Unit of Life notes Class 9In this article you will get CBSE Class 9 Science notes on chapter 5 ‘Is The Fundamental Unit of Life’ (Part-II) . These chapter notes are prepared by the subject experts and cover every important topic from the chapter. At the end of these notes you can try the questions asked from the discussed set of topics. These questions will help you to track your preparation level and get a hold on the subject.

CBSE Class 9 Science, The Fundamental Unit of Life: Chapter notes (Part-I)

Main topics covered in this part of CBSE Class 9 Science,  The Fundamental Unit of Life: Chapter Notes, are:

  • Diffusion
  • Osmosis
  • Hypotonic solution
  • Isotonic solution
  • Hypertonic solution
  • Mediated Transport
  • Endocytosis
  • Exocytosis

Key notes for Chapter- The Fundamental Unit of Life, are:

Transport of Substances through Plasma Membrane

Selectively Permeable Membrane:

Plasma membrane permits the entry and exit of certain materials in the cells. Therefore, it is named as selectively permeable membrane.

CBSE Class 9 Science Syllabus 2017-2018

Substances can pass through the plasma membrane by two processes:

1. Diffusion

2. Osmosis


It is the process of movement of substances from a region of high concentration to the region of low concentration until uniform concentration is finally acheived.

For example:

  • During the respiration, due to the difference of the concentration of carbon dioxide (CO2), inside and outside of the cell, CO2 is given out a waste product, moving out from region of high concentration to region of low concentration.
  • Similarly, oxygen (O2) enters the cell by the process of diffusion when level of concentration of O2 inside the cell decreases.


It is the process of movement of water from its high concentration region to its low concentration region through semi-permeable membrane.

Different types of solutions exhibiting osmosis are:

(i) Hypotonic Solution:
If the medium surrounding the cell has a higher water concentration than the cell, i.e., if the solution is very dilute solution, then the cell will gain water by osmosis. Such dilute solution is called Hypotonic solution.

Endomosis: Though water molecules are free to pass across plasma membrane in both sides but more water will enter inside the cell. The cell will therefore swell up and increase in volume. This process is called Endosmosis.

(ii) Isotonic Solution:

If the medium surrounding the cell is of exactly the same water concentration as inside the cell, there will be no net movement of water across membrane resulting in no change in size of cell. Such solution is called Isotonic solution.

(iii) Hypertonic solution:

If the medium surrounding the cell has a lower water concentration than inside the cell, i.e., if the solution is highly concentrated, then the cell will lose water through osmosis. Such concentrated solution is called Hypertonic solution.

Exomosis: When the water moves out of the cell placed in a highly concentrated solution, it wall cause the cell to shrink. This process is known as Exosmosis.

Plasmolysis and Cytolysis:

The biological phenomena of exosmosis when a cell or tissue is placed into a strong hypertonic solution, is termed as plasmolysis, whereas the reverse process is cytolysis, which occurs if the cell is placed in a hypotonic solution resulting in a lower external osmotic pressure and a net flow of water into the cell.

Difference between Diffusion and Osmosis:

S. No.




It can take place in any medium.

It can take place only in liquid medium.


Semipermeable membrane is not required.

Semipermeable membrane is required.


Here the diffusing molecules may be solids, liquids or gases.

It involves the movement of solvent molecules only.


It is dependent on the free energy of the molecules of the diffusing substance only.

Here the diffusion of the solvent molecules is affected by the presence of other substances (solute) in the system.


An equilibrium in the free energy of the diffusing molecules is achieved.

Equilibrium in the free energy of the solvent molecules is never achieved.

Mediated Transport:

Plasma membrane renders the transposrt of many molecules of biological significance. Such essential molecules are moved across the membrane by special proteins called transport proteins or permeases. This process of forced diffusion of certain substances through the plasma membrane is called mediated transport.

Note: Permeases used in the process are quite specific to the substance they transport.

Types of Mediated Transport:

It is of the following two types:

(i) Facilitated transport/diffusion: Here, the permeases assists a molecule to diffuse through the membrane that it cannot otherwise penetrate.

(ii) Active transport: In this case, the energy is supplied to the system to transport the molecules in a direction opposite to a concentration gradient.


It is the process of ingestion of materials by the cells through the plasma membrane.

It describes all three similar processes: phagocytosis (cell eating), potocytosis (cell drinking) and receptor-mediated endocytosis.

Phagocytosis: It is a method of intake of food materials by certain organisms like protozoa (Amoeba). The flexibility of the cell membrane enables the cell to engulf the solid particles of food and other materials from its external environment.

phagocytosis in Amoeba


In this process the membrane of a vesicle can fuse with plasma membrane and extrude its contents to the surrounding medium. This process is also named as cell vomiting.

Cells exhibit exocytosis to:

  • Remove the undigested residues od=f substances brought in by endocytosis.
  • Secrete substances such as hormones, enzymes
  • Transport a substance completely across a cellular barrier.

Try the following questions:

Q1. Why is the plasma membrane called a selectively permeable membrane?

Q2. How do substances like CO2 and water move in and out of the cell? Discuss.

Q3. How does an Amoeba obtain its food?

Q4. What is Plasmolysis?

Q5. Differentiate between endocytosis and exocytosis.

Q6. Why does the skin of our hands shrink when we wash clotehs for a long time?

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