The Fundamental Unit of Life

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04 Plasma Membrane


A cell consists of:

  • The cell membrane or plasma membrane, and cell wall
  • The nucleus, and
  • The cytoplasm.

Plasma membrane or cell membrane:

  • The cell surface membrane or the plasma membrane is the outermost covering of each cell.
  • It is present in the cells of plants, animals and microorganisms.


  • Plasma membrane is made up of organic molecules called lipids and proteins.
  • It is a living, thin, delicate, elastic and, selectively permeable membrane.


  • Plasma membrane is called a selectively permeable membrane because it permits the entry and exit of only some materials in the cells.
  • It performs the following functions:
    • Diffusion: Some substances such as carbon dioxide (CO2), oxygen (O2), water (H2O), etc., can move across the plasma membrane through a process called diffusion.
    • Osmosis: Water too, follows the law of diffusion. The spontaneous movement of water molecules through a selective permeable membrane (e.g. plasma membrane) is called osmosis.
    • It protects the integrity of the interior cell.
    • It provides support and maintains the shape of the cell.
    • It helps in regulating the cell growth through the balance of endocytosis and exocytosis.
    • The cell membrane also plays an important role in cell signaling and communication.
    • It acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.
  • One of the following three things could happen if we put an animal cell or a plant cell into a solution of sugar or salt in water:
    • If the medium surrounding the cell has a higher water concentration than the cell i.e. the outside solution is a very dilute solution, the cell will gain water by osmosis and such a dilute solution is called hypotonic solution.
    • If the medium surrounding the cell is of exactly the same water concentration as the cell, the net movement of water across the plasma membrane is zero. And such a solution is called isotonic solution.
    • If the medium has a lower concentration of water than the cell, i.e., the outside solution is a very concentrated solution, the cell will lose water by osmosis. And such a concentrated solution is called hypertonic solution.

Differences between diffusion and osmosis:

Diffusion Osmosis
Diffusion can occur in any medium. Osmosis occurs only in liquid medium.
The diffusing molecules may be solids, liquids or gases. Osmosis involves movement of solvent molecules only.
Semipermeable membrane is not required. Semipermeable membrane is required.
Diffusion depends only upon the free energy of the molecules of the diffusing substance. The presence of any other substances in the system is of no importance. Though osmosis is the diffusion of solvent molecules only, it influenced by the presence of other substances (solutes) in the system.
An equilibrium in the free energy of diffusion molecules is achieved in the system. Equilibrium in the free energy of solvent diffusion molecules is never achieved.

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