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Diffusion: Types & Factors Affecting Rate of Diffusion


  • Diffusion is downhill movement of a substance along its electrochemical gradient.

Types of Diffusion

Simple Diffusion
  • The molecule does not make any chemical interaction with the passage through which it passes.

  • Through intermolecular space in lipid bilayer

    • Lipid-soluble substances like oxygen, carbon dioxide, etc diffuse through this route.

    • The more the lipid solubility → the faster the diffusion.

  • Through pore/channels

    • Water and other lipid-insoluble substance e.g. sodium ions diffuse through pores or channels.

    • The larger the molecule → the slower the diffusion.

  • Rate of simple diffusion depends on:

    • Amount of substance available

    • Velocity of kinetic motion of molecules

    • Number of openings

    • Size of openings

Facilitated Diffusion
  • Occurs through carrier proteins → called carrier-mediated diffusion.

  • The molecule becomes bound to carrier chemically during its passage.

  • Carrier protein open at one side → Binds with a molecule that they carry → closes from the open side → Opens on the other side → Releases the molecule on that side → Closes from that side → This cycle is repeated to carry more molecules.

  • E.g. diffusion of glucose through glucose transporters (GLUT)

  • Speed is limited by the speed with which the carrier protein can undergo conformational changes.

  • When all the carrier proteins are occupied → the rate of diffusion reaches maximum → there cannot be any further increase in rate of diffusion with increasing concentration gradient.

Factors Affecting Rate of Diffusion

Concentration Gradient
  • The higher the concentration gradient → the faster the diffusion

Electrical Gradient
  • The more favorable the electrical gradient → the faster the diffusion

Pressure Gradient
  • The more the pressure gradient → the faster the diffusion


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