Diffusion: Simple Diffusion, Facilitated Diffusion & Factors Affecting Rate of Diffusion

Diffusion: Types & Factors Affecting Rate of Diffusion

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

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

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.