3.2.3 - Passive Transport

Wednesday 8th November ‘23

Simple diffusion: net (passive) movement of molecules or ions from an area of high concentration to an area of low concentration (down their concentration gradiation). This often happens across a partially permeable membrane and occurs for molecules which are non-polar and not too large.

• Factors affecting this: temperature; concentration gradient; surface area; membrane thickness

Larger molecules (e.g amino acids, glucose) and charged particles (e.g ions and polar molecules) would move very slowly through the membrane so they need to diffuse through specialised carrier or channel proteins instead – this is facilitated diffusion. The proteins “help” or “facilitate” the movement of the particles through the membrane. This is much faster than simple diffusion.

• Factors affecting this: concentration gradient - positive correlation until all proteins in use or once equilibrium is reached; number of channel / carrier proteins - positive correlation until all proteins in membrane becomes a limiting factor.

Carrier proteins

• Move large molecules acrosss the membrane

• Different proteins facillitate the diffusion of different large molecules

• Large molecules attach and the the protein changes shape releasing the molecule on the other side of the membrane

Channel proteins

• Form pores for charged particles to diffuse through

• Different protein channels facillitate the diffusion of different charged particles

• No changing shape, simple tunnel which allows charged particles to cross the membrane