Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration - meaning that particles diffuse down the concentration gradient.
This is a passive process - no energy is required for the process to happen. Sometimes particles can diffuse directly through cell membranes - this is simple diffusion.
What affects the rate of diffusion?
- Concentration gradient - a steeper concentration gradient means that diffusion can occur quicker.
- Thickness of exchange surface - the thinner the exchange surface, the shorter the diffusion pathway and this means that diffusion will occur faster.
- Surface area - larger surface area, faster rate of diffusion
Facilitated diffusion
Simple diffusion is not the only type of diffusion - some molecules need need to diffuse through the membrane, they diffuse through facilitated diffusion.
These are:
- Large molecules
- Polar molecules
- Ions
Instead, they are transported through channel proteins or carrier proteins. However, like simple diffusion it is a passive process and therefore energy is not needed. The molecules move down the concentration gradient.
Carrier proteins
These proteins move large molecules across a membrane, down the concentration gradient. How do they work?
- A molecule attaches to the carrier protein
- The carrier protein changes shape
- The molecule is released on the opposite side of the membrane
Channel proteins
The other type of transport protein is called channel proteins. These form pores in the membrane and charged particles diffuse through, as they travel down the concentration gradient. However, different channel proteins transport different particles.
What affects facilitated diffusion?
- Concentration gradient - higher concentration gradient means that there will be a faster rate of facilitated diffusion
- Number of carrier/channel proteins - when all transport proteins are saturated, facilitated diffusion cannot occur any faster, even if the concentration gradient is increased.