Diffusion

This lesson covers:

  1. The process of simple diffusion 
  2. The process of facilitated diffusion 
  3. How different factors affect the rate of diffusion

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. 

Diagram showing diffusion with particles moving from high concentration to low concentration.

Particles diffuse down a concentration gradient and eventually reach equilibrium in which all particles are evenly distributed (but still moving). 


Diffusion does not require energy and so is described as a passive process. 

You need to know about two types of diffusion: 

  1. Simple diffusion 
  2. Facilitated diffusion

Simple diffusion

Some molecules can diffuse directly across cell membranes in a process known as simple diffusion.

Diagram showing simple diffusion of molecules across a cell membrane with phospholipids.

Oxygen and carbon dioxide can diffuse into cells via simple diffusion.


This is because they are: 

  • Small - This means that they can pass through the spaces between phospholipids.
  • Non-polar - This means that they can dissolve in the hydrophobic core of the cell membrane. 

Facilitated diffusion

Large or polar molecules can cross cell membranes in a process known as facilitated diffusion.


Facilitated diffusion involves two types of proteins:

  1. Carrier proteins
  2. Channel proteins


Each carrier or channel protein is highly specific, allowing only one or two types of molecule to pass through. 

Carrier proteins

Carrier proteins transport large molecules across the cell membrane.


They work in the following way: 

Illustration of facilitated diffusion showing a carrier protein transporting a molecule across the cell membrane.
  1. A large molecule attaches to a carrier protein.

Channel proteins

Channel proteins transport ions across the cell membrane. 

Illustration of facilitated diffusion showing a carrier protein transporting a molecule across a cell membrane.

These proteins form pores in the cell membrane, which ions can travel through. 

Factors affecting the rate of diffusion

  1. Temperature - At higher temperatures, particles have more kinetic energy and diffuse faster. 
  2. Concentration gradient - The steeper the concentration gradient, the faster the rate of diffusion. 
  3. Thickness of membrane - The particles travel shorter distances through thin exchange surfaces, so diffuse faster. 
  4. Surface area - Larger surface areas mean more particles can cross the membrane at once, making diffusion faster.
  5. Number of carrier or channel proteins - The more of these proteins, the faster the rate of facilitated diffusion.