Diffusion (AQA GCSE Biology: Combined Science)

• In order for any organism to function properly, it needs to exchange substances such as food molecules and waste products between itself and its environment
• This exchange of substances occurs across the cell membrane
• There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport

What is Diffusion?

• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration
• This means:
• Particles in a solution or a gas are always moving about randomly
• If there happen to be more particles in one area/region, then there tends to be an overall (net) movement of particles from this region of higher concentration to a region where there are fewer particles (an area of lower concentration)

Diffusion of perfume particles through a room is one of the simplest examples of diffusion

• Diffusion is an entirely passive process; the movement of particles in a fluid is dependent on how much kinetic energy they have
  • The higher the temperature, the more kinetic energy particles have and therefore the faster they can move by diffusion

• All cells are surrounded by a cell membrane which separates the inside of the cell from its outside environment
• Substances have to be able to diffuse through the cell membrane to enter or exit the cell unaided (there are other ways for substances to get in)
• Not all substances are able to diffuse across the cell membrane which is why it is described as being ‘partially permeable’; some substances are able to enter or leave the cell whilst others aren’t
  • For example, oxygen and carbon dioxide can diffuse across the cell membrane during gas exchange whereas the carbohydrate starch cannot (it’s too big)

The cell membrane acts as a barrier across which substances have to be able to move to get into or out of a cell

• Examples of substances that enter cells by diffusion:
  • Oxygen must diffuse into cells for aerobic respiration
  • Carbon dioxide must diffuse into photosynthesising plant cells to be available for photosynthesis

• Examples of substances that leave cells by diffusion:
  • Liver cells break down excess amino acids into the waste product urea which is highly toxic and must diffuse out of cells to be excreted by the kidneys
  • Carbon dioxide produced by aerobic respiration must diffuse out of cells

• In both the transport of substances into and out of a cell by diffusion, the movement is driven by a concentration gradient existing between the cell and its immediate environment.
• If the direction of that concentration gradient changes, the movement of a substance by diffusion will also change