Factors that Affect Diffusion (AQA GCSE Biology): Revision Note

Diffusion: Factors that affect rate

• The following factors affect the rate of diffusion:

Difference in concentrations (concentration gradient)

• The greater the difference in concentration between two regions, the faster the overall rate of diffusion.

Temperature

• The higher the temperature, the more kinetic (movement) energy the particles of that substance will have.

• They will move/spread faster compared to when at a lower temperature, when they have less kinetic energy.

Surface area of a membrane separating two regions

• A membrane with a greater surface area will have a greater rate of diffusion across it.

• Think of there being more ‘entry or exit points’ for particles to cross.

Surface area to volume ratio

• The surface area : volume ratio of an organism affects how easily substances can be exchanged between it, and its environment

• Most bacteria are single-celled organisms.

• Bacteria have a relatively large surface area in comparison to their volume

  • This means that the distance between the cell membrane at a bacterial cell's surface, and the centre of the cell, is relatively low

  • Substances do not have to travel very far to get where they are needed, so transport by diffusion, osmosis or active transport alone is sufficient for the cell to meet its needs

Diffusion in a bacterial cell

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Calculating surface area to volume ratios

• Being able to calculate and compare surface area to volume ratios is important

• The effect of increasing size on surface area to volume ratio can be modelled using simple cubes

• Larger organisms with smaller surface area to volume ratios need exchange surfaces and transport systems to ensure their cells obtain the materials they need

• Surface area to volume ratio is calculated:

$\mathrm{surface}\mathrm{area}:\mathrm{volume}=\frac{\mathrm{surface}\mathrm{area}}{\mathrm{volume}}$

Use this image