Factors that Affect Diffusion (AQA GCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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Diffusion: Factors that Affect Rate
You need to be able to explain how the following factors affect the rate of diffusion:
Diffusion factors
Examiner Tips and Tricks
Remember that diffusion is a passive process, so when it occurs in a living organism, the cells of that organism do not provide the particles involved with energy to diffuse. The particles that are moving about randomly have their own kinetic energy.
Surface Area : Volume Ratio
The surface area to 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
Unicellular organisms can exchange materials directly with their environment
Calculating Surface Area to Volume Ratios
You should be able to calculate and compare surface area to volume ratios
We can model the effect of how increasing size affects surface: volume ratio using simple cubes
Larger organisms with smaller surface area: volume ratios need exchange surfaces and transport systems to ensure their cells obtain the materials they need
As the size of an organism increases, surface area: volume ratio decreases
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