# 1.3.2 Factors that Affect Diffusion

### Diffusion: Factors that Affect Rate

• You need to be able to explain how the following factors affect the rate of diffusion:

Diffusion factors

#### Exam Tip

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

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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