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Explain the relationship between cell size and "surface area to volume ratio"
- Nutrients and wastes are moved across a cell’s surface by the process of diffusion. Diffusion works well for small cells but not for large cells. The reason for this is explained by geometry.
- When a cell grows its rate of metabolism increases at the same rate as its volume. This means that the rate of diffusion – which provides the materials for metabolism -must increase proportionally with a cell’s growth. However, this can’t happen because a cell’s surface area increases at a much slower rate than its volume.
- Therefore, as a cell grows – and its surface-to-volume ratio decreases - it becomes increasingly difficult for the cell to obtain nutrients and expel wastes by diffusion. And at some point it simply becomes impossible. When cells grow too large they can divide in two by the process of mitosis.
- Thus cells are small because they can’t be large. A large cell – if one existed – would have a big volume requiring lots of nutrients, and producing lots of wastes – but its surface area would be too small for diffusion to be able to meet the cell’s needs.
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