SA:V ratio

The Challenges of Size:

- All organisms must exchange substances with their surroundings.
- For instance, in human gas exchange, O₂ enters our cells and CO₂ exits them.

- Elephants → Multicellular 
- Paramecium cells → Single-celled
- Both these organisms have similar requirements.
- What are the differences in the needs of these organisms for the following?
1) Specialised exchange surfaces
2) Internal transport system
- An elephant needs lungs (1) and a circulatory system (2).
- A paramecium cell does not require specialization in its cell membrane.

Importance: In determining whether an organism needs specialised exchange surfaces and a transport system or not?
- Size: A big organism will need specialised exchange surfaces and a transport system, but a small organism won't.
- Level of activity
- Surface area to volume ratio

SA:V Ratio

- Cell B has a larger surface area.
- However, oxygen will get to cell A’s middle faster because of the surface area to volume ratio.
- For example, a 1cm x 1cm x 1cm cube will have an SA of 6cm² and a volume of 1cm³.
- Therefore, the SA:V ratio is 6:1.
- For example, a 10 cm x 10 cm x 10 cm cube will have an SA of 600 cm² and a volume of 1000 cm³.
- Therefore, the SA:V ratio is 600:1000 or 0.6:1.
- The larger the cube, the smaller the SA:V ratio.

Change of SA:V Ratio in Cubes:

- The SA:V ratio decreases as the cell size increases.

- Diffusion/Exchange is fast in cell A but slower in cell B.