Why do organisms exchange materials?
Ready made food
Heat - in ectotherms
Urea and other nitrogenous substances
Carbon dioxide during daylight
Oxygen at night
Oxygen during daylight
Carbon dioxide at night
Surface area : volume
- When it comes to IBDP Biology, an organism’s requirement to obtain or remove materials depends on its metabolic activity.
- Metabolic activity increases with volume.
- The rate of exchange of substances and heat depends on the organism's surface area.
Fick’s Law: Rate of Diffusion a Surface area x concentration difference
- The ability to meet the requirements depends on the surface area : volume ratio.
- As organisms get bigger their volume and surface area both get bigger.
- Volume increases much more than surface area.
- Therefore their surface area: volume decreases
- Simple organisms are smaller so have a larger SA:Vol
- Complex organisms are larger so have smaller SA:Vol
- With a small surface area and relatively large volume, rate of diffusion and heat exchange is low.
- It becomes more difficult for them to exchange materials and heat with their surroundings.
To increase rate of exchange:
- Increase surface area of exchange surface:
- Folding/ branching
- Gill filaments
- Tracheoles in insects
- Being longer and thinner:
- Camels vs polar bears.
- Flat worms - flat shapes evolved.
- Jelly fish - hollow centres evolved.
- Folding/ branching
- Maintain high concentration gradients:
- Use of cilia by protozoa.
- Ventilation mechanisms.
- Movement of villi.
- Active uptake by exchange surface.
- Use substance as soon as it is taken up by cell eg phosphorylation of glucose.
- Efficient transport mechanisms the far side of the cell - capillary network.
- Minimise the distance over which exchange occurs:
- Thin leaves
- Single layer of epithelium
- Close proximity of endothelium
Exchange in a single celled organism
- Large cells, (approx 200 micrometers), but very small organisms.
- Irregular in shape with many pseudopodia.
- Consequently, large SA:vol.
- No specialised gas exchange surface.
- Gas exchange is across the plasma membrane by diffusion.
- Requires a concentration gradient.
- Respiration uses up glucose and oxygen internally.
- Carbon dioxide produced internally.
- Concentration gradient established for each.
- Carbon dioxide and oxygen are small molecules, with no charge distribution.
- They are very small molecules so pass easily through pores in the membrane.