In this topic of IBDP Biology, we will learn about adaptations of exchange systems.

• Respiration
• Nutrition
• Excretion

Animals:

IN

Ready made food

Oxygen

Heat - in ectotherms

OUT

Carbon dioxide

Urea and other nitrogenous substances

Heat

Plants:

IN

Carbon dioxide during daylight

Water

Minerals

Oxygen at night

Heat

OUT

Oxygen during daylight

Water

Carbon dioxide at night

Heat

• 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

Distance

• 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
    • Villi
    • Microvilli
    • Alveoli
    • Gill filaments
    • Tracheoles in insects
  • Being longer and thinner:
    • Leaves
    • Camels vs polar bears.
    • Flat worms -  flat shapes evolved.
    • Jelly fish -  hollow centres evolved.

• 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

When it comes to IBDP Biology, there is an example Amoeba proteus – a protoctistan

• 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.

That's all!

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