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2.6.1Additional lessons

Gas Exchange in Organisms — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 27 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 5Why Gas Exchange Is Needed

2.6.1 Gas Exchange in Organisms
Aerobic respiration
  • Cells oxidise glucose using oxygen to release energy as ATP and produce carbon dioxide as a waste product.
Exchange with environment
  • Oxygen must be taken in and carbon dioxide released, and this exchange occurs by diffusion across a surface.
Plants
  • In daylight plants take in carbon dioxide for photosynthesis and release oxygen, but they also respire and exchange gases.
Diffusion rate
  • Rate depends on surface area, concentration gradient and diffusion distance, so large surface areas and short distances speed it up.
Small organisms
  • Unicellular organisms have a high surface area to volume ratio, so diffusion across the cell surface is enough.

Part 2 of 5Properties of Gas Exchange Surfaces

Gas exchange surfaces in different organisms share the same features.

Properties of Gas Exchange Surfaces
Large surface area
  • Many small folds or air sacs increase the area available for diffusion, as in alveoli.
Thin barrier
  • Walls are one cell thick, which keeps the diffusion distance short.
Moist surface
  • Gases dissolve in a film of moisture before they diffuse across the cell membrane.
Permeable
  • The surface must allow gases to pass freely, unlike the waterproof outer layer of most organisms.
Steep gradient
  • Ventilation and blood flow carry gases towards and away from the surface to keep a concentration gradient.

Part 3 of 5Maintaining Concentration Gradients

1
Fresh air is brought to the surface by ventilation, which replaces air that has lost oxygen.
2
Oxygen diffuses from the air in the alveoli into the blood, where it binds haemoglobin and is carried away.
3
The blood flow removes oxygen from the surface and keeps the oxygen concentration in blood low.
4
Carbon dioxide diffuses the other way, from blood to alveolar air, and is removed by exhaling.
5
Constant ventilation and circulation therefore keep a steep gradient for both gases.

Part 4 of 5Worked Example

Question

  • An exchange surface doubles in area and its thickness is halved.
  • If the concentration gradient is unchanged, by what factor does the diffusion rate change?

  1. Diffusion rate is proportional to surface area × concentration gradient ÷ diffusion distance.
  2. Doubling the area multiplies the rate by 2.
  3. Halving the distance multiplies the rate by another 2, so overall 2 × 2.

Answer The diffusion rate increases four times.

Part 5 of 5Exam Checklist & Tips

2.6.1 – Key Points for the Exam

  • Gas exchange Diffusion of oxygen into, and carbon dioxide out of, an organism across a surface.
  • Diffusion Net movement of particles from higher to lower concentration.
  • Concentration gradient A difference in concentration between two regions.
  • Ventilation Movement of air or water over a gas exchange surface.

Common Mistakes

  • Saying gas exchange is the same as breathing; breathing is ventilation, while gas exchange is diffusion.
  • Stating that oxygen moves into blood by active transport; it diffuses.
  • Forgetting to mention moisture as a property of the gas exchange surface.
  • Describing a concentration gradient as a difference in pressure of liquids rather than in particle concentration.

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