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2.1.1Unit 2: Cells, Development, Biodiversity and Conservation

Properties of Gas Exchange Surfaces — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 26 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5Surface Area to Volume Ratio

2.1.1 Properties of Gas Exchange Surfaces

The rate of gas exchange depends on the size of the exchange surface relative to the volume of the organism.

Gas exchange
  • All aerobic organisms must take in oxygen and remove carbon dioxide across an exchange surface by diffusion.
Small organisms
  • They have a large surface area compared with their volume, so diffusion across the body surface is sufficient.
Large organisms
  • Their ratio is small and the diffusion distance to inner cells is large, so they need specialised exchange surfaces.
Increasing area
  • Alveoli in lungs, gill lamellae in fish and mesophyll air spaces in leaves greatly increase the surface area available.

Part 2 of 5Features of an Efficient Surface

Features of an Efficient Surface
Large surface area
  • A large area lets many molecules cross at the same time, speeding up the rate of diffusion.
Short diffusion pathway
  • The surface is often one cell thick and flattened, so gases only need to travel a very small distance.
Steep concentration gradient
  • Ventilation and blood flow constantly replace the gas on each side of the surface, so a high gradient is maintained.
Permeability
  • The membranes are thin and permeable to the respiratory gases, and are kept moist so gases can dissolve.

Part 3 of 5Fick’s Law and Examples

1
Fick’s law states that the rate of diffusion is proportional to surface area × concentration difference ÷ diffusion distance.
2
In the lungs, the walls of the alveoli and capillaries are each one cell thick, giving a very short diffusion distance.
3
Breathing movements refresh the air in the alveoli, while the circulation carries oxygen away and brings carbon dioxide in.
4
In fish, blood flows in the opposite direction to water over the gills, which keeps a gradient along the whole lamella.
5
In plants, stomata open to allow carbon dioxide to diffuse into the leaf and into the air spaces around the mesophyll cells.

Part 4 of 5Worked Example

Question Compare the surface area to volume ratio of a cube of side 2 cm with one of side 4 cm.

  1. Cube of side 2 cm: surface area = 6 × 2² = 24 cm²; volume = 2³ = 8 cm³; ratio = 24 ÷ 8 = 3 : 1.
  2. Cube of side 4 cm: surface area = 6 × 4² = 96 cm²; volume = 4³ = 64 cm³; ratio = 96 ÷ 64 = 1.5 : 1.
  3. Doubling the side halves the ratio.

Answer The 2 cm cube has a ratio of 3 : 1 and the 4 cm cube 1.5 : 1, so the ratio falls as size increases.

Part 5 of 5Exam Checklist & Tips

2.1.1 – Key Points for the Exam

  • Gas exchange surface A thin surface across which gases diffuse between an organism and its environment.
  • Surface area to volume ratio Surface area divided by volume.
  • Diffusion pathway The distance gases must diffuse across.
  • Concentration gradient A difference in concentration between two regions.

Common Mistakes

  • Saying that larger organisms have a higher surface area to volume ratio.
  • Writing that a thicker exchange surface increases the rate of diffusion.
  • Forgetting that the gradient must be maintained by ventilation and circulation.
  • Stating that gases are actively transported across exchange surfaces; they diffuse.

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