Rate of diffusion and Fick's law

Overview

Surface area, concentration gradient and diffusion distance, the villi and scarred lungs as examples, and Fick's law calculations of relative rate from area, difference and thickness.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Name the three factors that affect how quickly a substance diffuses across a surface.

   Model answer

   The surface area, the concentration gradient and the diffusion distance all affect the rate.

2. 2

   State the effect of increasing the surface area on the rate of diffusion.

   Model answer

   A larger surface area increases the rate of diffusion.

3. 3

   Explain why a steeper concentration gradient speeds up diffusion.

   Model answer

   A bigger difference in concentration means more particles move from the more concentrated side each second, so net movement is faster.

4. 4

   State the effect of a thicker exchange surface on the rate of diffusion.

   Model answer

   A thicker surface means a longer diffusion distance, so the rate of diffusion falls.

5. 5

   Suggest why the lining of the small intestine is folded into millions of villi.

   Model answer

   The folding gives a very large surface area, so digested food is absorbed much faster.

6. 6

   Suggest why someone whose lung tissue has become scarred and thickened absorbs oxygen more slowly.

   Model answer

   The thickened wall lengthens the diffusion distance and the damage destroys alveoli, reducing the surface area.

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