Factors that Influence Diffusion (Cambridge International IGCSE Biology)

Spec point — 3.1.2 · Investigate the factors that influence diffusion, limited to: surface area, temperature, concentration gradient and distance

Factors that influence diffusion

Surface area to volume ratio

• The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

  • Bigger structures have proportionally less surface area relative to their volume, which limits diffusion efficiency

• Many cells which are adapted for diffusion have increased surface area in some way, for example:

  • root hair cells in plants (which absorb water and mineral ions)
  • cells lining the ileum in animals (which absorb the products of digestion)

*Cell adaptations for diffusion *

The highly folded surface of the small intestine increases its surface area

Distance

• The smaller the distance molecules have to travel, the higher the rate at which diffusion can occur
• This is why blood capillaries and alveoli have walls which are only one cell thick, to ensure the rate of diffusion across them is as high as possible

Temperature

• The higher the temperature, the faster molecules move as they have more kinetic energy
• There is therefore a faster net rate of diffusion of molecules from a high to low concentration at higher temperatures

Concentration gradient

• The greater the difference in concentration on either side of the membrane, the greater the rate of diffusion
• This is because there are more molecules on the higher-concentration side, so more molecules move from high to low concentration by random motion than move from low to high - giving a faster net rate of diffusion in that direction

Exam Hint You should have carried out investigations into the factors that influence the rate of diffusion and as so should be able to use the information above to explain experimental results in an exam. You should also be able to plan and carry out an experiment which can investigate the effect of one of these factors.