Factors Affecting the Rate of Movement of Substances (Edexcel IGCSE Biology)

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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

  • Many cells which are adapted for diffusion have increased surface area in some way - e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

Cell Adaptations for Diffusion, IGCSE & GCSE Biology revision notes

Cell adaptations for diffusion

The highly folded surface of the small intestine increases its surface area, IGCSE & GCSE Biology revision notes

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

  • You should be able to calculate and compare surface area to volume ratios

  • You can model the effect of how increasing size affects surface area to volume ratio using simple cubes:

SA V ratio calculation, downloadable IGCSE & GCSE Biology revision notes

Calculating the surface area to volume ratio

Diffusion distance

  • The smaller the distance molecules have to travel the faster transport will occur

  • This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible

Temperature

  • The higher the temperature, the faster molecules move as they have more energy

  • This results in more collisions against the cell membrane and therefore a faster rate of movement across them

Concentration gradient

  • The greater the difference in concentration on either side of the membrane, the faster movement across it will occur

  • This is because on the side with the higher concentration, more random collisions against the membrane will occur

Summary of Diffusion Factors Table

Factor

How it affects diffusion

Concentration gradient

The greater the difference in concentration between two regions, the faster the overall rate of diffusion

Temperature

The higher the temperature, the more kinetic (movement) energy the particles of that substance will have

They will move/spread faster, compared to at a lower temperature when they have less kinetic energy

Surface area

A membrane with a greater surface area will have a greater rate of diffusion across it

Examiner Tips and Tricks

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.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.