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

Factors that Influence Diffusion

• The movement of substances like oxygen and carbon dioxide into and out of cells happens mainly by diffusion

• The rate of diffusion in organisms depends on several factors, including surface area to volume ratio, distance, temperature and concentration gradient

Surface area to volume ratio

• Surface area refers to the total area of the surface through which substances can diffuse

• As a cell or organism increases in size, its surface area to volume ratio decreases

  • This means there is less surface area available for each unit of volume, so diffusion alone becomes less efficient at supplying cells with essential substances and removing waste products

  • To overcome this, larger organisms have evolved specialised exchange surfaces (such as alveoli in the lungs or villi in the small intestine) that increase surface area and often have thin walls and good blood supply to maintain a steep concentration gradient for efficient diffusion

• In addition, 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

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:

Calculating the surface area to volume ratio

Diffusion distance

• The shorter the distance that molecules have to travel, the faster diffusion can occur

  • This is why alveoli in the lungs and capillary walls are only one cell thick — to minimise the diffusion distance for gases

  • A shorter diffusion distance allows oxygen and carbon dioxide to diffuse rapidly and efficiently between the air in the alveoli and the blood in the capillaries

Temperature

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