Diffusion (Edexcel GCSE Biology: Combined Science)

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration
• Molecules move down a concentration gradient, as a result of their random movement

Diffusion across the cell membrane

Diffusion in living organisms

• For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane, which can restrict the free movement of the molecules
• The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
  • The simplest sort of selection is based on the size of the molecules (i.e. smaller molecules can diffuse across the membrane but larger molecules cannot)

• Diffusion helps living organisms to:
  • Obtain many of their requirements
  • Get rid of many of their waste products
  • Carry out gas exchange for respiration

Examples of diffusion in living organisms

• You will need to learn examples of substances that organisms obtain by 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

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 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