Revision Notes

3.1 Diffusion

Diffusion in Living Organisms

  • 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, IGCSE & GCSE Biology revision notesDiffusion across the cell membrane


  • 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
  • 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
  • Don’t forget that plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature)

Brownian Motion

  • All particles move randomly at all times
  • This is known as Brownian motion
  • The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

Brownian motion, IGCSE & GCSE Biology revision notesBrownian motion

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 – eg 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 notesCell adaptations for diffusion


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


Three factors that affect the rate of diffusion and therefore the movement of molecules through membranes:


  • 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


  • 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 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

Exam Tip

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.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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