Diffusion

Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)

What is Diffusion?

Diffusion is one of the most important transport processes in living organisms. It requires no energy input from the cell.

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement
The energy for diffusion comes from the kinetic energy of random movement of molecules and ions - no energy is needed from the cell
Particles move randomly in all directions, but the net (overall) movement is always down the concentration gradient
Diffusion continues until equilibrium is reached (equal concentrations on both sides); particles keep moving, but with no further net change
Some substances move into and out of cells by diffusion through the cell membrane

Particles move randomly, but the net movement is always from high to low concentration until equilibrium is reached

Diffusion of gases and solutes is essential to many vital processes:

Oxygen diffuses from the alveoli into the blood capillaries in the lungs, supplying O₂ for aerobic respiration in all body cells
Carbon dioxide diffuses from respiring cells into the blood, and from the blood capillaries into the alveoli, removing this waste product of respiration from the body
Glucose and amino acids diffuse from the small intestine into the blood capillaries after digestion, delivering nutrients to cells for respiration, growth and repair
Carbon dioxide diffuses from the air into leaf mesophyll cells through the stomata, providing the carbon source for photosynthesis
Oxygen diffuses out of leaf mesophyll cells through the stomata, removing the excess O₂ produced during photosynthesis
Urea diffuses from liver cells into the blood plasma, carrying this metabolic waste to the kidneys for excretion

Four factors influence how quickly diffusion occurs. For each, you must be able to describe the effect and explain the reason.

Concentration gradient: a steeper gradient gives faster diffusion
- A greater difference in concentration means more particles move from the high-concentration side to the low-concentration side per unit time, so net movement is greater
Surface area: a larger surface area gives faster diffusion
- More area allows more particles to cross the membrane at the same time, so more particles diffuse per unit time
Temperature: a higher temperature gives faster diffusion
- Higher temperature gives particles more kinetic energy, so they move faster and cross the membrane more frequently
Distance (diffusion pathway / membrane thickness): a shorter distance gives faster diffusion
- Particles take less time to travel across a thinner membrane or a shorter pathway

A steeper concentration gradient, larger surface area, higher temperature and shorter distance all increase the rate of diffusion

❌ Thinking large organisms rely on diffusion alone, in the same way small organisms do
- ✓ small organisms have a large surface area to volume ratio, so diffusion alone is fast enough to supply their cells. Large organisms have a much smaller surface area relative to their volume, so they need specialised transport systems (e.g. a circulatory system) to move substances to cells far from the surface

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