Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It is passive, so it needs no energy from respiration. Oxygen and carbon dioxide diffuse through cell membranes. The rate is higher with a steeper concentration gradient, a higher temperature and a larger surface area.
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. The particles move down a concentration gradient. It is a passive process, so it does not need energy from respiration.
Oxygen, carbon dioxide and dissolved substances can diffuse into and out of cells through the cell membrane. The rate of diffusion is higher when:
the concentration gradient is steeper, because there is a bigger difference in concentration
the temperature is higher, because particles have more kinetic energy and move faster
the surface area is larger, because more particles can cross at once
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is the diffusion of water only, through a partially permeable membrane, from a dilute solution to a more concentrated solution. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, from a higher to a lower concentration. Both are passive processes, so neither needs energy from respiration.
Why does osmosis not require energy?
Osmosis does not require energy because it is a passive process. Water diffuses from a dilute solution to a more concentrated solution through a partially permeable membrane, so no energy from respiration is needed. Active transport is different: it moves substances against a concentration gradient, so it does need energy from respiration.
Why is active transport important in root hair cells?
Active transport lets root hair cells take in mineral ions from the soil even when the concentration of ions is higher inside the cell than in the soil. This movement is against the concentration gradient, so diffusion cannot do it. Active transport uses energy from respiration to move the ions into the cell.
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