Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. It is a passive process that does not need energy from respiration. Cell membranes are partially permeable, so cells gain or lose water by osmosis when the surrounding solution is more dilute or more concentrated than the cytoplasm.
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. A partially permeable membrane lets small molecules such as water through, but not larger molecules.
Osmosis is a passive process. It does not need energy from respiration. Cell membranes are partially permeable, so cells gain or lose water by osmosis when the solution around them is more dilute or more concentrated than the cytoplasm.
Osmosis is only about water. Do not write that 'particles' move by osmosis, and do not forget the partially permeable membrane.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is a special type of diffusion that involves only water moving through a partially permeable membrane. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, from a higher to a lower concentration. In osmosis, water moves from a dilute solution to a more concentrated solution. Neither process needs energy from respiration.
Why does osmosis not require energy?
Osmosis does not require energy because it is a passive process. Water simply diffuses from a dilute solution to a more concentrated solution through a partially permeable membrane, so no energy from respiration is used. Only active transport, which moves substances against a concentration gradient, needs energy from respiration.
Why is active transport needed in plant roots?
Active transport is needed in plant roots because root hair cells often contain a higher concentration of mineral ions than the soil. Diffusion would move ions out of the cell, so the cells must use energy from respiration to move ions in against the concentration gradient. This lets plants take in ions that are in short supply.
(opens full size in a new tab)