GCSE Biology: Osmosis

Osmosis Membrane


The water, represented above by the red dots can flow freely through the membrant and will diffuse until there is the same concentration on each side of the membrane. The starch will stay on the same side, it is too large to go through the membrane. This is shown by the large green particles on the above diagram. The sugar (white) molecules will pass through the membrane more slowly. When the concentration of sugar is equal the net movement will stop. The sugar molecules may move, but the balance will stay the same and so the concentration gradient is flat. Osmosis is the net movement of water molecules from an area of high concentration to an area of low concentration down a concentration gradient across a partially permiable membrane.

In pure water an animal cell swells as water diffuses into the cell via the semi permeable membrane. This is because there is a lower concentration of water inside the cell than there is outside of the cell. The cell may burst. When an animal cell is placed in a concentrated solution the cell shrinks as the water diffuses out of it. This is because there is a lower concentration of water outside the cell, and so the water molecules diffuse out of the cell through the semi permeable membrane via osmosis.

A Plant cell in pure water swells and becomed turgid. This is because water diffuses into the cytoplasm and vacuole through the selectively permeable cell membrane. If a plant cell is placed in a concentrated solution osmosis takes place and water diffuses out of the cytoplasm and vacuole. Firstly the cell shrinks slightly, and becomes flacid. Then the cell membrane pulls away from the cell wall, and the cell is plasmolysed.