Diffusion & Osmosis

Diffusion is the process by which substances move down a concentration gradient, from an area of high concentration to an area of low concentration. Diffusion happens in living systems, for example, it explains the movement of carbon dioxide in leaves.

Diffusion is a process in which material spreads throughout a liquid or a gas. It is the overall movement of a substance from a region of high to low concentration. It happens because of the random movement of individual particles. Diffusion is important to living things as it explains how useful materials and waste products can move into and out of cells. Diffusion gives the effect that the substance is moving from an area of high concentration to an area of low concentration (or 'down a concentration gradient').

The rate of diffusion is increased when:

• The distance is decreased
• The surface area is increased
• The concentration difference (concentration gradient) is increased

The Leaf

Materials go into and out of cells by diffusion through the cell membrane. These materials include oxygen, carbon dioxide and glucose. When a leaf cell is photosynthesising:

• The concentration of carbon dioxide outside the cell is greater than the concentration inside, so carbon dioxide diffuses into the cell
• The concentration of oxygen inside the cell is greater than the concentration inside, so oxygen diffuses out of the cell.

Osmosis is a type of diffusion that involves the random movement of water molecules. It is the movement of water across a partially-permeable membrane, from an area of high water concentration to an area of low water concentration.

Partially-Permeable

A partially-permeable membrane is a membrane that allows small particles such as water molecules through it, but not larger particles such as sugar molecules and ions from salts.

Water Concentration

A dilute solution has a high water concentration, while a concentrated solution has a low water concentration. For example, when salt is dissolved in water:

• A little dissolved salt produces a dilute solution with a high water concentration
• A lot of dissolved salt produces a concentrated solution with a low water concentration.