Diffusion, osmosis and active transport · Year 9
Cells must take in substances such as oxygen, glucose, water and mineral ions, and get rid of waste such as carbon dioxide and urea. These move across the cell membrane in three ways: diffusion, osmosis and active transport. The bigger an organism is, the smaller its surface area is compared with its volume, so large organisms need special exchange surfaces.
Key facts
- Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It needs no energy. Oxygen and carbon dioxide diffuse in the lungs, and urea diffuses from cells into the blood.
- Diffusion is faster with a steeper concentration gradient, a higher temperature and a larger surface area of membrane.
- Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
- Active transport moves substances from a more dilute solution to a more concentrated solution, against the concentration gradient. It needs energy from respiration. Root hair cells take in mineral ions this way, and the gut absorbs sugar into the blood.
- For a cube of side L, surface area = 6L² and volume = L³. Small organisms have a large surface area to volume ratio, so diffusion across their surface is enough; large organisms need exchange surfaces and transport systems.
- A good exchange surface has a large surface area, a thin membrane (a short diffusion path), a good blood supply and, in animals, ventilation. Examples are the alveoli, the villi, fish gills and leaves.
- Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100. Plant tissue gains mass in a dilute solution and loses mass in a concentrated one.