Transport into and out of cells

• Some substances move across cell membranes via diffusion. Diffusion is a spreading out and mixing process. Particles move from a region where they are in higher concentration to a region where their concentration is lower.
• Factors that affect the rate of diffusion across a membrane are: • the difference in concentration • the temperature • the surface area of the membrane.
• Water may move across cell membranes by osmosis. Cell membranes are partially permeable: they allow small molecules such as water through but not larger molecules. During osmosis water diffuses from where it is more concentrated (because the solute concentration is lower), through a partially permeable membrane to where water is less concentrated (because the solute concentration is higher).
• Some substances move across cell membranes via active transport. Active transport involves the movement of a dissolved substance from a region where it is less concentrated to a region where it is more concentrated. This requires energy from respiration.
• Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood with a higher sugar concentration.