Cells

Diffusion - is the spreading out and mixing of particles of any substance, in solution or gas, resulting in a net movement from a region of higher concentration to a region of lower concentration, down a concentration gradient.

The rate of diffusion is affected by the difference in concentrations, the temperature, and the available surface area.

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 through a partially permeable membrane to where it is less concentrated.

Higher concentration to lower concentration through a semi permeable membrane.

Plants:

Osmosis supports stems and leaves, turgor pressure makes cells hard and rigid.

If more water is lost by osmosis the vacuole and cytoplasm shrink and the cell membrane pulls away from the cell wall (plasmolysis). Plasmolysed cells die quickly unless the osmotic balance is restored.

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 plant root hairs to absorb mineral ions required for healthy growth from very dilute solutions in the soil against a concentration gradient.

Active transport enables sugar molecules used for cell respiration to be absorbed from lower concentrations in the gut into the blood where the concentration of the sugar is higher.