Substances need to move around the body of an organism – obviously. To do this they must cross biological membranes. There are a number of ways this can happen, including: passive transport, active processes and bulk transport.

An example of a passive transport is osmosis. Osmosis involves the diffusion of water molecules.

Osmosis is defined as the passive net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane.

At A-level rather than use the word concentration, we use the term water potential. Water moves from a higher water potential to a lower water potential by osmosis.

Pure water has the highest water potential. Because of osmosis if a cell was placed in pure water it would burst due to water moving into the cell.

Adding any soluble molecule to pure water will decrease its water potential. For instance sugar. However starch would have no impact as it is insoluble in water.

If a solution has the same water potential as the cell it is an isotonic solution, this means there is no net movement of water molecules.

If a cell is placed in a hypotonic solution water will move into the cell. Hypotonic solutions have a higher water potential than the cell in them. We know that water moves from a higher water potential to a lower water potential and so it will cause the cell to burst as it moves into the cell.

A hypertonic solution has a lower water potential as it contains a higher concentration of solutes. (hyper = high). This means that water will move out of the cell via osmosis.

Quick summary – a passive process uses no extra energy and diffuses particles down a concentration gradient and osmosis is the diffusion of water particles.

Plants use osmosis to move water into their roots, basically if you’re ever reading about the movement of water, always think about osmosis.