The Process of Osmosis
- All cells are surrounded by a cell membrane which is selectively permeable
- Water can move in and out of cells by osmosis
- Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a selectively permeable membrane
- In doing this, water is moving down its concentration gradient
- The cell membrane is selectively permeable which means it allows certain molecules (like water) through the bilayer
- Larger molecules (like solute molecules) require channel or carrier proteins, which may or may not exist in a certain piece of membrane
Osmosis Diagram
Osmosis and the selectively permeable membrane
- Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a selectively permeable membrane
- Water potential describes the tendency of water to move out of a solution
- This term is used to avoid confusion between water concentration and concentration of a solute
- A dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below)
How Osmosis Works Diagram
How osmosis works
- The water potential of pure water (without any solutes) at atmospheric pressure is 0 kPa, therefore any solution that has solutes will have a water potential lower than 0 kPa (it will be a negative value)
Examiner Tip
Water can pass through the phospholipid bilayer because water molecules are small molecules that can pass between phospholipids in the cell membrane. Although water molecules are polar, they can still pass through the bilayer because of their small size.When interpreting questions on water potential, remember – the more negative the water potential, the lower the water potential (the further it is away from pure water which has a water potential of 0 kPa).
