Water potential determines the direction of osmosis

1. Water potential describes the tendency of water molecules to move from one region to another.
2. A dilute solution has a higher water potential because it contains more free water molecules relative to dissolved solute.
3. A concentrated solution has a lower water potential because dissolved solute reduces the proportion of free water molecules.
4. Adding water raises water potential, while adding dissolved salts or sugars lowers water potential.
5. No numerical calculation of water potential is required here, but the direction of water movement must be predicted correctly.

Osmosis changes animal cell volume

1. Osmosis is the net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane.
2. The cell-surface membrane is partially permeable because water can cross it more readily than many dissolved substances.
3. Water moves in both directions, but the net movement is towards the side with lower water potential.
4. The movement continues until the water potentials become equal or another force prevents further net movement.

Dilute body fluid can cause cell lysis

1. If the body fluid outside an animal cell has a higher water potential than the cytoplasm, water enters the cell by osmosis.
2. The cytoplasm increases in volume and pushes the cell-surface membrane outwards.
3. Animal cells have no rigid cell wall, so a sufficiently large water gain can stretch the membrane until the cell bursts.
4. Bursting caused by water entry is called lysis.
5. Red blood cells placed in a very dilute solution are commonly used to illustrate this response.

Concentrated body fluid causes cells to shrink

1. If the body fluid outside a cell has a lower water potential than the cytoplasm, water leaves the cell by osmosis.
2. The cytoplasm loses volume and the cell-surface membrane becomes wrinkled as the cell shrinks.
3. A shrunken animal cell cannot function normally because its internal concentrations and membrane shape have changed.
4. Red blood cells become crenated when they lose water in a concentrated solution.

Equal water potentials prevent net change

1. If the body fluid and cytoplasm have equal water potentials, water still crosses the membrane in both directions.
2. The rates of movement are equal, so there is no net movement of water.
3. The cell therefore keeps approximately the same volume and shape.
4. Maintaining the water potential of body fluids within narrow limits protects cells from lysis and excessive shrinking.