1.10 - Osmosis

What osmosis is

• Osmosis is the net movement of water molecules across a partially permeable (also called selectively permeable) membrane.
• Water moves from a region of higher water concentration to a region of lower water concentration - i.e. from a more dilute solution to a more concentrated one.
• The same thing stated in terms of the dissolved solute: water moves from a lower solute concentration to a higher solute concentration.
• Osmosis is a special case of diffusion - the diffusion of water - so, like any diffusion, it is passive and needs no energy from respiration.

Partially permeable membranes

• A partially permeable membrane has very small holes in it - so small that only tiny molecules such as water can pass through, while bigger molecules such as sucrose cannot.
• A cell membrane is a partially permeable membrane.

The net flow of water

• Water molecules actually pass both ways through the membrane, because they move about randomly all the time.
• Overall there is a steady net flow from the dilute side (many water molecules) to the concentrated side (fewer water molecules).
• The effect is to even up the concentrations on the two sides: the more concentrated solution gradually becomes more dilute.

Osmosis in a potato

• Osmosis can be shown using pieces of plant tissue, such as cylinders cut from a potato, placed in sugar solutions of different concentrations.
• The cell membranes of potato cells are partially permeable, so water moves into or out of the cells by osmosis.
• The direction of movement depends on how the concentration of the surrounding solution compares with the concentration inside the cells.

How the cylinder responds to each solution

• More dilute solution (higher water concentration outside) - Water is drawn into the cells, so the cylinder gains mass and swells slightly.
• More concentrated solution (lower water concentration outside) - Water is drawn out of the cells, so the cylinder loses mass and shrinks slightly.
• Same concentration as the cells - The cylinder stays roughly the same, because there is no net movement of water.

The change in mass is a direct measure of which way, and how strongly, osmosis has moved the water.