2.5 - Osmosis

What osmosis is

• Osmosis is the net movement of water molecules across a partially permeable membrane, 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.
• A partially permeable membrane has holes 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.
• Water molecules move both ways through the membrane because they move randomly, but overall there is a net flow from the dilute side to the concentrated side.
• Osmosis is a special case of diffusion - the diffusion of water - so, like any diffusion, it is passive and needs no energy from respiration.

Osmosis in terms of water potential

• Water potential is the potential (the likelihood) of water molecules to diffuse out of or into a solution.
• A solution with a high water potential has a high concentration of water molecules; one with a low water potential has a low concentration of water molecules.
• Pure water has the highest water potential - every solution has a lower water potential than pure water.
• So osmosis can also be described as the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
  • The dilute solution has the higher water potential.
  • The concentrated solution has the lower water potential.

The effect of osmosis on plant cells

• Whether water moves in or out of a plant cell by osmosis depends on the water concentration of the surrounding solution compared with inside the cell.
• A plant cell can end up in one of three states: turgid, flaccid, or plasmolysed.

Turgid cells - Water moves in

• When the surrounding solution has a higher water concentration than inside the cell (e.g. a well-watered plant), water moves into the cell by osmosis.
• The cell becomes plump and swollen - it is turgid.
• The water inside presses outwards against the cell wall, creating turgor pressure.
• The cell wall is inelastic (rigid), so it stops the cell expanding too far and limits how much water can enter, preventing damage.
• When all a plant's cells are turgid, the turgor pressure on their walls supports the whole plant and keeps it upright.

Flaccid cells - Water moves out

• When the surrounding solution has a lower water concentration than inside the cell, water moves out of the cell by osmosis.
• The cell loses turgor, becomes limp, and is described as flaccid.
• A plant short of water wilts and droops, though the cell walls stop it collapsing completely.

Plasmolysed cells - Even more water lost

• If even more water is lost, the cytoplasm starts to shrink and the cell membrane pulls away from the cell wall.
• A cell in this condition is said to be plasmolysed, and the process is called plasmolysis.

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.