Osmosis

Where it happens

Osmosis happens across cell membranes, which are partially permeable, they let water through but not larger dissolved molecules. It controls the water content of cells.

In the body, osmosis moves water into root hair cells from soil water, from cell to cell across the root cortex, out of the kidney tubules back into the blood, and from the gut into the villi. In the laboratory the membrane is often visking tubing, which behaves like a cell membrane because its pores let water molecules through but hold back sucrose or starch.

Inputs and outputs

• Input: two solutions with different water potential, one hypotonic (more dilute) and one hypertonic (more concentrated) relative to the other.
• Input: a partially permeable membrane between them, such as the cell membrane or visking tubing.
• Input: the random kinetic energy of water molecules; no ATP is used.
• Output: a net flow of water into the hypertonic side until the two sides are isotonic or a wall stops further entry.
• Output in animal cells: swelling and haemolysis in a hypotonic solution, or crenation in a hypertonic one.
• Output in plant cells: turgor pressure in a hypotonic solution, or plasmolysis in a hypertonic one.

The steps

1. Two solutions of different concentration are separated by a partially permeable membrane.
2. The dilute side has a higher water potential, because a larger share of its molecules are free water molecules; the concentrated side has a lower water potential.
3. Water molecules move randomly and cross the membrane in both directions, while the larger solute molecules cannot pass.
4. More water molecules strike and cross the membrane from the dilute side than from the concentrated side, because there are more of them there.
5. The result is a net movement of water down the water potential gradient into the more concentrated solution.
6. The volume of the concentrated side increases and its concentration falls, while the dilute side loses volume.
7. Movement continues until the two sides are isotonic, or until pressure from a cell wall balances the inflow, at which point net movement stops.

Why it matters and how it is controlled

Osmosis controls whether cells gain or lose water. In a dilute solution animal cells swell and may burst, while plant cells become turgid; in a concentrated solution animal cells shrink and plant cells plasmolyse.

Osmosis is controlled by the water potential of the surroundings and by the cell's own structure. Animal cells have no wall, so the body keeps blood plasma isotonic to the cells through osmoregulation; if plasma became hypotonic, red blood cells would swell and burst.

Plant cells rely on the cell wall: as water enters, the wall pushes back with a pressure that rises until it stops further inflow, so the cell becomes turgid but does not burst. Turgor holds leaves and young stems upright, opens stomata, and drives the growth of plant cells, while loss of turgor causes wilting.

How it is examined

You may be asked to define osmosis, to predict whether a cell gains or loses water, or to explain a visking-tubing or potato-strip experiment.

Definition questions expect the phrases 'net movement of water', 'partially permeable membrane' and 'from a region of higher water potential to a region of lower water potential'. Practical-style questions give the mass change of potato strips in a series of sucrose solutions and ask you to plot a graph, find the concentration at which there is no mass change, and explain each region of the graph.

Diagram questions show a red blood cell or an onion epidermal cell in an unlabelled solution and ask you to identify whether the solution is hypotonic, isotonic or hypertonic and to name the state of the cell.

Common misconceptions

Worked exam-style question

Question. A student cut five potato cylinders of equal length, weighed each one, and placed them in sucrose solutions of 0.0, 0.2, 0.4, 0.6 and 0.8 mol per litre for thirty minutes. The cylinders in the 0.0 and 0.2 solutions gained mass, the cylinder in the 0.4 solution showed no change, and the cylinders in the 0.6 and 0.8 solutions lost mass and felt soft.

(a) Explain why the cylinder in the 0.0 solution gained mass. (b) State what the 0.4 result tells you about the potato cells.

(c) Explain why the cylinders in the 0.8 solution felt soft. (d) Suggest why the student blotted each cylinder before weighing it.

Model answer. (a) Distilled water has a higher water potential than the potato cell sap, so water enters the cells by osmosis across the partially permeable cell membrane; the cells become turgid and the cylinder gains mass. (b) The 0.4 solution is isotonic to the cell sap: there is no net movement of water, so the sap has the same water potential as a 0.4 mol per litre sucrose solution.

(c) The 0.8 solution is hypertonic, so water leaves the cells by osmosis; the cells lose turgor pressure, become flaccid and may plasmolyse, so the tissue is soft. (d) Blotting removes surface water so that the mass measured reflects only the water that has moved into or out of the cells, making the results a fair comparison.

Source:SRC-DSKP-EN

Frequently asked questions