Osmosis

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. Cells gain water in a more dilute solution and lose it in a more concentrated one. Plant cells swell and become firm, while animal cells can burst or shrivel, and pieces of potato show it as a change in mass.

Key facts

  • Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
  • Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100. Plant tissue gains mass in a dilute solution and loses mass in a concentrated one.
  • In pure water a plant cell swells and becomes firm (turgid), but its cell wall stops it bursting; an animal cell can burst.

Worked examples

Example 1

What is osmosis?

  1. Osmosis: the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.

Answer: the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane

Example 2

What happens to a plant cell placed in pure water?

  1. Pure water is more dilute than the cytoplasm, so water moves in by osmosis.
  2. So it swells and becomes firm (turgid), but the cell wall stops it bursting.

Answer: it swells and becomes firm (turgid), but the cell wall stops it bursting

Example 3

What happens to a red blood cell placed in a strong salt solution?

  1. The solution is more concentrated than the cytoplasm, so water moves out by osmosis.
  2. So it loses water and shrivels.

Answer: it loses water and shrivels

Example 4

A piece of potato lost mass after an hour in a sugar solution. Which explanation is right?

  1. The potato lost mass.
  2. So the solution was more concentrated than the cytoplasm, so water moved out of the cells by osmosis.

Answer: the solution was more concentrated than the cytoplasm, so water moved out of the cells by osmosis

Example 5

A potato cylinder had a mass of 2.50 g. After 30 minutes in a sugar solution its mass was 2.20 g. Calculate the percentage change in mass, and explain it.

  1. Change in mass: 2.20 − 2.50 = −0.30 g
  2. Divide by the starting mass and multiply by 100. −0.30 ÷ 2.50 × 100 = −12%
  3. The mass went down, so water moved out of the cells by osmosis.
  4. The sugar solution was more concentrated than the cytoplasm of the potato cells.

Answer: −12%: water left the cells by osmosis, because the solution was more concentrated than the cytoplasm

Example 6

Potato cylinders were left in sugar solutions of different concentrations. Use the graph to estimate the concentration of the solution inside the potato cells.

  1. Above the axis the potato gained mass; below it, it lost mass.
  2. Where the change in mass is 0%, there is no net movement of water.
  3. So the solution outside has the same concentration as the cytoplasm.
  4. The line crosses 0% at about 0.3 mol/dm³.

Answer: about 0.3 mol/dm³

Example 7

Cell A has a water potential of −500 kPa and cell B has a water potential of −600 kPa. Which way does water move by osmosis?

  1. −500 kPa is higher (less negative) than −600 kPa.
  2. Water moves down the water potential gradient: from cell A to cell B.

Answer: from cell A to cell B