Osmosis · Grade 11
In Grade 11 you learn: water potential.
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?
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?
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?
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?
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.
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.
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?
Answer: from cell A to cell B