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How do substances move into and out of cells, and which processes need energy?

Diffusion, osmosis and active transport as ways substances move across cell membranes, the factors affecting the rate of diffusion, the effect of osmosis on plant and animal cells, and calculating percentage change in mass in the osmosis practical.

A focused answer to the OCR Gateway GCSE Biology A topic B2 on diffusion, osmosis and active transport, covering the definitions, the factors affecting diffusion, the effect of osmosis on cells, the osmosis practical and percentage change calculations.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Diffusion
  3. Osmosis
  4. Active transport
  5. The osmosis practical (PAG B3)

What this dot point is asking

OCR wants you to define diffusion, osmosis and active transport, state which need energy, describe the factors affecting the rate of diffusion, explain the effect of osmosis on plant and animal cells, and carry out the percentage change calculation from the osmosis practical.

Diffusion

Examples in living things include oxygen and carbon dioxide moving in and out of cells during gas exchange, and urea diffusing from cells into the blood.

The rate of diffusion is faster when:

  • The surface area is larger (more area for particles to cross).
  • The concentration difference (gradient) is bigger.
  • The distance the particles travel is shorter (a thinner membrane).
  • (Also faster at higher temperature, because particles move faster.)

Osmosis

Osmosis affects cells:

  • A plant cell in a dilute solution takes in water by osmosis and becomes firm (turgid), supported by its cell wall. In a concentrated solution it loses water and becomes flaccid, and may plasmolyse (the membrane pulls away from the wall).
  • An animal cell has no cell wall. In a dilute solution it takes in water and may burst (lyse); in a concentrated solution it loses water and shrinks (crenates).

Active transport

Examples include the uptake of mineral ions by root hair cells (where the soil has a lower concentration than the cell) and the absorption of glucose from the gut into the blood when the gut concentration is lower.

The osmosis practical (PAG B3)

A common required practical investigates osmosis using potato cylinders. You cut cylinders of the same size, measure their starting mass, and place them in sugar (or salt) solutions of different concentrations for a set time. You then dry and reweigh them and calculate the percentage change in mass. Cylinders that gained mass took in water (the solution was more dilute than the cells); cylinders that lost mass lost water (the solution was more concentrated). The point where there is no change shows the concentration that matches the cell contents.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksCompare diffusion, osmosis and active transport. In your answer state, for each, what moves, the direction relative to the concentration gradient, and whether energy is needed.
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A 4-mark Compare question, so make paired points across all three.

Diffusion: the net movement of any particles from a higher to a lower concentration (down the gradient); no energy needed (passive).

Osmosis: the diffusion of water molecules across a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated solution (low water concentration); no energy needed (passive).

Active transport: the movement of substances from a lower to a higher concentration (against the gradient); requires energy from respiration and carrier proteins.

Markers reward the three processes correctly distinguished by what moves, direction and energy. A strong answer gives an example of each (oxygen by diffusion, water by osmosis, mineral ions into root hairs by active transport).

OCR 20214 marksPotato cylinders were placed in sugar solutions. One cylinder had a mass of 5.0 g at the start and 4.2 g at the end. Calculate the percentage change in mass, and explain what this result shows about the solution compared with the cells.
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A 4-mark calculation and explanation from the osmosis practical.

Calculation: percentage change =change in massoriginal mass×100=4.2−5.05.0×100=−0.85.0×100=−16%= \dfrac{\text{change in mass}}{\text{original mass}} \times 100 = \dfrac{4.2 - 5.0}{5.0} \times 100 = \dfrac{-0.8}{5.0} \times 100 = -16\%. Reward the negative sign showing a loss of mass.

Explanation: the cylinder lost mass, so water moved out of the potato cells by osmosis. This means the sugar solution was more concentrated than the cell contents (lower water concentration outside), so water moved from the cells (higher water concentration) to the solution. Markers reward the direction of water movement and the conclusion that the solution was more concentrated than the cells.

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