How do substances move into and out of cells by diffusion, osmosis and active transport?
Diffusion, osmosis and active transport as ways substances cross cell membranes, the factors affecting diffusion, the effect of osmosis on cells, and the osmosis required practical.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on movement across membranes, covering diffusion and the factors affecting it, osmosis and its effect on plant and animal cells, active transport and its energy demand, and the osmosis required practical.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to define diffusion, osmosis and active transport, describe the factors affecting diffusion, explain the effect of osmosis on cells, and describe the required practical on osmosis in plant tissue.
Diffusion
Diffusion happens because particles move randomly; over time, more particles move from the crowded side to the less crowded side. Examples include oxygen and carbon dioxide moving across the alveoli during gas exchange, and glucose moving from the gut into the blood.
The rate of diffusion is faster when:
- the concentration gradient is steeper (a bigger difference between the two regions),
- the temperature is higher (particles have more kinetic energy and move faster),
- the surface area is larger (more area for particles to cross),
- the distance to travel is shorter (a thinner membrane).
Osmosis
A partially permeable membrane lets water through but not larger dissolved molecules. Water always moves towards the more concentrated solution, evening out the concentrations. Osmosis is diffusion of water, so it needs no energy.
The effect of osmosis on cells
Plant cells. In a dilute solution, water moves into the cell by osmosis. The vacuole swells and pushes the cytoplasm against the cell wall, making the cell turgid (firm), which supports the plant. In a concentrated solution, water moves out, the cell becomes flaccid, and in extreme cases the membrane pulls away from the wall, which is plasmolysis.
Animal cells. Animal cells have no cell wall, so osmosis can damage them. In a dilute solution, water moves in and the cell may swell and burst (lysis). In a concentrated solution, water moves out and the cell shrinks (crenation).
Active transport
Because it works against the gradient, active transport needs energy, which is why cells that do a lot of it (such as root hair cells) have many mitochondria. Examples include the uptake of mineral ions by root hair cells from dilute soil water, and the reabsorption of glucose in the kidney tubules.
Comparing the three processes
| Process | Moves what | Direction | Needs energy? |
|---|---|---|---|
| Diffusion | Any particle | High to low (down gradient) | No |
| Osmosis | Water only | High to low water potential | No |
| Active transport | Dissolved substances | Low to high (against gradient) | Yes |
The osmosis required practical
A common practical investigates osmosis in potato tissue. You cut potato cylinders of equal size, measure their starting mass, and place each in a different concentration of sugar or salt solution. After a set time you dry and reweigh them, then calculate the percentage change in mass:
Cylinders in dilute solutions gain mass (water enters); those in concentrated solutions lose mass (water leaves). The concentration where there is no change equals the concentration inside the potato cells.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style6 marksA student places three identical pieces of potato into salt solutions of low, medium and high concentration. Describe what happens to the mass of each piece and explain the results using osmosis.Show worked answer →
A 6-mark extended question. Mark it for clear results linked to osmosis across all three solutions. In the low concentration (dilute) solution, there is a higher water potential outside than inside the potato cells, so water moves into the cells by osmosis and the piece gains mass. In the high concentration solution, there is a lower water potential outside, so water moves out of the cells by osmosis and the piece loses mass. In the medium solution, if the concentration matches the cells, there is no net movement and the mass stays about the same. A top answer states the direction of water movement in each case, links it to the difference in water potential (or concentration), and connects each movement to the change in mass.
WJEC style3 marksExplain why active transport, but not diffusion, requires energy from respiration.Show worked answer →
A 3-mark explanation comparing the two processes. Diffusion moves particles down a concentration gradient, from high to low concentration, so it does not need an input of energy. Active transport moves substances against the concentration gradient, from low to high concentration, so it needs energy released by respiration to do this work. Markers reward: diffusion is down the gradient and needs no energy; active transport is against the gradient; and it uses energy from respiration. Saying active transport "uses energy" without the idea of moving against the gradient does not fully explain why.
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