Contents: 8 sections
Cambridge IGCSE Biology 0610 · Core and Extended
Syllabus points
- Describe diffusion and explain its importance in living organisms.
- Describe osmosis in terms of water potential and a partially permeable membrane.
- Investigate and explain the effects of solutions of different concentrations on plant tissue and on animal cells.
- Describe active transport and explain its importance.
- State the factors that affect the rate of movement of substances.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion.
Three points carry most of the marks.
- It is passive. No energy from respiration is needed; the particles are moving anyway.
- It is net movement. Particles cross in both directions the whole time, and diffusion is the difference between the two flows. When concentrations are equal the movement continues but the net movement is zero.
- It happens in gases and in solution, so both oxygen entering a leaf and glucose crossing the gut wall are diffusion.
Examples worth having ready: oxygen and carbon dioxide across the alveoli, carbon dioxide into a leaf through the stomata, and digested food from the small intestine into the blood.
Osmosis
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Water potential is simply how much water is present relative to solute. A dilute solution has lots of water, so a high water potential. A concentrated solution has less water, so a low water potential. Water moves from dilute to concentrated, and it does so because only water can cross the membrane, not the solute.
Osmosis is a special case of diffusion: the same random motion, the same downhill direction, but restricted to water and requiring a partially permeable barrier.
Plant cells in different solutions
The cell wall changes what happens, which is why plant and animal cells behave so differently.
In a dilute solution (higher water potential outside), water enters. The vacuole swells and pushes the cytoplasm against the cell wall. The wall is strong and resists, so pressure builds and the cell becomes turgid. It does not burst. Turgor is what holds a non-woody plant upright.
In a concentrated solution (lower water potential outside), water leaves. The cell loses turgor and becomes flaccid, and the plant wilts. If enough water leaves, the cytoplasm and membrane pull away from the cell wall and the cell is plasmolysed.
Animal cells in different solutions
There is no wall, so nothing resists.
In a dilute solution, water enters until the cell swells and bursts. For a red blood cell this is called haemolysis.
In a concentrated solution, water leaves and the cell shrinks and crenates.
This is why the water potential of blood plasma must be kept steady, and why a drip must be the right concentration rather than pure water.
The potato experiment
A standard practical, and a standard question. Pieces of potato of equal size are left in sucrose solutions of different concentration and their mass is measured before and after.
- In dilute solutions, mass increases: water entered by osmosis.
- In concentrated solutions, mass decreases: water left by osmosis.
- At the concentration where mass does not change, the solution has the same water potential as the cell contents, so there was no net movement.
Percentage change in mass is used rather than change in mass, because the pieces are never exactly identical to start with:
percentage change = (change in mass ÷ starting mass) × 100
Active transport
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy from respiration.
The differences from diffusion are the ones to state.
| Diffusion and osmosis | Active transport | |
|---|---|---|
| Direction | down the gradient | against the gradient |
| Energy | none needed | needs energy from respiration |
| Carrier proteins | not required | required |
Because it needs energy, anything that stops respiration stops active transport. A cell starved of oxygen, or poisoned with cyanide, cannot carry it out, and that is the usual way an exam question proves active transport is involved.
Examples: ion uptake by root hair cells, where the soil is more dilute than the cell, and glucose absorption from the small intestine when the concentration in the gut has already fallen below that in the blood.
What affects the rate
For diffusion and osmosis:
- Steeper concentration gradient — faster, since the difference in the two flows is greater.
- Higher temperature — faster, since particles have more kinetic energy and move more quickly.
- Larger surface area — faster, since more particles can cross at once.
- Shorter distance — faster, which is why exchange surfaces are one cell thick.
For active transport, the rate depends on the rate of respiration and the number of carrier proteins, so it can be limited by oxygen supply and by temperature acting on enzymes.