Diffusion
Particles in liquids and gases are always jiggling about at random. Over time that random movement spreads them out from where there are lots of them to where there are few. That spreading is diffusion.
- Diffusion
- The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive, so it needs no energy from the cell.
In your body, oxygen diffuses from the air in your lungs into the blood because there is more oxygen in the air sacs than in the blood arriving there. Carbon dioxide diffuses the other way. Nothing is spending energy to push it: the gradient does the work.
Osmosis
Osmosis is really just diffusion of water, but with one twist: it happens across a partially permeable membrane, a barrier that lets small water molecules through but blocks bigger dissolved particles like sugar or salt.
- Osmosis
- The net movement of water molecules across a partially permeable membrane, from a region of higher water concentration (dilute) to a region of lower water concentration (concentrated).
Two words you must not mix up:
| Term | The solution is... | What happens to a cell placed in it |
|---|---|---|
| Hypotonic | More dilute than the cell (lots of water outside) | Water moves in; the cell swells. An animal cell may burst. |
| Hypertonic | More concentrated than the cell (little water outside) | Water moves out; the cell shrinks. A plant cell becomes flaccid. |
A memory hook
"HyPO" sounds like "PO" for "plenty of water", so water moves in. "HyPER" concentration pulls water out. Water always chases the saltier, more concentrated side.
Active transport
Sometimes a cell needs to move a substance the "wrong" way, from where there is little of it to where there is already lots. That is like walking up a hill, so it costs energy.
- Active transport
- The movement of particles across a membrane from a lower to a higher concentration, against the concentration gradient. It uses energy from respiration and carrier proteins in the membrane.
Root hair cells use active transport to pull mineral ions out of dilute soil water, even though the root already has more of those ions inside. That is why actively respiring roots take up minerals faster: more respiration means more energy available.
| Process | Direction | Needs energy? |
|---|---|---|
| Diffusion | High to low (down the gradient) | No |
| Osmosis | High to low water (down the gradient) | No |
| Active transport | Low to high (against the gradient) | Yes |
Where this is assessed
Explaining the three transport processes is Criterion A (knowing and understanding). Planning a fair osmosis experiment is Criterion B (inquiring and designing), and turning raw masses into a percentage change and a valid conclusion is Criterion C (processing and evaluating).
Factors affecting rate and a worked example
Diffusion and osmosis go faster when:
- the concentration gradient is steeper (a bigger difference in concentration),
- the temperature is higher (particles move faster),
- the surface area is larger, and
- the diffusion distance is shorter (a thinner membrane).
A classic experiment measures osmosis using potato cylinders left in different sugar solutions. You record the mass before and after and work out the percentage change, because starting masses are never identical.
A potato cylinder started at 5.0 g. After an hour in pure water it weighed 5.6 g. Find the percentage change in mass and explain it.
Check yourself
1. Define osmosis in one sentence. +
The net movement of water molecules across a partially permeable membrane, from a higher water concentration to a lower water concentration.
2. A cylinder started at 4.0 g and ended at 3.4 g in a strong salt solution. What is the percentage change, and why did it happen? +
Change = 3.4 - 4.0 = -0.6 g. Percentage change = (-0.6 ÷ 4.0) × 100 = -15%. The salt solution is hypertonic, so water left the cells by osmosis and the mass fell.
3. Why does active transport need energy but diffusion does not? +
Active transport moves particles against the concentration gradient (from low to high), which does not happen on its own, so energy from respiration is needed. Diffusion goes down the gradient, which particles do naturally, so no energy is required.
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