Active transport is the movement of particles against a concentration gradient, from lower to higher concentration, using energy from respiration. Diffusion is movement down a gradient with no energy from the cell. The direction and the energy are the two things to check.
This lesson follows explaining diffusion and osmosis in movement across membranes. You also need your ideas about cells from cells and microscopy.
What are the differences, side by side?
| Feature | Diffusion | Active transport |
|---|---|---|
| Direction | Down the concentration gradient | Against the concentration gradient |
| Energy from respiration | Not needed | Needed |
| Membrane proteins | Not needed for simple diffusion | Carrier proteins used |
| Effect of less oxygen | No direct effect on rate | Rate falls, because less respiration |
| Ends when | Net movement stops at equal concentration | Can build up a higher concentration inside than outside |
The last row is the key idea. Diffusion can only even out a difference. Active transport can create or maintain one.
Why would a cell bother?
A root hair cell needs mineral ions such as nitrate for making proteins. The soil water often holds only a low concentration of these ions. If the cell relied on diffusion, ions would only enter until the concentrations matched, and then stop.
By using energy to move ions in against the gradient, the cell can collect more than diffusion alone would allow. That is why root hair cells contain many mitochondria, which release energy by respiration.
Worked example
The numbers are invented for practice.
A cell sits in a solution containing 0.02 units of an ion. Inside the cell the concentration is 0.15 units. Over time the inside concentration rises to 0.20 units.
When the cell is given a substance that stops respiration, the uptake stops.
Question: Is the ion entering by diffusion or by active transport? Explain.
Step 1, compare concentrations. Outside is 0.02 and inside is 0.15, so the inside is higher.
Step 2, find the direction of movement. The ion is moving into the cell, which is the direction of higher concentration. That is against the gradient.
Step 3, find the extra evidence. Uptake stops when respiration stops. Diffusion would carry on without the cell’s energy, so this points to a process that needs energy.
Step 4, conclude. The ion enters by active transport, because it moves against the concentration gradient and depends on energy from respiration.
A good answer names both pieces of evidence, not only one.
The mistake to watch for
One mistake is to decide from the name of the substance instead of the data. Another is to call a process active just because it is fast.
Mistaken answer: The ion moves quickly into the cell, so it must be active transport.
Speed is not the test. Fast diffusion happens across steep gradients and short distances without any energy from the cell.
The correction is to use the two tests: does it move against the gradient, and does it depend on respiration? Rate alone tells you neither. If the movement is with the gradient and unaffected by respiration, it is diffusion.
Check yourself
Answer these, then open each solution.
1. The concentration of an ion is 0.50 units inside a cell and 0.10 units outside. It moves out of the cell and the rate does not change when oxygen is removed. Diffusion or active transport?
Show answer
Inside is higher than outside and the ion moves out, so it moves down the gradient. The rate does not depend on oxygen, so there is no need for respiration. This is diffusion.
2. Why does a root hair cell contain many mitochondria?
Show answer
Mitochondria release energy by aerobic respiration. The cell needs this energy to take up mineral ions by active transport against the concentration gradient.
3. An ion’s concentration is higher inside a cell than outside, but the cell continues to take it in. Give one piece of evidence you would want to confirm this is active transport.
Show answer
Show that the uptake slows or stops when respiration is reduced, for example with less oxygen or a respiration inhibitor. The movement against the gradient plus the energy dependence points to active transport.
Where this leads next
Next, practise reading numbers in experiments with interpreting a mass-change dataset, then test everything in the movement across membranes practice set.
Students often need to see the same idea in three or four different examples before the two tests become automatic. A teacher in online one-to-one Biology tuition can give you fresh examples at the pace you need.