How can a cell move substances against the concentration gradient, and why does this need energy?
Define active transport, explain why it requires energy, and describe examples such as mineral uptake by root hairs
A scaffolded answer to the N(A)-Level Biology outcome on active transport. Moving substances against the gradient using energy from respiration, how it differs from diffusion, and examples such as root hair mineral uptake and glucose absorption.
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What this dot point is asking
This outcome wants you to define active transport, explain clearly why it needs energy, and give examples in living things. The most important idea is that active transport moves substances against the concentration gradient, from where there is less to where there is more, which is the opposite direction to diffusion. Because this does not happen by itself, the cell must spend energy from respiration to make it happen.
The answer
What active transport is
Active transport is the movement of substances across a cell membrane against the concentration gradient, that is, from a region of lower concentration to a region of higher concentration, using energy. Diffusion only moves things down a gradient, so when a cell needs to take in a substance that is already more concentrated inside, it must use active transport instead.
Why energy is needed
Particles do not move from low to high concentration on their own, because that is against their natural spreading. To force them the other way, the cell must put in energy. This energy is released by respiration inside the cell. Special carrier proteins in the cell membrane use this energy to pick up the substance on one side and pump it across to the other.
How it differs from diffusion
| Feature | Diffusion | Active transport |
|---|---|---|
| Direction | Down the gradient (high to low) | Against the gradient (low to high) |
| Energy | None needed (passive) | Energy from respiration needed |
| Carrier proteins | Not always needed | Always needed (acting as pumps) |
Why it matters
Active transport lets cells take in useful substances even when there is very little of them outside. Root hair cells use it to absorb mineral salts from dilute soil water. Cells lining the small intestine use it to absorb the last of the glucose from the gut when the gut concentration has fallen below that of the blood.
Examples in context
Example 1. Root hairs feeding the plant. Soil water is very dilute, yet a plant needs minerals such as nitrate and potassium to grow. Root hair cells use active transport to pump these ions in from the dilute soil, against the gradient, which is why a healthy root system has many mitochondria to supply the energy.
Example 2. Mopping up the last of the glucose. Near the end of digestion, the glucose left in the gut can be more dilute than the blood. The cells lining the small intestine then use active transport to absorb this remaining glucose, so that none of the useful food is wasted.
Try this
Q1. Define active transport. [2 marks]
- Cue. The movement of substances across a membrane against the concentration gradient (low to high), using energy from respiration.
Q2. State one difference between diffusion and active transport. [1 mark]
- Cue. Diffusion needs no energy and moves substances down the gradient; active transport needs energy and moves them against the gradient.
Q3. Explain why root hair cells contain many mitochondria. [2 marks]
- Cue. Mitochondria release energy by respiration, and root hair cells need a lot of energy for active transport to take in mineral salts against the gradient.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksRoot hair cells take in mineral salts from the soil, even when the soil has a lower concentration of those salts than the cell. Name the process used and explain why it needs energy.Show worked answer →
The process is active transport.
The mineral salts are at a lower concentration in the soil than inside the root hair cell. This means they must be moved against the concentration gradient, from low to high concentration. Moving substances against a gradient does not happen on its own, so it requires energy. This energy is released by respiration in the cell and is used by carrier proteins in the membrane to pump the salts in.
What markers reward: naming active transport, the phrase against the concentration gradient (from low to high), and stating that the energy comes from respiration. A common slip is to say diffusion, which only moves substances down a gradient.
Original4 marksCompare diffusion and active transport. Refer to the direction of movement and the need for energy.Show worked answer →
Diffusion moves substances down the concentration gradient, from a higher to a lower concentration, and needs no energy because it is passive.
Active transport moves substances against the concentration gradient, from a lower to a higher concentration, and needs energy from respiration to do so.
Both processes move substances across the cell membrane. The key differences are the direction (down the gradient for diffusion, against it for active transport) and the energy requirement (none for diffusion, energy needed for active transport).
What markers reward: a clear contrast on both points, not two separate descriptions. The phrases down the gradient and against the gradient, and passive versus needs energy, are the marking points.
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