How can a cell move substances against the concentration gradient?
Define active transport and explain its role using energy, with examples in cells
A focused answer to the O-Level Biology outcome on active transport. The definition, why it needs energy from respiration, how it differs from diffusion, and examples such as mineral uptake in roots and glucose uptake in the gut.
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What this dot point is asking
SEAB wants a clear definition of active transport, an explanation of why it requires energy (from respiration), and how it differs from diffusion. You should be able to give examples, such as the uptake of mineral ions by roots and the absorption of glucose in the gut, and explain why diffusion alone could not achieve them.
The answer
What active transport is
Active transport is the movement of particles (molecules or ions) across a cell membrane from a region of lower concentration to a region of higher concentration, that is, against the concentration gradient, using energy released by respiration.
Because it works against the gradient, it cannot rely on the particles' own random movement. Instead, the cell uses carrier proteins in the membrane that act like pumps, and these need energy to work.
Why it needs energy
Moving particles from where they are scarce to where they are already plentiful is like pushing something uphill: it takes effort. The energy comes from respiration, which releases energy from glucose, mostly in the mitochondria. Cells that carry out a lot of active transport, such as root hair cells and cells lining the gut, have many mitochondria to supply this energy.
How active transport 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 | Always, acting as pumps |
The key contrast is the direction of movement and the need for energy.
Examples in organisms
- Mineral ion uptake by roots. Soil often has a lower concentration of ions such as nitrate than the root cell, yet the plant still absorbs them, by active transport.
- Glucose absorption in the small intestine. After most glucose has been absorbed, the remaining glucose is taken into the blood against its gradient by active transport, so none is wasted.
- Reabsorption in the kidney. Useful glucose is actively reabsorbed from the kidney tubule back into the blood.
Examples in context
Example 1. Loading sugar for transport. In a leaf, sugars made by photosynthesis are loaded into the phloem by active transport, even when sugar is already concentrated there. This loading drives the transport of food around the plant.
Example 2. The kidney tubule. As filtered fluid passes along the kidney tubule, useful glucose is reabsorbed into the blood by active transport, against its gradient, so that no glucose is lost in the urine of a healthy person.
Try this
Q1. Define active transport. [2 marks]
- Cue. The movement of particles across a membrane against the concentration gradient, from lower to higher concentration, using energy from respiration.
Q2. State one difference between diffusion and active transport. [1 mark]
- Cue. Diffusion moves substances down the gradient with no energy; active transport moves them against the gradient using energy from respiration.
Q3. Explain why a cell that carries out a lot of active transport has many mitochondria. [2 marks]
- Cue. Active transport needs energy, which is released by respiration in the mitochondria, so many mitochondria are needed to supply enough energy.
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.
Original5 marksRoot hair cells take up mineral ions from the soil even when the concentration of those ions is higher inside the cell than in the soil. (a) Name the process used. (b) Explain why diffusion cannot be responsible. (c) State where the energy for this process comes from.Show worked answer →
(a) The process is active transport.
(b) Diffusion only moves substances from a higher to a lower concentration (down the gradient). Here the ions are moving from a lower concentration in the soil to a higher concentration in the cell, that is, against the gradient, so diffusion cannot be responsible.
(c) The energy comes from respiration (the release of energy from glucose, mainly in the mitochondria).
Markers reward naming active transport, the explanation that movement is against the concentration gradient (which rules out diffusion), and respiration as the energy source.
Original3 marksCompare diffusion and active transport in terms of 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 from the cell (it is passive).
Active transport moves substances against the concentration gradient, from a lower to a higher concentration, and requires energy from respiration.
Markers reward a matched comparison: the opposite directions of movement, and that only active transport needs energy. Two separate descriptions without a clear contrast score less.
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