Active transport

In the syllabus and the exam

Active transport is the process the syllabus uses to explain situations where a substance must move against its concentration gradient, and it is most often examined through the example of mineral ion uptake by root hairs, or glucose reabsorption in the kidney's nephron, both of which require energy released by aerobic respiration in mitochondria. Structured questions typically give candidates a scenario in which a substance is more concentrated inside a cell than outside, and ask which of the three transport processes (diffusion, osmosis or active transport) must be occurring, expecting the answer 'active transport' along with a reason referencing the direction of movement against the gradient and the involvement of ATP and carrier proteins. A frequent error is describing active transport without mentioning that it requires energy, or describing it as always faster than diffusion, when in fact the defining feature examiners look for is the direction of movement relative to the gradient, not speed. Candidates are also commonly asked to explain why cells that carry out a lot of active transport, such as root hair cells or kidney tubule cells, contain unusually large numbers of mitochondria, linking this term directly back to cellular respiration and organelle structure covered in earlier chapters. Candidates sometimes struggle to explain why active transport is still needed even though diffusion could theoretically move the same substance; the answer examiners expect is that once the substance inside the cell becomes more concentrated than outside, diffusion alone can no longer move any more of it in, so only active transport can continue the uptake.

Example

Root hair cells absorb mineral ions such as nitrate from the soil by active transport, even when the ion concentration is already higher inside the root hair cell than in the surrounding soil water.

Related terms

DiffusionOsmosis

Source:SRC-DSKP-EN