Active transport
Moving a substance across a membrane against its concentration gradient, using energy (usually ATP). Carrier proteins called pumps do the work.
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Analogy
Pushing a trolley uphill: it needs effort.
Example
One cycle:3 Na+ out2 K+ in1 ATP used
In practice
The sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in, using one ATP each cycle. Nerve cells depend on it.
Practice
Flashcards
- How does active transport differ from diffusion?Active transport goes against the gradient and uses energy. Diffusion goes down the gradient and needs none.
Interview questions
- Explain how a cell moves water and ions across its membrane, including one example that needs energy.
- Water follows solute by osmosis
- Small gradients allow diffusion through channels
- Active transport needs ATP
- Sodium-potassium pump as example
Questions people ask
What is the difference between primary and secondary active transport?
Primary active transport uses ATP directly, as the sodium-potassium pump does. Secondary active transport uses the gradient that a pump has already built. For example, glucose enters gut cells together with sodium ions, which flow back down their gradient and drag the glucose in.
Why do cells spend so much energy on pumps?
Gradients store energy like a charged battery. Nerve signals, nutrient uptake and keeping cell volume stable all depend on them. In many animal cells, a large share of the ATP budget goes to the sodium-potassium pump alone, and in nerve cells it is especially high.