Describe active transport against a concentration gradient
Connect membrane transfer from lower to higher concentration with respiratory energy.
Section B3.3 · Active transport
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall diffusion direction and respiration as an energy-releasing process.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which transfer fits active transport?
What you will learn
- Connect membrane transfer from lower to higher concentration with respiratory energy.
- Apply the idea to a supplied biological example or investigation.
Recognise the against-gradient transfer
Active transport moves particles through a cell membrane from lower concentration to higher concentration, against their concentration gradient, using energy from respiration. The direction is defined for the named substance, not by whether an arrow points into or out of a drawing.
For inward active uptake, the cell can accumulate a substance already more concentrated inside than outside. Net diffusion alone would favour the opposite direction if a suitable passive pathway existed. Active transport therefore maintains differences that cannot be explained by inward diffusion down the gradient.
Use evidence and avoid overclaims
A cell can use energy from respiration for active transfer. If the respiratory energy supply is reduced, active transport may be reduced; the observation does not prove every membrane process stops at once. Different processes can occur simultaneously.
Moving into a cell is not by itself evidence of active transport. Oxygen can diffuse inward down its gradient, and water can enter by osmosis. Compare concentrations, the membrane condition and the energy requirement before naming the process.
Sources for this lesson
Pause and explain
An ion enters from concentration 3 to concentration 12 using respiration. Why is diffusion insufficient?
Worked example
A named solute is 2 units outside and 9 inside. A cell continues accumulating it inward using respiratory energy. Identify the process.
Show the worked solution
- The transfer crosses the membrane.
- It goes from 2 toward 9, against the solute gradient.
- Respiratory energy is supplied.
Answer Active transport.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outside ion concentration is 1 unit and inside is 6 units. Identify inward transfer using respiration.
Give me a hint
Check lower to higher.
Compare my reasoning
- 1 is lower than 6.
- Respiratory energy is used.
Active transport.
Look for these in your work
- Include against-gradient and energy.
A substance diffuses inward from 8 units outside to 2 inside. Is inward direction alone evidence of active transport?
Give me a hint
Follow the gradient.
Compare my reasoning
- 8 to 2 is down the gradient.
- Diffusion can explain permitted passive entry.
No; it is compatible with diffusion.
Look for these in your work
- Do not classify by inward direction alone.
A cell transfers an ion outward from concentration 2 inside to 7 outside using energy. Can this be active transport?
Give me a hint
Against-gradient transfer can point outward.
Compare my reasoning
- 2 to 7 is lower to higher.
- It crosses the membrane using respiratory energy.
Yes; active transport is defined relative to the gradient, not always inward.
Look for these in your work
- Give direction relative to concentration.
Common mistakes
- Inward movement is not always active.
- Follow the named substance gradient, not arrow orientation alone.
What can you explain now?
An ion enters from concentration 3 to concentration 12 using respiration. Why is diffusion insufficient?
Compare with the explanation
The inward movement is against the ion gradient.
Net diffusion would favour the opposite direction if permeable.
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Make it stick
State the active-transport definition with boundary, gradient and energy.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.