Explain active transport across membranes
Active transport uses membrane proteins and metabolic energy to move selected particles against an electrochemical gradient.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What two features identify active transport?
What you will learn
- Explain how carrier proteins and metabolic energy support active transport
- Distinguish active transport from passive diffusion
Transport against a gradient
Active transport moves selected molecules or ions against their concentration or electrochemical gradient, from lower to higher concentration in the relevant comparison. This direction requires an energy source from cellular metabolism.
Carrier proteins bind selected particles and change shape to move them across the membrane. The protein’s selectivity and energy-dependent cycle allow transport that would not occur by passive diffusion down the gradient.
Compare the processes, not just the protein
Simple and facilitated diffusion move net particles down a gradient and do not require direct metabolic energy. Facilitated diffusion uses a channel or carrier, so the presence of a transport protein alone does not prove active transport.
To classify a process, identify the direction relative to the gradient and whether metabolic energy is required. An active transporter can move one substrate while another particle moves down its gradient, but only make that claim when the provided model supports it.
Pause and explain
A carrier moves ions down their gradient without energy. Is it active transport?
Worked example
A cell accumulates a mineral ion from a solution where the external ion concentration is lower than inside. A membrane carrier uses ATP. Explain the transport.
Show the worked solution
- Net movement is against the concentration gradient.
- A carrier and metabolic energy are used.
Answer This is active transport: the ion is moved against its concentration gradient by a selective carrier protein using metabolic energy.
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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.
Explain how active transport can accumulate an ion inside a cell against its concentration gradient.
Give me a hint
Name the protein, direction and energy source.
Compare my reasoning
- A selective membrane carrier binds the ion.
- Metabolic energy supports movement against the gradient.
A selective carrier protein binds the ion and uses energy from cellular metabolism to change shape and move it into the cell against its concentration gradient.
Look for these in your work
- Carrier protein named
- Energy and against-gradient movement stated
A labelled membrane pathway uses a channel to move ions from high to low concentration without ATP. Classify it and explain the evidence.
Give me a hint
Use direction and energy, then account for the channel.
Compare my reasoning
- Movement is down the gradient.
- A channel is used without direct energy, so this is facilitated diffusion.
This is facilitated diffusion: the ions move down their concentration gradient through a selective channel protein without direct metabolic energy.
Look for these in your work
- Gradient direction used
- Protein-mediated passive process named
A student says that a membrane carrier proves ATP is used. Explain why this conclusion is not justified.
Give me a hint
Recall that carriers can also mediate a passive process.
Compare my reasoning
- Some carriers mediate facilitated diffusion.
- Classify by gradient direction and energy evidence.
A carrier may mediate facilitated diffusion, moving a particle down its gradient without direct ATP use. To identify active transport, evidence must show movement against a gradient and an energy requirement.
Look for these in your work
- Alternative explained
- Classification criteria stated
Common mistakes
- Calling protein-mediated movement active without checking the gradient
- Saying ATP itself carries the ion through the membrane
- Confusing active transport with vesicle transport
What can you explain now?
What two features identify active transport?
Compare with the explanation
Movement against a gradient using a membrane protein and energy
Direction and energy distinguish active transport.
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Make it stick
How does active transport differ from facilitated diffusion when both may use membrane proteins?
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.