Lesson 3 of 4 / Membranes and transport
Move a solute against its gradient
Where does the energy for uphill transport come from?
I can explain energy coupling in active transport and distinguish primary and coupled routes.
I can explain simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.Syllabus
Syllabus 9477, Core 1(l). Explain simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
About 6 min
Make a guess. You are not marked.
A cotransporter moves glucose into a gut cell against its gradient, but the cotransporter does not hydrolyse ATP. Where does the energy come from?
Show the answer
Sodium ions moving down their own gradient.
A sodium-potassium pump uses ATP to keep sodium low inside. Sodium flowing back in drives glucose uphill.
The key idea
Active transport couples an energetically unfavourable movement to an energy source, using selective membrane proteins.
Explore the idea
Trace the energy for uphill movement
ATP hydrolysis supports binding and conformational changes that move the ion uphill. This is a pump cycle, not an always-open channel. The diagram assumes no opposing electrical difference.
Original schematic. Molecular sizes and distances are illustrative, not to scale.
Explanation
A pump can move ions against their electrochemical gradient by coupling transport to ATP hydrolysis. Binding, phosphorylation and conformational changes alter which side a binding site faces and its affinity for the ion. The protein is not a permanently open pore through which ions simply diffuse uphill.
ATP regeneration depends on metabolism. Inhibiting respiration can therefore reduce sustained active transport, although stored ATP may allow brief continued activity. An experiment must allow for this delay and for effects on cell viability before interpreting transport changes.
Some uphill transport is coupled to another solute moving downhill. A sodium gradient established by ATP-dependent pumping can drive a cotransporter that brings another solute into a cell. Such secondary active transport is energy dependent indirectly; it is inaccurate to draw every glucose carrier as a pump directly hydrolysing ATP.
Direction alone is not enough unless the full gradient is known, especially for ions. Compare uptake with gradients, metabolic inhibition and specificity. Passive movement can continue when active pumping stops, so removing ATP does not imply that all membrane traffic or molecular motion ceases.
Step by step
- 1
State the uphill movement
Identify which gradient is opposed.
- 2
Trace the energy source
Distinguish direct ATP use from a stored ion gradient.
- 3
Predict a controlled change
Consider pump inhibition without claiming all motion stops.
Worked example
Work through the evidence
A cell stops pumping an ion when ATP becomes depleted, but the ion still leaks inward through channels. Explain.
One way to explain it
Active transport has lost its energy supply. Channel-mediated passive movement can continue down the ion's electrochemical gradient.
Why this answer works
- Different proteins support the two fluxes.
- Net ion content can change as the balance of fluxes changes.
Watch out for this
A student says: "An open channel can use ATP to push ions uphill without a conformational cycle." What is wrong with this?
Show the answer
A passive channel offers a downhill route; active pumping needs coupling and regulated protein changes.
Check your understanding
A cotransporter brings glucose uphill while sodium moves downhill. Which is best?
Show the answer
The sodium gradient supplies energy for coupled transport.
Indirect ATP dependence differs from a direct ATP-driven glucose pump.
Quick recall
Where does the energy for uphill transport come from?