General Biology I · Active transport

Explain how pumps establish gradients and how a second transporter can use that stored energy to move cargo uphill.

Before you start

Review concentration, diffusion, and the selective permeability of membranes. Know that opposite electrical charges attract and that ATP hydrolysis can supply energy to a coupled process.

Review diffusion and osmosis

Which direction is downhill?

For an uncharged solute, moving from higher to lower concentration is downhill. For an ion, concentration and membrane voltage both contribute to the electrochemical gradient. If these effects oppose one another, their relative strengths determine which direction is favorable. Movement also requires a permeable route.

Two ways to pay for uphill movement

Primary active transport couples movement directly to an energy source such as ATP hydrolysis. Secondary active transport couples uphill cargo movement to the downhill movement of a driver ion. The two movements must be linked by the transporter, and the energy released by the downhill movement must cover the uphill cost for forward transport to be favorable.

ATP hydrolysis powers a pump

The pump builds an ion gradient

Ion moves downhill while cargo moves uphill

Energy pathway across two transporters. The pump stores energy in the gradient. The coupled transporter uses energy as the ion moves down that gradient.

Worked examples

Follow a pump through four cycles

In one cycle, the sodium–potassium pump uses one ATP to move three sodium ions out and two potassium ions in. Four cycles use four ATP, export 12 sodium ions, and import eight potassium ions. The pump therefore moves four net positive charges outward. This counts the pump’s contribution only. Other membrane pathways can also move charge.

Balance an energy budget

Suppose moving one cargo molecule in the proposed direction costs +7 units of free energy, and one downhill driver releases 4 units. With one driver the total is +7 − 4 = +3. With two drivers it is +7 − 8 = −1. Only the two-driver example has a negative total for the proposed direction. The calculation assumes fixed gradients and complete coupling.

Common mistakes

Protein-assisted movement can be passive. Check the gradient and the energy source.
Same-direction transport is called symport. That label describes direction and does not identify the energy source.
For ions, a concentration difference alone can give the wrong prediction if voltage matters.

Practice

Count ATP and ions moved in three sodium–potassium pump cycles.
Cargo costs 9 units and each downhill ion releases 4. What is the smallest whole number of ions that makes the total negative?

Worked solutions

Multiply the cycle counts by three: 3 ATP, 9 sodium ions out, and 6 potassium ions in. Net outward charge is 9 − 6 = 3.
Two ions give 9 − 8 = +1. Three give 9 − 12 = −3, so three is the minimum. This is a model prediction, not a claim about a particular protein.

Assessment · 10 points

4 points. A pump uses ATP to move an ion uphill. A second protein uses the downhill return of that ion to move cargo uphill. Classify both proteins and identify their immediate energy sources.
6 points. Cargo costs 8 units. Two driver ions each release 4 units. Compute the total and explain the result. Repeat if each ion releases only 3 units.

Answer key and scoring

Question 1: the pump is primary and uses ATP hydrolysis. The second protein is secondary and uses the ion’s electrochemical gradient. Award 1 point for each classification and each source.

Question 2: 8 − 2 × 4 = 0, so there is no net thermodynamic drive in the proposed direction. Then 8 − 2 × 3 = +2, so forward transport is unfavorable under the model assumptions. Award 2 points per calculation and 1 per interpretation.

Free reading

Read the electrochemical gradient and primary and secondary active transport sections. Trace the energy source in each mechanism.

OpenStax Biology 2e · 5.3 Active Transport

Virtual lab · Budget transport energy

Predict whether an idealized coupled cycle has a favorable energy budget.

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Procedure

Set cargo cost to 8, energy released per driver to 4, and driver count to 1. Predict and run the result. Repeat with 2 and 3 drivers. Keep 3 drivers and reduce release per driver to 2. When is the total negative? Use the sign to explain each result.

Expected results and answer check

With 1, 2, and 3 drivers at 4 units each, the totals are +4, 0, and −4. Only the last is negative. Reducing release to 2 with three drivers gives +2, so the proposed direction is unfavorable. Zero means no net driving force in this model.

This synthetic model checks the sign of the free-energy change under fixed conditions. It cannot predict a transport rate or measure a membrane gradient. Those require experimental measurements.