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Positive Delta G: Nonspontaneous, Endergonic and How Cells Still Do It

By StudyStore · Published 2026-11-05 · 2 min read · 452 words

A positive delta G means the reaction is nonspontaneous as written: it is endergonic, and it will not proceed forward on its own under those conditions. A negative delta G is spontaneous (exergonic). Zero is equilibrium.

What it does and does not mean

It means the forward reaction needs an input of free energy.
It does not mean the reaction cannot happen. Coupling, concentration changes and temperature can all change the outcome.
It does not mean slow. Delta G says direction, not rate. Rate depends on the activation energy and on enzymes.

Delta G vs delta G naught

Symbol	Conditions	Use
ΔG°	Standard state, all 1 M	Compare reactions, find K
ΔG	Actual concentrations	Decide the real direction

ΔG = ΔG° + RT ln Q. A reaction with a positive ΔG° can have a negative ΔG if Q is small enough, meaning products are kept low. This is how many metabolic steps run forward in cells. Also ΔG° = -RT ln K, so a positive ΔG° means K is below 1.

Coupling to ATP hydrolysis

ATP hydrolysis has a ΔG° of about -30 kJ/mol (-7.3 kcal/mol). Free energies of coupled reactions add:

Glucose + Pi → glucose-6-phosphate: ΔG° about +14 kJ/mol
ATP → ADP + Pi: ΔG° about -30 kJ/mol
Net (hexokinase): about -16 kJ/mol, spontaneous

The two reactions must share an intermediate, usually a phosphorylated enzyme or substrate. This is what hexokinase does in the first step of glycolysis.

When a positive delta G can flip with temperature

Use ΔG = ΔH - TΔS. If ΔH is positive and ΔS is positive, ΔG is positive at low T and negative at high T (ice melting above 0 degrees C). If ΔH is positive and ΔS is negative, ΔG is positive at every temperature. The full sign table is in the Gibbs free energy note.

Common traps

Equating positive ΔG with a rate of zero. It is about direction.
Using ΔG° to decide direction in a cell. Use ΔG with actual concentrations.
Adding coupled reactions' K values. Add ΔG values; multiply K values.
Mixing up the sign in electrochemistry. A galvanic cell has E positive and ΔG negative.

One-minute recap

Positive ΔG is endergonic and nonspontaneous as written. It says nothing about speed and it can be overcome by coupling to a strongly negative ΔG such as ATP hydrolysis, by keeping Q low, or sometimes by changing temperature.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. What does a positive delta G tell you about a reaction?

It is nonspontaneous and endergonic as written, so the forward reaction needs free energy input.

2. Does a positive delta G mean the reaction is slow?

No. Delta G gives direction, not rate; rate depends on activation energy.

3. A reaction has delta G naught of +14 kJ/mol and is coupled to ATP hydrolysis at -30 kJ/mol. What is the net?

-16 kJ/mol, which is spontaneous.

4. How can a reaction with a positive delta G naught run forward in a cell?

Delta G also depends on Q. Keeping products low makes Q small and delta G negative.

5. What is the relationship between a positive delta G naught and K?

K is less than 1, because delta G naught = -RT ln K.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.

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