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Wize AP Biology Textbook > Energy & Metabolism

ATP Energy and the Proton Motive Force (PMF)

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ATP and the Proton Motive Force (PMF)

Adenosine TriPhosphate is an energy storage molecule. It stores potential energy (PE) between the negative charges of the phosphate groups.
Photo by OpenStax College / CC BY

Wize Tip
Keeping those negatively charged phosphates next to one another is like keeping three people who really hate each other locked in a small closet.


ATP hydrolysis releases free energy (negative ΔG).
ATP(aq)+H2O(l)→ADP(aq)+Pi(aq)ATP_{(aq)}+H2O_{(l)}\rightarrow ADP_{(aq)}+Pi_{(aq)}


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How Does the Cell Create ATP?

  • ATP is not the only way cells store energy. Another way is by generating an electrochemical gradient. Example: During cellular respiration in our cells, electron energy is used to pump H+ across the plasma membrane to generate an electrochemical gradient called the proton motive force (PMF).
  • The PMF can then be used to synthesize ATP.


The amount of energy stored across a membrane can be calculated using the formula:
ΔG=RTln([x]final/[x]initial)\varDelta G = RT ln ([x]_{final} / [x]_{initial})

R = gas constant (8.314 J/mol K)
T = temperature in K
[x]initial[x]_{initial}= molar concentration of X on the outside of the membrane
[x]final[x]_{final}= molar concentration of X on the inside of the membrane

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Example: ATP Synthesis

Sketch and estimate the Gibbs free energy required to create an ATP molecule from ADP.

If breaking down ATP releases 52 kJ/mol of energy, we have that creating a molecule of ATP from the same materials (ADP and Pi) will require that amount of energy.

See video solution for sketch of Gibbs free energy graph.

Practice: ATP Energy

Which of the following is true?