3.4 Cellular Energy

ATP is the energy currency

Cells run on ATP (adenosine triphosphate). Energy is released when ATP is hydrolyzed to ADP + a phosphate group. Cells constantly recycle ADP back into ATP using energy from food. ATP powers cellular work like transport, movement, and building molecules.

Exergonic and endergonic reactions

Exergonic reactions release energy (like breaking down glucose). Endergonic reactions absorb energy (like building large molecules). Cells use energy coupling: the energy from exergonic reactions (via ATP) powers endergonic ones. Together, all these reactions make up the cell's metabolism.

Key terms

ATP

Adenosine triphosphate — the cell's main energy currency.

Exergonic

A reaction that releases energy (e.g., breaking down glucose).

Endergonic

A reaction that absorbs energy (e.g., building macromolecules).

Energy coupling

Using energy from an exergonic reaction to power an endergonic one, often via ATP.

Metabolism

All of a cell's chemical reactions — anabolic (building) plus catabolic (breaking down).

Try an AP-style question

The synthesis of glutamine from glutamic acid is endergonic. How do cells make it proceed?

1. They lower the activation energy with an enzyme until ΔG is negative
2. They couple it to ATP hydrolysis, so the combined process is exergonic
3. They remove water so the reaction releases energy on its own
4. They raise the temperature until the reaction becomes spontaneous