AP Biology / Cellular Energetics
Lesson
Charging and spending ATP
Respiration and photosynthesis are complementary energy stories, not opposites in every detail.
Learning goals
- Outline how glucose oxidation leads to ATP.
- Outline how light reactions and the Calvin cycle share jobs.
Explanation
In cellular respiration, cells break down glucose in steps. Glycolysis happens in the cytosol. In eukaryotes, the mitochondria finish the oxidation and run an electron transport chain that pumps protons. ATP synthase lets protons back through and makes ATP. Oxygen is a final electron acceptor in aerobic respiration—not a vague “energy vitamin.”
Photosynthesis in chloroplasts captures light, splits water, and uses the resulting energy carriers to build sugar from carbon dioxide. The light reactions and the sugar-building cycle depend on each other. Plants still respire. Photosynthesis does not cancel the need for ATP in the dark.
Key terms
- ATP. The cell’s short-term energy currency, spent to drive otherwise unfavorable work.
- Electron transport chain. A series of membrane proteins that pass electrons and use the energy to build a proton gradient.
Common mistakes
- Writing that plants only photosynthesize and animals only respire.
- Skipping the proton gradient and saying “electrons make ATP” as if by magic.
Practice
Original Marlow Works items. Check the answer explanation after you try.
Explain why a cell that cannot carry out oxidative phosphorylation can still produce a small amount of ATP from glucose.
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
If a poison blocks ATP synthase, why do electron carriers back up even if oxygen is present?
Answer. The proton gradient cannot be relieved, so the chain stops accepting electrons and NADH stays reduced.
The steps are coupled. Blocking the last energy step stalls the earlier ones.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source