The light-dependent reactions are the first stage of photosynthesis, where sunlight is converted into chemical energy. They take place in the thylakoid membranes inside chloroplasts. These reactions matter because they produce ATP and NADPH, the energy carriers needed to build sugars in the Calvin cycle.
They also release oxygen gas as a byproduct from the splitting of water.
Key Facts
- Overall light reaction summary: 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light energy -> O2 + 2 NADPH + 3 ATP
- Photosystem II absorbs light and replaces lost electrons by splitting water: 2 H2O -> O2 + 4 H+ + 4 e-
- Electrons flow from Photosystem II to plastoquinone, cytochrome b6f, plastocyanin, Photosystem I, ferredoxin, and NADP+ reductase.
- The cytochrome b6f complex helps pump H+ from the stroma into the thylakoid lumen, building a proton gradient.
- ATP synthase uses proton flow from the lumen to the stroma to make ATP: ADP + Pi -> ATP.
- Photosystem I re-energizes electrons with light so NADP+ can be reduced: NADP+ + 2 e- + H+ -> NADPH.
Vocabulary
- Thylakoid membrane
- The chloroplast membrane where the light-dependent reactions occur and where photosystems, electron carriers, and ATP synthase are embedded.
- Photosystem
- A protein and pigment complex that captures light energy and transfers excited electrons into an electron transport chain.
- Photolysis
- The splitting of water by light-driven reactions to replace electrons in Photosystem II and release oxygen and protons.
- Proton gradient
- A difference in H+ concentration across a membrane that stores potential energy for ATP production.
- NADPH
- A high-energy electron carrier made in the light reactions and used to help build sugars in the Calvin cycle.
Common Mistakes to Avoid
- Putting the Calvin cycle inside the thylakoid membrane is wrong because the Calvin cycle occurs in the stroma, while the light-dependent reactions occur in the thylakoid membrane.
- Saying oxygen comes from carbon dioxide is wrong because the oxygen released in photosynthesis comes from the splitting of water in Photosystem II.
- Reversing proton movement through ATP synthase is wrong because H+ flows from the thylakoid lumen back into the stroma to drive ATP formation.
- Thinking NADPH and ATP are final food products is wrong because they are temporary energy carriers used later to make carbohydrates.
Practice Questions
- 1 If 4 water molecules are split during photolysis, how many O2 molecules, H+ ions, and electrons are produced?
- 2 A chloroplast makes 18 ATP during the light-dependent reactions. Using the ratio 3 ADP + 3 Pi -> 3 ATP in the summary equation, how many sets of the overall light reaction does this represent?
- 3 Explain why blocking the cytochrome b6f complex would reduce ATP production even if Photosystem II still absorbed light.