The light reactions are the first stage of photosynthesis and take place in the thylakoid membranes of chloroplasts. They capture light energy and convert it into chemical energy stored in ATP and NADPH. Students need this cheat sheet because the process has many connected parts, including photosystems, electron carriers, and proton movement.
A clear reference helps show how energy flows from sunlight to chemical products.
The core idea is that light excites electrons in chlorophyll, and those electrons move through an electron transport chain. Water is split to replace lost electrons, releasing oxygen gas as a byproduct. Electron transport pumps H+ ions into the thylakoid space, creating a concentration gradient.
ATP synthase uses this gradient to make ATP, while NADP+ accepts electrons and H+ to form NADPH.
Key Facts
- The overall purpose of the light reactions is to convert light energy into ATP and NADPH for the Calvin cycle.
- The light reactions occur in the thylakoid membranes inside chloroplasts.
- Photolysis splits water using light energy: 2 H2O -> 4 H+ + 4 e- + O2.
- Photosystem II absorbs light first and replaces its lost electrons with electrons from water.
- Electrons move from Photosystem II through an electron transport chain to Photosystem I.
- The electron transport chain pumps H+ ions into the thylakoid space, building a proton gradient.
- ATP synthase makes ATP as H+ ions move down their gradient from the thylakoid space to the stroma.
- Photosystem I helps reduce NADP+ to NADPH using the reaction NADP+ + 2 e- + H+ -> NADPH.
Vocabulary
- Thylakoid
- A flattened membrane sac inside a chloroplast where the light reactions of photosynthesis occur.
- Photosystem
- A cluster of chlorophyll and proteins that absorbs light and excites electrons.
- Photolysis
- The splitting of water by light energy to produce electrons, hydrogen ions, and oxygen gas.
- Electron Transport Chain
- A series of proteins that passes electrons along and uses their energy to pump H+ ions.
- Chemiosmosis
- The process of making ATP as H+ ions flow through ATP synthase down their concentration gradient.
- NADPH
- An energy-carrying molecule that provides high-energy electrons for the Calvin cycle.
Common Mistakes to Avoid
- Saying the light reactions make glucose directly is wrong because glucose is made later during the Calvin cycle using ATP and NADPH.
- Putting the light reactions in the stroma is wrong because they occur in the thylakoid membranes, while the Calvin cycle occurs in the stroma.
- Forgetting that oxygen comes from water is wrong because O2 is released when water is split during photolysis, not from carbon dioxide.
- Mixing up Photosystem II and Photosystem I is wrong because Photosystem II acts first, even though it has the number II.
- Thinking ATP synthase uses electrons directly is wrong because ATP synthase uses the flow of H+ ions down a proton gradient to make ATP.
Practice Questions
- 1 If 4 water molecules are split during photolysis, how many O2 molecules are produced?
- 2 If 12 electrons leave Photosystem II, how many water molecules must be split to replace them?
- 3 Write the balanced photolysis equation for the splitting of water in the light reactions.
- 4 Explain why a plant would struggle to run the Calvin cycle if its thylakoid membranes could no longer build an H+ gradient.