SECTION 5: Light Reactions of Photosynthesis

Q: Where does photosynthesis occur in the cell?

A: In the chloroplasts, specifically within thylakoid membranes (light reactions) and the stroma (Calvin Cycle).

Q: What is the origin of chloroplasts?

A: Endosymbiosis — they evolved from free-living cyanobacteria.

Q: What are the two stages of photosynthesis?

A: Light reactions (convert light → chemical energy) and Calvin Cycle (use CO₂ and chemical energy to make sugar).

Q: What happens in Photosystem II?

A: Chlorophyll absorbs photons, exciting electrons that are passed to the Electron Transport Chain (ETC). Water is split to replace lost electrons, producing O₂ and H⁺.

Q: Is chlorophyll oxidized or reduced when it loses an electron?

A: Oxidized (it loses energy/electron).

Q: What molecule replaces chlorophyll’s lost electron?

A: Water (H₂O) — it is oxidized, releasing O₂, H⁺, and e⁻.

Q: What is the purpose of the ETC in light reactions?

A: To transfer electrons, releasing energy that pumps H⁺ into the lumen, creating a proton gradient for ATP synthesis.

Q: What happens in Photosystem I?

A: Electrons are re-energized by light and used to reduce NADP⁺ to NADPH.

Q: What does ATP synthase do?

A: Uses proton gradient energy to synthesize ATP from ADP + Pi.

Q: What are the main products of light reactions?

A: ATP, NADPH, and O₂ (released).

Q: 🔴Why is chlorophyll green?

A: 🔴Because it reflects green light and absorbs mostly red and blue wavelengths, which drive photosynthesis.