Turn Lecture Notes into Student Quizzes

Lecture 4 — Photosynthesis: Light-Dependent Reactions

Photosynthesis converts light energy into chemical energy stored in
glucose. The light-dependent reactions occur in the thylakoid
membrane of chloroplasts and produce ATP and NADPH.

Photosystem II (PSII) absorbs light at 680 nm. The energy excites
electrons that travel down the electron transport chain. Water is
split (photolysis) to replace electrons in PSII, releasing oxygen
as a by-product. Photosystem I (PSI) absorbs light at 700 nm and
transfers electrons to NADP+ to form NADPH.

The proton gradient established across the thylakoid membrane drives
ATP synthase, producing ATP from ADP and inorganic phosphate. This
process is called photophosphorylation.

1. Recall
   1. In which structure of the chloroplast do the light-dependent reactions occur?
   • A. Stroma
   • B. Thylakoid membrane ✓
   • C. Outer membrane
   • D. Intermembrane space
   Rationale: The light-dependent reactions occur in the thylakoid membrane.
2. Recall
   2. What is the source of electrons used to replace those lost from Photosystem II?
   • A. NADPH
   • B. Carbon dioxide
   • C. Water (via photolysis) ✓
   • D. Glucose
   Rationale: Photolysis of water replaces electrons in PSII and releases O₂.
3. Application
   3. If a chemical inhibitor blocked ATP synthase in the thylakoid membrane, which outcome would you predict?
   • A. NADPH production would stop immediately
   • B. The proton gradient would build up but ATP would not form ✓
   • C. Oxygen production would increase
   • D. Chlorophyll would no longer absorb light
   Rationale: Blocking ATP synthase prevents protons from flowing back, so the gradient builds up but ATP synthesis halts.