AP Biology: Photosynthesis – Light?Dependent Reactions (Photosystems II & I, ETC, NADPH, ATP)

Photosynthesis – Light?Dependent Reactions (Photosystems II & I, ETC, NADPH, ATP)

Concept Summary

• Photosynthesis: Process converting light energy into chemical energy (glucose), occurring in chloroplasts of plants, algae, and cyanobacteria.
• Light-dependent reactions: First stage of photosynthesis, producing ATP and NADPH using light energy, occurring in the thylakoid membrane.
• Photosystem II (PSII): Protein-pigment complex that absorbs light (P680), splits water (photolysis), and initiates the electron transport chain (ETC).
• Electron Transport Chain (ETC): Series of protein complexes (e.g., cytochrome b6f) that pump protons into the thylakoid lumen, creating a proton gradient for ATP synthesis.
• Photosystem I (PSI): Protein-pigment complex (P700) that re-energizes electrons to reduce NADP? to NADPH via ferredoxin.

Core Questions

WHAT (definitional)

Q: What is photolysis? A: The splitting of water molecules by PSII into O?, protons (H?), and electrons (e?) using light energy. Trap/Clarification: Photolysis is not the same as the Calvin cycle—it’s the source of electrons for the ETC and O? as a byproduct.

Q: What is the role of NADP? reductase? A: Enzyme that catalyzes the reduction of NADP? to NADPH using electrons from PSI and protons from the stroma. Trap/Clarification: NADP? reductase does not produce ATP—it only generates NADPH.

WHY (causal/explanatory)

Q: Why is the proton gradient important in the light-dependent reactions? A: The proton gradient (high H? in thylakoid lumen) drives ATP synthesis via ATP synthase (chemiosmosis). Trap/Clarification: The gradient is not used directly for NADPH production—it’s solely for ATP.

Q: Why does PSII come before PSI in the ETC? A: PSII’s P680 has a higher redox potential, allowing it to oxidize water and pass electrons to PSI (P700), which has a lower redox potential. Trap/Clarification: Electrons flow downhill energetically from PSII-PSI, not the reverse.

HOW (process/application)

Q: How is ATP generated in the light-dependent reactions? A: Protons (H?) flow down their gradient through ATP synthase (chemiosmosis), catalyzing ADP + P?-ATP. Trap/Clarification: ATP is not made by substrate-level phosphorylation here—it’s oxidative phosphorylation via a proton motive force.

Q: How do electrons move from PSII to PSI? A: Electrons are passed through the ETC (plastoquinone-cytochrome b6f-plastocyanin) to PSI, losing energy to pump protons. Trap/Clarification: The ETC does not directly produce NADPH—it only sets up the proton gradient.

CAN (conditions/possibilities)

Q: Can the light-dependent reactions occur without water? A: No—water is the electron donor for PSII; without it, the ETC stalls, halting ATP/NADPH production. Trap/Clarification: Some bacteria use H?S instead of H?O, but plants require water for photolysis.

Q: Under what conditions does cyclic electron flow occur? A: When NADPH levels are high, electrons cycle back from PSI to the ETC (via ferredoxin) to produce only ATP (no NADPH). Trap/Clarification: Cyclic flow does not involve PSII or water splitting—it’s a "short circuit" for ATP.

Quick Facts & Traps

• Fact: P680 (PSII) absorbs light at 680 nm, while P700 (PSI) absorbs at 700 nm—this difference drives the linear ETC.
• Trap: "Oxygen is produced by PSI"-Reality: O? comes from PSII’s photolysis of water, not PSI.
• Fact: Cytochrome b6f is the proton pump in the ETC, analogous to Complex III in mitochondria.
• Trap: "ATP synthase is part of the ETC"-Reality: ATP synthase uses the proton gradient after the ETC creates it.
• Fact: Ferredoxin transfers electrons from PSI to NADP? reductase (non-cyclic) or back to the ETC (cyclic).
• Trap: "NADPH is used in the Calvin cycle to make ATP"-Reality: NADPH provides reducing power (electrons), not ATP.

Rapid-Fire True/False

• Statement: The light-dependent reactions produce glucose. Answer: FALSE Why the common mistake happens: Confusion with the Calvin cycle (light-independent reactions), which uses ATP/NADPH to fix CO? into glucose.

• Statement: Electrons from PSII replace those lost by PSI. Answer: FALSE Why the common mistake happens: Electrons from water (via PSII) replace PSII’s lost electrons; PSI’s electrons come from the ETC (not PSII directly).

• Statement: ATP and NADPH are produced in equal amounts during non-cyclic electron flow. Answer: FALSE Why the common mistake happens: The stoichiometry varies—typically more ATP is needed for the Calvin cycle, so cyclic flow supplements ATP production.