Light Reactions and the Calvin Cycle

By the end of this lesson, you will be able to trace the flow of electrons through the light reactions, explain how ATP and NADPH are produced, describe the three stages of the Calvin cycle, and connect both stages.

Light Reactions: Tracing the Electrons

Photosystem II (PSII) → Photosystem I (PSI) (counterintuitive naming — PSII comes first).

Step-by-step:

1. Light hits PSII → chlorophyll is excited, high-energy electrons released
2. Water is split (photolysis): $2H_2O \to 4H^+ + 4e^- + O_2$. O₂ is released; electrons replace those lost from PSII
3. Electrons move through the electron transport chain (plastoquinone, cytochrome complex, plastocyanin)
4. H⁺ pumped into thylakoid space → builds a proton gradient
5. H⁺ flows through ATP synthase → ATP produced (chemiosmosis)
6. Electrons reach PSI → re-energized by light
7. Ferredoxin transfers electrons to NADP⁺ reductase → NADPH produced

Net products of light reactions: ATP, NADPH, O₂

Calvin Cycle: Three Stages

Located in the stroma. Uses ATP and NADPH from light reactions.

Stage 1: Carbon Fixation

• CO₂ combines with RuBP (5-carbon) via RuBisCO enzyme → unstable 6-carbon compound → two molecules of 3-PGA (3-phosphoglycerate)

Stage 2: Reduction

• ATP and NADPH convert 3-PGA → G3P (glyceraldehyde-3-phosphate)
• G3P is the actual sugar product — can be used to make glucose, starch, or other organic molecules

Stage 3: Regeneration of RuBP

• Most G3P molecules used to regenerate RuBP (5-carbon acceptor)
• Requires ATP
• Cycle continues

The "Balance Sheet" for One Glucose

To produce one molecule of glucose ($C_6H_{12}O_6$) requires the Calvin cycle to run 6 times:

• 6 CO₂ fixed
• 12 ATP used in reduction; 6 ATP used in RuBP regeneration → 18 ATP total
• 12 NADPH used
• 1 G3P "exported" (the other 5 used to regenerate RuBP)

The Connection Between the Two Stages

The light reactions are not just a setup for the Calvin cycle — they are coupled: