Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light energy. Sugars are used at once or stored (e.g. as starch).
- Origins: photosynthesis first evolved in prokaryotes. Cyanobacteria first oxygenated the atmosphere (sharp rise about 2.4 billion years ago). Chloroplasts descend from cyanobacteria, so eukaryotic photosynthesis is built on the prokaryotic pathway.
- Light reactions (thylakoid membranes, grana): light boosts electrons in photosystems II and I; water is split to replace PSII’s electrons and O₂ is released; electrons pass PSII → ETC → PSI → NADP⁺, forming NADPH.
- Proton gradient: ETC redox reactions pump H⁺ into the thylakoid space. High H⁺ inside, low H⁺ in the stroma. H⁺ flows back through ATP synthase, making ATP: chemiosmosis, here called photophosphorylation.
- Calvin cycle (stroma): ATP and NADPH power the building of carbohydrate from CO₂. ADP, Pᵢ and NADP⁺ return to the light reactions.
- ETCs also run in mitochondria and across prokaryotic plasma membranes.
- Not needed: Calvin cycle steps, molecule structures, enzyme names (except ATP synthase), names of ETC carriers.
Key relationships
| Stage | Where | In | Out |
|---|---|---|---|
| Light reactions | thylakoid membranes (grana) | light, H₂O, ADP + Pᵢ, NADP⁺ | O₂, ATP, NADPH |
| Calvin cycle | stroma | CO₂, ATP, NADPH | carbohydrate, ADP + Pᵢ, NADP⁺ |
| Calculation | Rule |
|---|---|
| Moles | moles = mass ÷ molar mass (CO₂ 44.01, H₂O 18.02, O₂ 32.00, glucose 180.16 g mol⁻¹) |
| Ratios | 6 CO₂ : 6 H₂O : 1 glucose : 6 O₂ |
| pH difference | each pH unit = tenfold change in H⁺ concentration |
Assumptions behind the numbers
- Equation calculations give the maximum sugar; a real plant also uses sugar in respiration.
- Glucose stands in for all the carbohydrate made.
Mistakes to avoid
- Saying O₂ comes from CO₂ (it comes from water).
- Putting the Calvin cycle in the thylakoids, or the light reactions in the stroma.
- Saying ATP synthase pumps protons (the ETC pumps; ATP synthase lets them flow back).
- Putting the high H⁺ concentration in the stroma.
- Saying the Calvin cycle runs in the dark.
- Forgetting the 6 : 1 ratio between CO₂ and glucose.
Quick self-check
- A plant fixes 0.12 mol CO₂ into glucose. What mass of glucose and of O₂? (0.12 ÷ 6 = 0.020 mol glucose × 180.16 = 3.60 g; 0.12 mol O₂ × 32.00 = 3.84 g)
- Thylakoid space pH 5, stroma pH 8. How many times more H⁺ inside? (10³ = 1000 times)
- Why does the Calvin cycle stop soon after the light is switched off? (ATP and NADPH from the light reactions run out.)
Next: practice questions.