§ B-10 Chloroplast Photosynthesis: Z-Scheme Electron Transport & Calvin Cycle CO₂ Fixation (Grade 9–11 · Photosynthesis & Calvin Cycle)
Thylakoid Light Reactions (PSII, Water Photolysis, PSI) & Stroma Calvin Cycle (RuBisCO). “Thylakoid chlorophylls transduce photon energy hν into NADPH and ATP while splitting water to release O₂, enabling RuBisCO in the stroma to fix atmospheric CO₂ into hexose sugars.”
Theory Summary & Scientific Mechanism
Photosynthesis in chloroplasts comprises the light-dependent reactions on thylakoid membranes and the Calvin–Benson carbon-fixation cycle in the stroma. Chlorophylls a and b absorb strongly in the blue (430–450 nm) and red (660–680 nm) spectral bands while reflecting green light (sim 540 nm).
Along the non-cyclic Z-scheme, photon absorption excites Photosystem II (P680), which extracts electrons from water at the Mn₄CaO₅ oxygen-evolving complex (water photolysis: 2H₂O xrightarrow4hν O₂↑ + 4H⁺ + 4e⁻, the sole source of evolved O₂). Electrons pass through plastoquinone and cytochrome b₆f (building a lumenal H⁺ gradient for CF₀CF₁-ATP synthase) to Photosystem I (P700), which reduces NADP⁺ to NADPH.
In the stroma, RuBisCO catalyzes CO₂ carboxylation of ribulose-1,5-bisphosphate. Synthesizing 1 molecule of glucose (C₆H₁₂O₆) from 6 CO₂ in the Calvin cycle consumes 18 ATP and 12 NADPH.
Everyday Analogy
A chloroplast runs a two-stage factory: thylakoid solar panels capture red and blue photons, split water (releasing the O₂ we breathe!), and charge ATP and NADPH batteries. Then the Calvin cycle sugar mill spends 18 ATP and 12 NADPH to assemble 6 CO₂ into one glucose molecule!
Interactive Question & Derivation
During non-cyclic photophosphorylation in the chloroplast (the Z-scheme) followed by the Calvin cycle, one molecule of glucose C₆H₁₂O₆ is synthesized. Which substrate donates the electrons and all evolved molecular oxygen O₂, and how many ATP and NADPH molecules are consumed by the Calvin cycle to fix 6 CO₂?
- A: Molecular O₂ is cleaved directly from CO₂ in the Calvin cycle without water
- B: All evolved O₂ originates from water photolysis at Photosystem II (2H₂O → O₂ + 4H⁺ + 4e⁻), and synthesizing 1 glucose from 6 CO₂ in the Calvin cycle requires 18 ATP and 12 NADPH
- C: The Calvin cycle takes place inside the thylakoid lumen and emits photons
- undefined: Green light (λ ≈ 540 nm) is absorbed ten times more strongly by chlorophyll than blue (440 nm) or red (670 nm) light
Study Hint: All evolved O₂ comes from water photolysis at PSII, and fixing 6 CO₂ into 1 glucose in the Calvin cycle consumes 18 ATP and 12 NADPH.
Proof & Explanation: Isotopic tracer experiments with H₂¹⁸O proved that all photosynthetic O₂ comes from water splitting at the Mn₄CaO₅ oxygen-evolving complex of Photosystem II (P680): 2H₂O xrightarrow4hν O₂↑ + 4H⁺ + 4e⁻. Electrons travel via plastoquinone, cytochrome b₆f, and plastocyanin to Photosystem I (P700) to reduce NADP⁺ to NADPH, while the lumen-to-stroma proton gradient drives CF₀CF₁-ATP synthase. In the stroma, fixing each CO₂ molecule in the Calvin–Benson cycle requires 3 ATP and 2 NADPH, totaling 18 ATP and 12 NADPH per molecule of glucose ( extC₆H₁₂O₆).
Key Equations & 3D Simulation Parameters
vₚhoto = vₘax (I)/(I + K_I) ([CO₂])/([CO₂] + K_C) α(λ)
- Photosynthetically active irradiance PAR I: 50 … 1800 μmol/(m²·s) (default: 800 μmol/(m²·s))
- Ambient carbon dioxide concentration [CO₂]: 100 … 1000 ppm (default: 420 ppm)
- Illumination wavelength λ (chlorophyll peaks at 440 nm & 670 nm): 420 … 700 nm (default: 660 nm)
- Leaf temperature t (RuBisCO optimum ~25 °C): 10 … 40 °C (default: 25 °C)