Biology · Ch 12 — Photosynthesis
Dark Reaction
Dark Reaction
Carbon fixation, the dark reaction, occurs in the stroma through a series of enzyme-catalysed steps that use up the ATP and NADPH2 the light reaction produced in the thylakoids. The complete route — from CO2, through several intermediate compounds, to sugar and starch — is the Calvin cycle (also called the C3 pathway), worked out by Melvin Calvin, Andrew Benson and their co-workers by feeding radioactively labelled 14CO2 to the unicellular green alga Chlorella and tracing where the label ended up; Calvin was awarded the 1961 Nobel Prize for this work. The cycle has three stages — carboxylation, reduction (glycolytic reversal) and regeneration of the CO2-acceptor molecule — described individually below.
Adding up the light and dark reactions together gives the balanced overall accounting of one full turn (scaled to fix 6 CO2):
Light reaction (in the granum): 24H₂O → 24OH⁻ + 24H⁺; 24OH⁻ → 24OH + 24e⁻; 24e⁻ + 24H⁺ + 12NADP⁺ → 12NADPH₂; 18ADP + 18Pᵢ → 18ATP; 24OH → 12H₂O + 6O₂↑
Dark reaction (in the stroma): 6CO₂ + 18ATP + 12NADPH₂ → C₆H₁₂O₆ + 6H₂O + 18ADP + 18Pᵢ + 12NADP⁺
Combined: 6CO₂ + 24H₂O → (Light; Chloroplast) C₆H₁₂O₆ + 18H₂O + 6O₂↑ …
What this figure shows. A schematic linking the two halves of photosynthesis inside a chloroplast: light and water enter the thylakoid/granum region where the light reactions occur, producing ATP (from ADP), NADPH (from NADP+) and oxygen; the ATP and NADPH are then shown passing into the stroma to power the dark reactions, which take in carbon dioxide and output glucose, with ADP and NADP+ shown cycling back to the light …
What this figure shows. A circular metabolic map divided into three labelled arcs — Carboxylation, Reduction and Regeneration — tracing 6 molecules of CO2 combining with 6 RuBP (via Rubisco) into an unstable 6-carbon intermediate that immediately splits into 12 molecules of 3-phosphoglycerate; these are converted, using 12 ATP and then 12 NADPH, through 1,3-diphosphoglycerate into 12 molecules of 3-phosphoglyceraldehyde. Of these 12, two are drawn branching off toward Fructose-1,6-diphosphate, Fructose-6-phosphate, Glucose-6-phosphate and finally Glucose (the Synthesis branch), while the remaining ten are shown cycling through Xylulose-5-phosphate, Erythrose-4-phosphate, Sedoheptulose-7-phosphate and Ribose-5-phosphate intermediates, consuming further A …