Biochemistry
Photosynthesis Dark Reactions (Calvin Cycle)
CO₂ fixation into sugar — uses ATP and NADPH from light reactions
The Calvin cycle (also called dark reactions or light-independent reactions) fixes CO₂ into organic carbon. Occurs in chloroplast stroma. Uses ATP and NADPH (from light reactions) to drive synthesis. Three phases: (1) Carbon fixation — RuBisCO attaches CO₂ to RuBP (5-carbon sugar) → 6-carbon intermediate → splits to 2 × 3PG. (2) Reduction — 3PG → G3P (sugar) using ATP and NADPH. (3) Regeneration — most G3P used to regenerate RuBP. 6 cycles needed to net produce 1 glucose. RuBisCO is most abundant protein on Earth.
- LocationChloroplast stroma
- Key enzymeRuBisCO (most abundant protein on Earth)
- InputsCO₂, ATP, NADPH (from light reactions)
- OutputG3P (3-carbon sugar; precursor to glucose)
- Per glucose6 CO₂ fixed; 18 ATP, 12 NADPH used
- DiscoveryMelvin Calvin (Nobel 1961)
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Why Calvin cycle matters
- Carbon fixation. Pulls CO₂ from atmosphere.
- Food production. Sugar made from inorganic carbon.
- Climate. Major sink for CO₂.
- Crop yields. RuBisCO is rate-limiting.
- Synthetic biology. Engineering better RuBisCO.
- Photorespiration. Major loss in C3 plants.
- Plant ecology. Adaptation to climate.
Common misconceptions
- Dark reactions only at night. Light-independent; work daytime.
- Direct glucose production. G3P first; combined to make glucose.
- RuBisCO efficient. Slow and prone to errors.
- One CO₂ = one G3P. 3 cycles for one G3P from net.
- All plants C3. C4 and CAM also exist.
- Calvin cycle only in plants. Cyanobacteria, algae too.
Frequently asked questions
How does the Calvin cycle work?
Three phases. (1) Carbon fixation: RuBisCO catalyzes CO₂ + RuBP (5C) → unstable 6C → splits to 2 × 3PG (3C). (2) Reduction: 3PG + ATP → 1,3-BPG → G3P (using NADPH). (3) Regeneration: most G3P → RuBP (using more ATP). Per turn: 1 CO₂ fixed; 1 G3P out (1/6 needed for net glucose). 6 turns produce enough G3P for one glucose (6C).
What is RuBisCO?
Ribulose-1,5-bisphosphate carboxylase/oxygenase. Catalyzes CO₂ + RuBP → 2 × 3PG. Most abundant protein on Earth (50% of leaf protein). But: slow (3 reactions/sec) and inefficient — also catalyzes O₂ instead of CO₂ (photorespiration). C3 plants accept this loss; C4 and CAM plants evolved to overcome.
Why is it called "dark"?
Doesn't directly require light — works in light or dark. But: depends on ATP and NADPH from light reactions. So functionally requires light most of time. Better term: "light-independent reactions." Despite name, occurs during day in plants. "Dark" refers to no direct light involvement, not nighttime.
What's photorespiration?
RuBisCO sometimes binds O₂ instead of CO₂ → produces phosphoglycolate (waste). Glycolate must be processed via complex pathway involving 3 organelles, releasing CO₂. Wastes energy and carbon. Worse at high T (O₂/CO₂ ratio favorable to O₂). Reduces photosynthesis efficiency by ~25% in C3 plants. Reason for evolution of C4 and CAM photosynthesis.
How does this relate to glucose?
G3P (3-carbon sugar) is direct product. Net per turn: 1 G3P. 6 G3P combine to make 1 glucose (6C) via gluconeogenesis-like steps. Some G3P also used for: starch synthesis, sucrose synthesis (for transport to other plant parts), other biosynthesis. Glucose is one of many products from photosynthesis.
What ATP and NADPH balance?
Per glucose: 18 ATP, 12 NADPH consumed. Light reactions must produce these in correct ratio. ATP:NADPH = 18:12 = 1.5:1. PSII + PSI together: 2 H₂O → 1 O₂ + 4 e⁻ + 4 H⁺ → produces 2 NADPH and ~3 ATP per O₂. Calvin cycle thus consumes ~6 O₂ worth of light reactions per glucose. Cyclic electron flow (PSI alone) can provide extra ATP if needed.
What's C3 photosynthesis?
Standard mechanism. CO₂ fixed directly to 3-carbon compound (3PG). Used by ~85% of plants — wheat, rice, soybean. Inefficient at high T due to photorespiration. Evolved earlier; works fine in cool, wet conditions. Most familiar plants are C3.