CO2 Fixation Calculators

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CO₂ fixation is the biological or chemical process of converting inorganic carbon dioxide into organic carbon compounds. In biology, photosynthetic CO₂ fixation is the foundation of virtually all life on Earth — plants, algae, and cyanobacteria use the Calvin cycle (light-independent reactions) to incorporate CO₂ into 3-carbon compounds that are then used to synthesize carbohydrates, lipids, and amino acids. CO₂ fixation also describes industrial carbon capture processes and is central to understanding climate change, the global carbon cycle, and strategies for carbon sequestration.

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What Is CO₂ Fixation?

CO₂ fixation is the incorporation of inorganic CO₂ into organic molecules. In biology, this is done by autotrophs:

  • Photoautotrophs (plants, algae, cyanobacteria): use light energy via photosynthesis
  • Chemoautotrophs (some Archaea and Bacteria): use chemical energy from oxidation of inorganic compounds (hydrogen, sulfur, ammonia)

The Calvin Cycle (C3 Photosynthesis)

The Calvin cycle (light-independent reactions) occurs in the stroma of chloroplasts in three stages:

  1. Carbon fixation: RuBisCO enzyme catalyzes: CO₂ + RuBP (5-C) → 2 × 3-phosphoglycerate (3-PGA, 3-C). The carboxylation of ribulose-1,5-bisphosphate is the rate-limiting step of photosynthesis globally.
  2. Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH from the light reactions
  3. Regeneration: Most G3P is used to regenerate RuBP; some exits for glucose synthesis

Net: 3CO₂ + 9ATP + 6NADPH → G3P

C4 and CAM Pathways

  • C4 plants (corn, sugarcane): pre-fix CO₂ as 4-carbon compounds (malate) in mesophyll cells; transport to bundle sheath where CO₂ is released to RuBisCO at high concentration — reducing photorespiration in hot, bright conditions
  • CAM plants (cacti, succulents): fix CO₂ at night (stomata open) as malate; release and fix again via Calvin cycle during the day (stomata closed) — minimizes water loss in arid environments

Global CO₂ Fixation

Global terrestrial NPP (photosynthesis minus respiration) fixes ~120 Pg C/year; marine phytoplankton fix ~50 Pg C/year. Total biological CO₂ fixation ~170 Pg C/year — the dominant flux removing CO₂ from the atmosphere. This is balanced by respiration, decomposition, and combustion releasing CO₂ back.

Glossary

CO₂ Fixation
The incorporation of inorganic CO₂ into organic carbon compounds. In biology, performed by autotrophs via the Calvin cycle using RuBisCO. Foundation of virtually all food webs and the primary mechanism removing CO₂ from the atmosphere.
RuBisCO
Ribulose-1,5-bisphosphate carboxylase/oxygenase — the enzyme catalyzing CO₂ fixation in the Calvin cycle: CO₂ + RuBP → 2 × 3-PGA. The most abundant enzyme on Earth. Also catalyzes photorespiration by reacting with O₂ instead of CO₂.
Calvin Cycle
The light-independent reactions of photosynthesis occurring in the chloroplast stroma. Three stages: (1) CO₂ fixation by RuBisCO; (2) reduction of 3-PGA to G3P using ATP and NADPH; (3) regeneration of RuBP. Net: 3CO₂ → one G3P (triose phosphate).

Frequently Asked Questions

CO₂ fixation is the incorporation of atmospheric CO₂ into organic molecules during the Calvin cycle. The enzyme RuBisCO catalyzes the key carboxylation reaction: CO₂ + RuBP (5-carbon) → 2 molecules of 3-phosphoglycerate (3-carbon). This 3-PGA is then reduced using ATP and NADPH (from the light reactions) to produce glyceraldehyde-3-phosphate (G3P) — the organic carbon precursor for glucose, amino acids, and lipids.

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme catalyzing the fixation of CO₂ in the Calvin cycle. It is the most abundant enzyme on Earth — estimated at ~700 million tonnes globally — partly because it is catalytically slow (~3 carboxylation reactions/second) and must be present in huge amounts to meet photosynthetic demand. RuBisCO also catalyzes a competing oxygenation reaction (photorespiration) that wastes energy — a problem C4 and CAM plants have evolved mechanisms to minimize.

C3: CO₂ is fixed directly by RuBisCO into 3-carbon 3-PGA — in all plants. Susceptible to photorespiration at high temperature/low CO₂. C4 (corn, sugarcane): CO₂ is first fixed into 4-carbon malate in mesophyll cells, then concentrated around RuBisCO in bundle sheath cells — effectively a CO₂ pump that suppresses photorespiration. CAM (cacti, succulents): CO₂ is fixed at night as malate (stomata open), then re-released to Calvin cycle in daytime (stomata closed) — minimizing daytime water loss in arid environments.

Biological carbon sequestration is the long-term storage of atmospheric CO₂ in biological systems — primarily in plant biomass, soil organic matter, and oceanic organisms. Forests are the largest terrestrial carbon sink (~2–3 Pg C/year net uptake). Soils store more carbon than all living vegetation combined (~1,500 Pg C in the top 1 meter). Ocean phytoplankton and the biological pump transfer ~10 Pg C/year to deep ocean sediments. Protecting and restoring forests and soils is a primary natural climate solution strategy.