9.2 Calvin cycle and carbon fixation

The Calvin Cycle

The Calvin cycle is the part of photosynthesis that actually builds sugar from $CO_2$. While the light reactions capture solar energy as $ATP$ and $NADPH$, the Calvin cycle spends that energy to fix carbon into organic molecules. It takes place in the stroma of the chloroplast and runs in three stages: carbon fixation, reduction, and regeneration of $RuBP$.

Steps of the Calvin Cycle

1. Carbon Fixation

The enzyme RuBisCO attaches a molecule of $CO_2$ to a 5-carbon sugar called ribulose bisphosphate (RuBP), forming an unstable 6-carbon compound. That compound immediately splits into two molecules of 3-phosphoglycerate (3-PGA), each with 3 carbons. This is the actual moment inorganic carbon becomes part of an organic molecule.

2. Reduction

Energy from the light reactions is used here. First, $ATP$ donates a phosphate group to each 3-PGA (catalyzed by phosphoglycerate kinase), producing 1,3-bisphosphoglycerate. Then $NADPH$ donates electrons to reduce 1,3-bisphosphoglycerate into glyceraldehyde 3-phosphate (G3P) (catalyzed by G3P dehydrogenase). G3P is the direct sugar output of the Calvin cycle.

3. Regeneration of RuBP

For every 3 turns of the cycle (fixing 3 $CO_2$), 6 molecules of G3P are produced, but only 1 G3P exits the cycle as net product. The other 5 are rearranged through a series of enzyme-catalyzed reactions (involving transketolase, aldolase, and phosphatases) to rebuild 3 molecules of $RuBP$. The final step uses the enzyme phosphoribulokinase, which adds a phosphate from $ATP$ to ribulose 5-phosphate, regenerating $RuBP$ so the cycle can continue.

To build one 6-carbon glucose molecule, the cycle must turn 6 times, fixing 6 $CO_2$.

Carbon Fixation and RuBisCO

Carbon fixation is the conversion of inorganic $CO_2$ into an organic compound. The enzyme responsible is RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), and it's the most abundant protein on Earth. That abundance is partly because RuBisCO is remarkably slow, fixing only about 3–10 $CO_2$ molecules per second. Plants compensate by producing a lot of it.

RuBisCO has a significant limitation: it can also bind $O_2$ instead of $CO_2$, triggering a wasteful process called photorespiration. Photorespiration produces no $ATP$ or sugar and actually costs the plant energy to recover from. This is why RuBisCO's full name includes "oxygenase." On hot, dry days when stomata close and $O_2$ builds up relative to $CO_2$ inside the leaf, photorespiration becomes a bigger problem.

Despite this inefficiency, RuBisCO is essential for photosynthesis in plants, algae, and cyanobacteria, and it plays a central role in the global carbon cycle by pulling $CO_2$ out of the atmosphere.

Regeneration of RuBP

Regeneration is what makes the Calvin cycle a cycle. Without it, the cell would run out of $RuBP$ and carbon fixation would stop after a single round.

• 5 out of every 6 G3P molecules are recycled back into $RuBP$
• Enzymes like transketolase and aldolase shuffle the carbon skeletons of various sugar phosphates (3-, 4-, 5-, 6-, and 7-carbon intermediates) to eventually produce ribulose 5-phosphate
• Phosphatases remove phosphate groups where needed during these rearrangements
• In the final step, phosphoribulokinase uses $ATP$ to phosphorylate ribulose 5-phosphate into $RuBP$

This regeneration phase consumes 3 of the 9 $ATP$ used per 3 turns of the cycle.

Significance of the Light-Independent Reactions

The Calvin cycle is sometimes called the "light-independent reactions," though it still depends on the light reactions for its supply of $ATP$ and $NADPH$. It doesn't use light directly, but it can't run without the products of the reactions that do.

• Carbon source for life: The Calvin cycle is the primary pathway that converts atmospheric $CO_2$ into organic carbon. The G3P it produces is used to synthesize glucose, sucrose, starch, amino acids, and fatty acids.
• Energy for ecosystems: Glucose and other organic molecules built from Calvin cycle products fuel the plant itself through cellular respiration and feed every organism up the food chain.
• C3 pathway: The Calvin cycle is the main carbon fixation pathway in C3 plants, which make up the majority of plant species. (C4 and CAM plants use additional steps to concentrate $CO_2$ before it enters the Calvin cycle, reducing photorespiration.)
• Climate regulation: By removing $CO_2$ from the atmosphere and incorporating it into biomass, the Calvin cycle is a critical part of the global carbon cycle and helps regulate atmospheric $CO_2$ levels.