Photosynthesis in Higher Plants · Lesson 6 of 10
The Calvin Cycle: Carbon Fixation, Reduction and Regeneration
“Explain how ATP and NADPH support RuBP carboxylation, triose-phosphate production and sugar formation.”
• Explain the experiments that revealed 3-PGA as an early stable product. • Trace carboxylation, reduction and regeneration in stroma. • Explain why RuBP rather than a two-carbon compound accepts CO₂. • Calculate ATP and NADPH requirements for three and six CO₂ molecules.
Where do ATP and NADPH go after the light reactions?
The light reactions release O₂ and generate ATP and NADPH. These two molecules allow the stroma to carry out carbon-reduction reactions that convert inorganic CO₂ into organic compounds. The Calvin cycle is a cyclic set of enzyme-catalysed reactions, also called the Calvin–Benson cycle. It does not directly require photons for each reaction step, but it uses products of light reactions and is regulated in ways that generally favour operation in the light. Calling it a reaction that occurs only in darkness is therefore incorrect.
Melvin Calvin and colleagues used radioactively labelled carbon dioxide to trace the earliest products formed by photosynthetic organisms such as algae. One early stable labelled product was 3-phosphoglycerate, abbreviated 3-PGA, a three-carbon compound. This evidence led scientists to search for the carbon acceptor and the route by which sugars are formed. Unexpectedly, the principal acceptor is not a two-carbon molecule but the five-carbon sugar bisphosphate ribulose-1,5-bisphosphate, abbreviated RuBP.
Step one: carboxylation by RuBisCO
RuBP contains five carbon atoms. The enzyme RuBisCO catalyses addition of one CO₂, giving a transient six-carbon intermediate that immediately splits into two molecules of 3-PGA, each containing three carbons. The carbon balance is therefore 5C + 1C → 2 × 3C. This is the initial stable product of the Calvin cycle in both ordinary C₃ plants and C₄ plants, though C₄ species also use an earlier mesophyll fixation step that produces oxaloacetate.
RuBisCO is an abbreviation for ribulose-1,5-bisphosphate carboxylase/oxygenase. Its carboxylase activity adds CO₂ to RuBP; its oxygenase activity competes with that reaction and leads toward photorespiration. At this stage focus on successful carboxylation: CO₂ is chemically incorporated into an organic acid, but an energy-rich sugar has not yet been completed. The subsequent reduction and regeneration steps are essential if fixation is to continue repeatedly.
Step two: reduction to triose phosphate
The 3-PGA molecules are processed using ATP and reducing equivalents from NADPH. For each CO₂ molecule fixed, the reduction phase accounts for the use of two ATP and two NADPH in the conventional net accounting. ATP provides energy for phosphorylation, while NADPH supplies electrons that reduce the carbon intermediate to a three-carbon sugar phosphate such as glyceraldehyde-3-phosphate (G3P). Some of this triose phosphate can leave the cycle for production of sucrose, starch and other carbon compounds.
The word reduction has a chemical meaning: gaining electrons or, in this context, acquiring reducing equivalents supplied by NADPH. The reduction stage is not simply the reduction of CO₂ gas into a finished glucose molecule in one step. Multiple transformations and carbon rearrangements occur while ATP and NADPH are consumed and ADP, Pi and NADP⁺ are returned for reuse. The stroma must continually receive new ATP and NADPH from the illuminated thylakoid system.
Step three: regenerate the five-carbon acceptor
If every three-carbon product were removed from the cycle, RuBP would soon be exhausted. Most triose phosphate is therefore rearranged through a sequence of enzyme reactions to rebuild RuBP. Regeneration consumes another ATP per net CO₂ fixed. Carbon conservation explains the need for the cycle: fixation creates more organic carbon, but the acceptor must be recreated to enable the next carboxylation event. RuBP regeneration is not a separate source of carbon; it reorganises part of the recently fixed carbon and pre-existing cycle intermediates.
A precise accounting of molecules
For three CO₂ molecules, three RuBP molecules each accept one carbon dioxide and produce a total of six 3-PGA molecules. These are converted into six triose-phosphate equivalents. Five triose phosphates are reorganised to regenerate three RuBP molecules, leaving one net three-carbon triose-phosphate equivalent available for biosynthesis. The net energy cost for three CO₂ is nine ATP and six NADPH. Doubling this to six CO₂ yields two net three-carbon equivalents that can form one six-carbon hexose equivalent, consuming eighteen ATP and twelve NADPH.
A net hexose equivalent is not always released as a free molecule of glucose at the end of six turns. Sugar phosphates are subsequently converted into sucrose, starch or other compounds through additional metabolism. The conventional 'one glucose from six CO₂' accounting captures carbon and energy requirements rather than specifying a single final sugar product in all cell types. This distinction is useful when a diagram shows triose phosphate leaving instead of drawing glucose directly inside the cycle.
| Net carbon input | Main net export | ATP cost | NADPH cost |
|---|---|---|---|
| 1 CO₂ | One-sixth hexose equivalent | 3 | 2 |
| 3 CO₂ | One triose-phosphate equivalent | 9 | 6 |
| 6 CO₂ | Two triose equivalents / one hexose equivalent | 18 | 12 |
Why C₃ and C₄ plants both use the Calvin cycle
C₃ plants obtain CO₂ directly for RuBisCO-mediated carboxylation in mesophyll chloroplasts. In C₄ plants, an earlier carbon-concentrating pathway delivers CO₂ to bundle sheath cells, where the same Calvin-cycle chemistry operates. The terms C₃ and C₄ refer to the identity of the first stable fixation product of the initial CO₂-capture pathway, not to entirely different final sugar-synthesis cycles. Both need ATP, NADPH and RuBP regeneration to assimilate carbon into carbohydrate.
Each net CO₂ fixed through the Calvin cycle costs 3 ATP and 2 NADPH. Therefore six net CO₂ require 18 ATP and 12 NADPH for one hexose equivalent.
Quick Quiz
Quiz
Which molecule is the direct CO₂ acceptor in the Calvin cycle?
What are the first stable products after one RuBP is carboxylated?
Which compounds supply energy and reducing electrons to the Calvin cycle?
How many ATP molecules are needed per net CO₂ fixed in conventional Calvin-cycle accounting?
How many NADPH molecules are required for one six-carbon hexose equivalent from six CO₂?
Practice Problems
- Explain each stage of the Calvin cycle and its purpose.
- Why did scientists initially search for a two-carbon acceptor, and what did they discover?
- Calculate ATP and NADPH required to assimilate six CO₂ through Calvin cycle.
- Explain why the cycle would stop if RuBP were not regenerated.
- Why is the Calvin cycle not accurately described as a process that requires darkness?
Model Answers
1. Carboxylation uses RuBisCO to add CO₂ to RuBP and forms 3-PGA. Reduction consumes ATP and NADPH to produce triose phosphate. Regeneration consumes ATP to restore RuBP and allow repeated fixation.
2. Because the first stable product had three carbons, a two-carbon acceptor seemed plausible. Isotope-tracing and pathway analysis instead identified five-carbon RuBP, which yields an unstable six-carbon intermediate that splits into two three-carbon products.
3. Six CO₂ × 3 ATP = 18 ATP; six CO₂ × 2 NADPH = 12 NADPH, yielding one six-carbon carbohydrate equivalent through two net triose-phosphate products.
4. Each carboxylation consumes a RuBP acceptor; without ATP-powered regeneration, the pool of five-carbon acceptors would be depleted and further CO₂ fixation could not continue.
5. The cycle does not use photons directly but depends on ATP and NADPH and light-regulated enzymes, so it generally runs during illumination rather than being confined to darkness.
Key Takeaways
• RuBisCO adds CO₂ to the five-carbon acceptor RuBP, giving two 3-PGA molecules. • Reduction consumes ATP and NADPH to form triose phosphate; regeneration uses ATP to rebuild RuBP. • Three CO₂ yield one net triose equivalent at a cost of 9 ATP and 6 NADPH. • Six CO₂ cost 18 ATP and 12 NADPH for one net hexose equivalent. • The Calvin cycle operates in both C₃ and C₄ photosynthesis, although CO₂ delivery differs.