9.6
The Calvin Cycle
In autotrophic plants, the Calvin Cycle starts when atmospheric carbon dioxide eventually diffuses into the stroma of the chloroplast.
Here, one carbon atom from the carbon dioxide is added or fixed to a five-carbon acceptor sugar molecule, ribulose bisphosphate, or RuBP, in a reaction catalyzed by the enzyme Ribulose 1,5-bisphosphate-carboxylase-oxygenase, or RuBisCo for short. The resulting six-carbon molecule is highly unstable and splits into two three-carbon molecules of 3-phosphoglyceric acid, 3-PGA.
With ATP providing the energy, and NADPH affixing one hydrogen to each, the three PGA chains are converted into another three-carbon intermediate called glyceraldehyde-3-phosphate. One G3P then exits the cycle and waits for another one to build glucose with six carbon atoms.
Meanwhile, the remaining G3P must wait for four more cycles as carbons accumulate and ATP provides more energy to regenerate the RuBP acceptors. Overall, six turns of the Calvin Cycle fix six carbon dioxides from the atmosphere using the energy and reducing power of 18 ATPs and 12 NADPHs, respectively, to generate one molecule of glucose and rebuild RuBP to continue the loop.
Overview
Oxygenic photosynthesis plays a central role in the global carbon and oxygen cycles. The carbohydrates produced support nearly all food webs, while the oxygen by‑product enables aerobic life.
Light‑dependent and light‑independent reactions
Photosynthesis occurs in two main stages, each in a different part of the chloroplast: light‑dependent reactions and light‑independent reactions, also called the Calvin‑Benson cycle or simply the Calvin cycle.
Light‑dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy, which is used to split water molecules, releasing O₂ and generating ATP and NADPH. These energy carriers then move to the stroma, where the Calvin‑Benson cycle occurs.
The Calvin‑Benson cycle uses ATP and NADPH to reduce atmospheric CO₂ into carbohydrates. ADP and NADP⁺, produced as by‑products, return to the thylakoids to be recycled in the light reactions.
Calvin‑Benson Cycle
The Calvin‑Benson cycle consists of three phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor (RuBP).
- Carbon fixation: CO₂ enters the leaf through stomata. In the stroma, the enzyme RuBisCO attaches each CO₂ molecule to ribulose‑1,5‑bisphosphate (RuBP), forming an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA).
- Reduction: Each 3‑PGA molecule is phosphorylated by ATP to form 1,3‑bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde‑3‑phosphate (G3P), a three‑carbon sugar. For every six CO₂ molecules fixed, two G3P molecules exit the cycle. These G3P molecules help form glucose and other organic compounds.
- Regeneration: The remaining G3P molecules undergo a series of reactions that regenerate RuBP, using additional ATP. This regeneration step allows the cycle to continue fixing CO₂.
In autotrophic plants, the Calvin Cycle starts when atmospheric carbon dioxide eventually diffuses into the stroma of the chloroplast.
Here, one carbon atom from the carbon dioxide is added or fixed to a five-carbon acceptor sugar molecule, ribulose bisphosphate, or RuBP, in a reaction catalyzed by the enzyme Ribulose 1,5-bisphosphate-carboxylase-oxygenase, or RuBisCo for short. The resulting six-carbon molecule is highly unstable and splits into two three-carbon molecules of 3-phosphoglyceric acid, 3-PGA.
With ATP providing the energy, and NADPH affixing one hydrogen to each, the three PGA chains are converted into another three-carbon intermediate called glyceraldehyde-3-phosphate. One G3P then exits the cycle and waits for another one to build glucose with six carbon atoms.
Meanwhile, the remaining G3P must wait for four more cycles as carbons accumulate and ATP provides more energy to regenerate the RuBP acceptors. Overall, six turns of the Calvin Cycle fix six carbon dioxides from the atmosphere using the energy and reducing power of 18 ATPs and 12 NADPHs, respectively, to generate one molecule of glucose and rebuild RuBP to continue the loop.
From Chapter 9:
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