OCR Specification focus:
‘Explain how products of the light-dependent stage drive the Calvin cycle to form triose phosphate, referencing RuBP, RuBisCO and GP.’
The light-independent reactions, also known as the Calvin cycle, are central to photosynthesis, using ATP and reduced NADP from the light-dependent stage to fix carbon dioxide into organic molecules essential for life.
Overview of the Calvin Cycle
The Calvin cycle occurs in the stroma of the chloroplast, the fluid-filled space surrounding the thylakoid membranes. It is termed “light-independent” because it does not require light directly, though it relies entirely on the ATP and reduced NADP (NADPH) produced during the light-dependent reactions. These reactions supply the energy and reducing power needed to convert inorganic carbon dioxide into triose phosphate (TP), a three-carbon compound that forms the basis of carbohydrates and other biomolecules.
The Calvin cycle can be divided into three main stages:
Carbon fixation (incorporating CO₂ into organic molecules)
Reduction (conversion of fixed carbon into triose phosphate)
Regeneration (reformation of RuBP to continue the cycle)
Each stage is catalysed by specific enzymes and tightly regulated to maintain balance between synthesis and regeneration.
Overview of the Calvin–Benson cycle in the chloroplast stroma. CO₂ is fixed to RuBP by RuBisCO, generating GP, which is reduced to TP using ATP and reduced NADP (NADPH) from the light-dependent stage. Most TP regenerates RuBP, with a small proportion leaving for carbohydrate synthesis. Source.
Stage 1 – Carbon Fixation
Carbon dioxide from the atmosphere diffuses into the stroma and combines with ribulose bisphosphate (RuBP), a 5-carbon compound. This reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase).
RuBisCO (spinach enzyme shown) catalyses the carboxylation of RuBP, yielding two molecules of GP. The active site coordinates Mg²⁺ and facilitates CO₂ addition, initiating carbon fixation. Extra structural detail (subunits and tertiary structure) exceeds OCR requirements but clarifies the enzyme’s identity and importance. Source.
RuBisCO: The enzyme that catalyses the fixation of CO₂ to RuBP in the Calvin cycle, producing two molecules of glycerate 3-phosphate (GP).
The reaction is highly specific but relatively slow, which is why plants must produce large amounts of RuBisCO. For each molecule of CO₂ fixed, an unstable 6-carbon intermediate is formed, which immediately splits into two molecules of glycerate 3-phosphate (GP), each containing three carbons.
This fixation of carbon is the first step in converting inorganic carbon into an organic compound that can be utilised in metabolism.
Stage 2 – Reduction of GP to Triose Phosphate
Each molecule of GP is reduced to triose phosphate (TP) using energy and reducing power from the light-dependent stage:
ATP from the light-dependent reactions provides energy for the conversion.
Reduced NADP (NADPH) donates hydrogen ions to reduce GP.
During this step:
2 ATP molecules are hydrolysed per CO₂ fixed.
2 NADPH molecules are oxidised to NADP⁺, which are then recycled back to the thylakoids for reuse in the light-dependent stage.
Triose phosphate (TP): A 3-carbon compound produced in the Calvin cycle; a key intermediate that can form carbohydrates, lipids, and amino acids.
This reduction step transforms GP into TP, increasing the energy content of the molecule and making it suitable for biosynthesis.
Stage 3 – Regeneration of RuBP
Only a small proportion of TP leaves the cycle to form organic molecules such as glucose. The majority of TP is used to regenerate RuBP, ensuring the cycle can continue. For every six molecules of TP formed:
One TP molecule exits the cycle for carbohydrate synthesis.
Five TP molecules (containing 15 carbons in total) are rearranged using ATP into three molecules of RuBP (each containing five carbons).
This regeneration phase consumes additional ATP, again linking the light-dependent and light-independent stages through energy transfer.
EQUATION
—-----------------------------------------------------------------
Overall Simplified Calvin Cycle Equation (per CO₂ fixed)
CO₂ + 2 NADPH + 3 ATP → (CH₂O) + 2 NADP⁺ + 3 ADP + 3 Pi
CO₂ = Carbon dioxide (inorganic carbon source)
NADPH = Reduced nicotinamide adenine dinucleotide phosphate (reducing agent)
ATP = Adenosine triphosphate (energy molecule)
(CH₂O) = General formula of carbohydrate unit
—-----------------------------------------------------------------
Although this equation represents a single turn of the cycle, six turns are required to produce one molecule of hexose sugar (C₆H₁₂O₆), consuming 18 ATP and 12 NADPH in total.
Energy and Control in the Calvin Cycle
The Calvin cycle’s operation depends entirely on the continuous supply of ATP and NADPH from the light-dependent stage. Without light, these molecules cannot be regenerated, and the cycle soon halts. This interdependence demonstrates the link between the two stages of photosynthesis.
Control mechanisms include:
Enzyme regulation – RuBisCO and other Calvin cycle enzymes are activated by light indirectly via changes in stromal pH and Mg²⁺ concentration.
Product feedback – Accumulation of sugars can slow down the cycle by reducing the availability of inorganic phosphate needed to regenerate ATP.
Molecular Organisation within the Chloroplast
The spatial organisation of chloroplasts ensures efficient coordination:
Thylakoid membranes house the light-dependent reactions producing ATP and NADPH.
Stroma contains the enzymes for the Calvin cycle.
Transport mechanisms allow rapid movement of these molecules between compartments, maintaining balance between the two stages.
Carbon Balance and Biosynthetic Roles of TP
The triose phosphate produced serves multiple essential roles:
Carbohydrate synthesis – Two TP molecules combine to form hexose sugars such as glucose, which can then polymerise into starch or cellulose.
Amino acid formation – TP can enter pathways producing amino acids when combined with nitrogen from nitrates.
Lipid synthesis – TP provides the glycerol backbone for triglycerides and phospholipids.
However, TP cannot accumulate indefinitely. Its regeneration into RuBP is vital to sustain CO₂ fixation, maintaining a dynamic equilibrium between growth and energy storage.
Relationship to the Light-Dependent Reactions
The Calvin cycle is often described as the “dark stage,” yet this is misleading. It is light-independent, not light-insensitive. It depends directly on the continuous output of ATP and NADPH from the light-dependent reactions. When light ceases:
ATP and NADPH supplies fall.
GP accumulates because it cannot be fully reduced to TP.
Regeneration of RuBP slows or stops, halting further CO₂ fixation.
This relationship highlights photosynthesis as a single, integrated process rather than separate phases. The products of one stage are the essential inputs of the next, forming a biochemical cycle of energy transformation that sustains autotrophic life.
FAQ
Although the Calvin cycle does not directly require light, it depends on ATP and reduced NADP produced in the light-dependent stage. When light intensity decreases, these energy carriers are produced more slowly.
Additionally, some Calvin cycle enzymes, including RuBisCO activase, are indirectly activated by light through changes in stromal pH and magnesium ion concentration. Reduced light therefore lowers enzyme activity, slowing carbon fixation.
When CO₂ becomes limiting:
RuBisCO cannot fix carbon efficiently, reducing RuBP carboxylation.
GP levels fall because less is produced from RuBP and CO₂.
TP levels also fall, as GP production slows.
RuBP accumulates, as it is not being converted to GP at the normal rate.
This pattern helps scientists diagnose which factor is limiting photosynthesis in experiments
RuBisCO is the most abundant enzyme on Earth, yet it works slowly—catalysing only a few reactions per second. It can also bind oxygen instead of carbon dioxide, leading to photorespiration, which wastes energy and reduces carbon fixation efficiency.
Plants compensate by producing large amounts of RuBisCO to maintain sufficient rates of photosynthesis despite the enzyme’s sluggish and sometimes wasteful activity.
In the reduction phase, ATP provides energy to convert GP to TP alongside hydrogen from reduced NADP.
In the regeneration phase, ATP powers the rearrangement of five TP molecules into three RuBP molecules, restoring the CO₂ acceptor and ensuring the cycle can continue.
Thus, ATP acts both as an energy source for reduction and as a driving force for carbon recycling within the cycle.
Each turn of the Calvin cycle fixes one molecule of CO₂, producing two molecules of TP, but only one-sixth of TP leaves the cycle.
To produce one glucose molecule (six carbons), the cycle must turn six times, fixing six CO₂ molecules and generating 12 TP molecules. Of these, two TP molecules combine to form one glucose, while ten TP molecules regenerate six RuBP molecules, maintaining the cycle’s continuity.
Practice Questions
Question 1 (2 marks)
State the roles of ATP and reduced NADP (NADPH) in the Calvin cycle.
Mark Scheme:
1 mark for identifying that ATP provides energy for the reduction of GP to TP.
1 mark for identifying that reduced NADP provides hydrogen (reducing power) for converting GP into TP.
Question 2 (5 marks)
Describe the main stages of the Calvin cycle, outlining the key compounds involved and explaining how it depends on the light-dependent reactions of photosynthesis.
Mark Scheme:
Award up to 5 marks as follows:
1 mark: CO₂ combines with RuBP, catalysed by RuBisCO, to form an unstable 6-carbon intermediate that splits into two molecules of GP (glycerate 3-phosphate).
1 mark: GP is reduced to TP (triose phosphate) using ATP and reduced NADP from the light-dependent reactions.
1 mark: ATP provides energy and reduced NADP provides hydrogen for this reduction process.
1 mark: Most TP is used to regenerate RuBP, using additional ATP.
1 mark: Some TP leaves the cycle to form carbohydrates (or other organic molecules).
Accept references to the cycle occurring in the stroma of chloroplasts and dependence on ATP and NADPH from the light-dependent stage for full credit.
