UNIT 2.1.2

The Calvin Cycle (Dark Reactions)

Fixing carbon dioxide into organic molecules

🎯 After this unit, you will be able to:

Explain the three phases of the Calvin cycle
Describe the role of rubisco in carbon fixation
Understand how ATP and NADPH from light reactions are used
Calculate the energy requirements for producing one glucose molecule

🔄 From Light Energy to Organic Carbon

The Calvin cycle (also called the Calvin-Benson-Bassham cycle) is the set of light-independent reactions that fix carbon dioxide into organic molecules. It takes place in the stroma of chloroplasts and uses the ATP and NADPH produced by the light reactions .

3 CO₂ + 9 ATP + 6 NADPH → G3P (glyceraldehyde-3-phosphate) + 9 ADP + 8 Pi + 6 NADP⁺

Key concept: Although called "dark reactions," the Calvin cycle doesn't require darkness—it simply doesn't need light directly. However, it usually runs during the day because it depends on ATP and NADPH from the light reactions .

🔄 [Diagram: Overview of Calvin cycle with three phases — to be inserted]

📋 The Three Phases of the Calvin Cycle

PHASE 1

Carbon Fixation

CO₂ is attached to a 5-carbon acceptor molecule (RuBP) by the enzyme rubisco, forming an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglycerate (3-PGA) .

PHASE 2

Reduction

3-PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. This is the actual "fixing" of carbon into a sugar molecule .

PHASE 3

Regeneration of RuBP

Most of the G3P (5 out of 6 molecules) is used to regenerate RuBP, the CO₂ acceptor. This requires additional ATP and allows the cycle to continue .

Phase 1: Carbon Fixation

The Calvin cycle begins when CO₂ enters the stroma and is attached to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme rubisco (ribulose bisphosphate carboxylase/oxygenase)—the most abundant protein on Earth .

🌿 About Rubisco: Rubisco is a large enzyme (16 subunits in plants) with a slow catalytic rate—it fixes only about 3 CO₂ molecules per second. Plants compensate by producing enormous quantities of rubisco, which can constitute up to 50% of soluble protein in leaves .

The 6-carbon intermediate is highly unstable and immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound .

🔗 [Diagram: Carbon fixation step - CO₂ + RuBP → 2 × 3-PGA — to be inserted]

🌍 Did you know? Rubisco is often considered a "slow and confused" enzyme because it can react with either CO₂ or O₂. When it fixes O₂ instead of CO₂, it initiates a wasteful process called photorespiration (covered in Unit 2.1.3) .

Phase 2: Reduction

In this phase, the 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a true sugar. This requires energy from both ATP and NADPH, which were produced in the light reactions .

The reduction phase has two steps:

Phosphorylation: ATP donates a phosphate to 3-PGA, forming 1,3-bisphosphoglycerate
Reduction: NADPH donates electrons, reducing 1,3-bisphosphoglycerate to G3P

For every 3 CO₂ molecules fixed, the cycle produces 6 molecules of G3P. But only 1 G3P is exported as net gain—the other 5 are used to regenerate RuBP .

⚡ [Diagram: Reduction phase - 3-PGA → G3P using ATP and NADPH — to be inserted]

Phase 3: Regeneration of RuBP

To keep the cycle running, the CO₂ acceptor (RuBP) must be regenerated. This involves a complex series of reactions that rearrange 5 molecules of G3P (15 carbons total) into 3 molecules of RuBP (also 15 carbons). This process requires additional ATP .

5 G3P + 3 ATP → 3 RuBP + 3 ADP

After regeneration, the cycle is ready to accept more CO₂.

🔄 [Diagram: Regeneration phase - G3P → RuBP — to be inserted]

📊 Energy Accounting: The Cost of Making Sugar

To produce one molecule of G3P (a 3-carbon sugar), the Calvin cycle requires:

3 CO₂ molecules
9 ATP molecules (3 for fixation? Actually: 3 in reduction + 6 in regeneration? Let's be precise)
6 NADPH molecules

To make one molecule of glucose (a 6-carbon sugar), two G3P are needed, so the requirements double:

Product	CO₂ used	ATP used	NADPH used
1 G3P (3C)	3	9	6
1 Glucose (6C)	6	18	12

Energy investment: This explains why plants need such extensive light-harvesting systems—they require enormous amounts of ATP and NADPH to build sugars from CO₂ .

🔆 Regulation: Why the Calvin Cycle Runs in Light

Although the Calvin cycle doesn't require light directly, it's tightly regulated to run primarily during the day. Several mechanisms ensure this:

Light activates enzymes: Several Calvin cycle enzymes (including rubisco activase) are activated by light via the ferredoxin/thioredoxin system .
pH and Mg²⁺ changes: Light-driven proton pumping into thylakoids increases stromal pH and Mg²⁺ concentration, which activates rubisco and other enzymes .
Substrate availability: ATP and NADPH are only produced in light .

🌙 What Happens in Darkness?

In darkness, Calvin cycle enzymes are inactivated. Some intermediates are converted to storage forms (starch) during the day and mobilized at night. This prevents futile cycling and conserves energy .

🧑‍🌾 Horticultural Implications

CO₂ Enrichment in Greenhouses

Since CO₂ is the substrate for the Calvin cycle, increasing CO₂ concentration can boost photosynthesis—up to a point. Greenhouse growers often supplement CO₂ to 800-1000 ppm (compared to ambient ~400 ppm) to increase crop yields, especially in high-light conditions .

🍅 Benefits of CO₂ Enrichment

Increases rubisco carboxylation rate
Suppresses photorespiration (more CO₂ means less O₂ binding)
Enhances growth and yield in many crops
Most effective when light is not limiting

Temperature Effects

The Calvin cycle is enzyme-driven, so it's sensitive to temperature. Optimal temperatures for most crops are 25-30°C. High temperatures can:

Increase photorespiration (rubisco fixes more O₂)
Denature Calvin cycle enzymes
Close stomata, limiting CO₂ entry

🌡️ Did you know? Some crops like wheat and rice show reduced Calvin cycle efficiency at temperatures above 35°C, partly due to rubisco activase denaturation. Heat-tolerant varieties have more stable rubisco activase .

🔬 Recent Research: Engineering a Better Calvin Cycle

Scientists are working to improve photosynthetic efficiency by modifying the Calvin cycle. Two promising approaches:

1. Improving Rubisco

Researchers are trying to:

Replace plant rubisco with faster versions from algae
Engineer rubisco to better discriminate CO₂ from O₂
Improve rubisco activase stability at high temperatures

2. Bypassing Photorespiration

Synthetic biology approaches create shortcuts that metabolize the toxic byproducts of photorespiration more efficiently, boosting productivity by 20-40% in test crops .

These "photosynthesis hacking" approaches could significantly increase crop yields in a warming world.

📌 Unit Summary

Phase	Inputs	Outputs	Key enzyme
Carbon fixation	3 CO₂ + 3 RuBP	6 3-PGA	Rubisco
Reduction	6 3-PGA + 6 ATP + 6 NADPH	6 G3P	G3P dehydrogenase
Regeneration	5 G3P + 3 ATP	3 RuBP	Various

Net gain: For every 3 CO₂, the cycle produces 1 G3P (which can be used to make glucose, sucrose, starch, and other organic molecules).

Reflection question: A greenhouse grower in Ethiopia wants to increase tomato yields during the dry season when light is abundant but temperatures are high (often >35°C). Based on your understanding of the Calvin cycle, what advice would you give about CO₂ enrichment, temperature management, and variety selection?

📌 Key terms introduced

Calvin cycle Rubisco RuBP 3-PGA G3P Carbon fixation Reduction phase Regeneration phase Stroma Photorespiration CO₂ enrichment

✅ Check your understanding

What are the three phases of the Calvin cycle, and what happens in each?
Why is rubisco considered both essential and problematic?
How many ATP and NADPH molecules are needed to produce one molecule of glucose?
Why does the Calvin cycle primarily run during the day, even though it doesn't directly require light?
A greenhouse operator increases CO₂ to 1000 ppm. Explain why this might increase yield, and under what conditions it would be most effective.

Discuss your answers in the course forum.

Plant Biochemistry for Horticulture · HORT 202 · Dilla University · Last updated March 2026