🌚Dark Reactions — Calvin Cycle, C4 Pathway, CAM, and Photorespiration

From Field to Lab — Why Sugarcane Out-yields Wheat

In the previous lesson, we explored the light reactions of photosynthesis — how chlorophyll captures light energy and converts it into ATP and NADPH₂. Now we move to the second half: the dark reactions, where that stored energy is used to fix CO₂ into carbohydrates.

A wheat farmer in Punjab achieves about 4–5 tonnes per hectare. A sugarcane farmer next door harvests 70–80 tonnes of cane. Why the massive difference? Part of the answer lies in the dark reactions of photosynthesis. Wheat is a C3 plant — it uses the Calvin cycle, which loses energy through photorespiration. Sugarcane is a C4 plant — it uses the Hatch-Slack pathway with a CO₂-concentrating mechanism that eliminates photorespiration, enabling much higher photosynthetic efficiency in hot, sunny conditions.

This lesson covers:

1. Dark reactions overview — definition, location, and the three CO₂ fixation pathways
2. Calvin Cycle (C₃) — the universal carbon fixation pathway
3. Blackman’s Law — limiting factors in photosynthesis
4. Hatch-Slack Cycle (C₄) — the tropical efficiency pathway with Kranz anatomy
5. C₃ vs C₄ comparison — the master table for exams
6. CAM Pathway — how desert plants fix CO₂ at night
7. Photorespiration — why C₃ plants waste energy

Understanding C3 vs C4 vs CAM pathways is one of the highest-weightage topics in agriculture exams.

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What Are Dark Reactions?

The dark phase of photosynthesis is the CO₂ fixation step — also called Blackman’s reaction or Path of Carbon.

Three Pathways of CO₂ Fixation

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Calvin Cycle (C₃ Cycle)

Found predominantly in Wheat, Rice, Barley, Pulses, Soybean, Cotton, Sunflower — these are called C₃ plants.

How it works:

1. 6 molecules of RuBP combine with 6 molecules of CO₂ (catalysed by RuBisCO)
2. This produces 6 molecules of hexose (6-carbon sugar)
3. 1 hexose is consumed as food (net gain)
4. 5 hexoses are reconverted to 6 RuBP (regeneration phase, requires ATP)

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Blackman’s Law of Limiting Factors

If light intensity is doubled but CO₂ concentration remains constant, there will be no increase in photosynthetic rate — CO₂ becomes the limiting factor.

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Hatch-Slack Cycle (C₄ Cycle)

Found in Sugarcane, Maize, Sorghum, Bajra, Amaranthus, and other tropical grasses — called C₄ plants.

• Discovered by Kortschak, Hartt & Burr (1965, sugarcane) and confirmed by M.D. Hatch & C.R. Slack (1967, Australia)
• A subtropical species of Atriplex rosea shows C₄ while the temperate species shows C₃ — demonstrating adaptation

Kranz Anatomy — The C₄ Advantage

C₄ plants have a distinctive leaf structure with two types of chloroplasts:

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C₃ vs C₄ Plants — The Master Comparison Table

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CAM — Crassulacean Acid Metabolism

Desert and arid-zone plants face a dilemma: opening stomata during the hot day to absorb CO₂ means losing precious water. CAM plants solve this by separating CO₂ fixation and the Calvin cycle temporally — they fix CO₂ at night and run the Calvin cycle during the day.

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Photorespiration — The C₃ Energy Leak

Photorespiration is a wasteful process in which RuBisCO fixes O₂ instead of CO₂, producing a toxic 2-carbon compound (phosphoglycolate) that must be salvaged at the cost of energy. It occurs only in C₃ plants because their RuBisCO is exposed to atmospheric O₂ without a CO₂-concentrating mechanism.

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Summary Table — Key Facts at a Glance

Summary Cheat Sheet