Key Photosynthesis Reactions

Why This Matters

Photosynthesis isn't just one reaction—it's an elegantly coordinated series of energy transformations that you're expected to trace from photon to glucose. Exams test whether you understand how light energy becomes chemical energy, where each step occurs within the chloroplast, and why plants evolved alternative pathways like C4 and CAM. You'll need to connect electron flow to ATP synthesis, explain why photorespiration is a problem, and compare how different plants solve the same environmental challenges.

The reactions covered here demonstrate core principles: chemiosmosis, enzyme specificity, redox chemistry, and evolutionary adaptation to environmental stress. Don't just memorize that the Calvin cycle happens in the stroma—know why it needs the ATP and NADPH generated in the thylakoid membranes, and what happens when $CO_2$ availability drops. Master the mechanisms, and you'll be ready for any FRQ that asks you to trace energy flow or compare photosynthetic strategies.

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Light-Dependent Reactions: Capturing Energy

The light-dependent reactions transform light energy into chemical energy carriers. These reactions occur in the thylakoid membranes, where embedded protein complexes capture photons and use that energy to split water, move electrons, and generate a proton gradient.

Photolysis of Water

• Water splitting—light energy breaks $2H_2O$ into $O_2$, $4H^+$, and $4e^-$, providing the electrons that power the entire light reaction
• Oxygen release is a byproduct, not the goal—the real products are the electrons and protons that drive downstream reactions
• Occurs at Photosystem II (PSII), making this complex the entry point for electrons into the transport chain

Electron Transport Chain

• Protein complexes (PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI) shuttle electrons through the thylakoid membrane
• Proton pumping occurs as electrons move—cytochrome b6f actively transports $H^+$ into the thylakoid lumen, building the gradient
• Redox reactions drive the process; each carrier is reduced then oxidized as electrons pass through, releasing energy incrementally

NADPH Production

• Final electron acceptor—$NADP^+$ receives electrons at the end of PSI, becoming $NADPH$ (a powerful reducing agent)
• Ferredoxin-NADP+ reductase catalyzes the final transfer, adding both electrons and a proton
• Essential for carbon fixation—without $NADPH$, the Calvin cycle cannot reduce 3-phosphoglycerate to G3P

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ATP Generation: Powering the Cycle

ATP synthesis in chloroplasts follows the same chemiosmotic principle as mitochondria—a proton gradient stores potential energy that ATP synthase converts into chemical bond energy.

ATP Synthesis (Chemiosmosis)

• Proton gradient—$H^+$ ions accumulate in the thylakoid lumen (low pH inside, high pH in stroma), creating electrochemical potential
• ATP synthase spans the membrane, allowing protons to flow down their gradient while catalyzing $ADP + P_i \rightarrow ATP$
• Coupling mechanism—electron transport and ATP production are linked; blocking one stops the other

Photophosphorylation

• Light-driven ATP formation—distinguishes this from oxidative phosphorylation in mitochondria
• Cyclic photophosphorylation uses only PSI, recycling electrons to produce ATP without $NADPH$ or $O_2$—useful when the Calvin cycle needs extra ATP
• Non-cyclic photophosphorylation is the standard pathway, producing both ATP and $NADPH$ while releasing $O_2$

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The Calvin Cycle: Building Sugar

The Calvin cycle uses ATP and $NADPH$ from light reactions to fix carbon dioxide into organic molecules. This occurs in the stroma, where enzymes are dissolved in the aqueous environment surrounding the thylakoids.

Carbon Fixation

• $CO_2$ incorporation—inorganic carbon is attached to the 5-carbon sugar RuBP, producing two 3-carbon molecules (3-PGA)
• First organic product is 3-phosphoglycerate, making this a C3 pathway (the "3" refers to this three-carbon compound)
• Rate-limiting step—carbon fixation is typically the slowest reaction, making it a key control point for photosynthesis

RuBisCO Activity

• Most abundant enzyme on Earth—plants invest heavily in RuBisCO because it's slow and inefficient
• Dual function problem—RuBisCO can bind either $CO_2$ (carboxylase activity) or $O_2$ (oxygenase activity), leading to photorespiration
• Specificity factor determines how strongly the enzyme prefers $CO_2$ over $O_2$—this ratio decreases at high temperatures, worsening photorespiration

Calvin Cycle Overview

• Three phases: fixation ($CO_2$ → 3-PGA), reduction (3-PGA → G3P using ATP and $NADPH$), and regeneration (G3P → RuBP using ATP)
• Net equation: $3CO_2 + 9ATP + 6NADPH \rightarrow G3P + 9ADP + 8P_i + 6NADP^+$
• One G3P exits per three turns; the rest regenerate RuBP to keep the cycle running

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Photorespiration: The Efficiency Problem

When RuBisCO binds oxygen instead of $CO_2$, plants lose fixed carbon and waste energy. This becomes significant under conditions of high light, high temperature, and low $CO_2$—exactly when stomata close to conserve water.

Photorespiration

• Oxygenase reaction—RuBisCO adds $O_2$ to RuBP, producing one 3-PGA and one 2-carbon phosphoglycolate (a metabolic dead end)
• Energy cost—recovering phosphoglycolate requires ATP and releases previously fixed $CO_2$, reducing photosynthetic efficiency by 25-50%
• Temperature sensitivity—$O_2$ solubility decreases less than $CO_2$ solubility as temperature rises, shifting RuBisCO toward oxygenase activity

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Alternative Carbon Fixation: Evolutionary Solutions

C4 and CAM pathways represent independent evolutionary solutions to photorespiration. Both strategies concentrate $CO_2$ around RuBisCO, but they separate the initial fixation step differently—spatially in C4 plants, temporally in CAM plants.

C4 Carbon Fixation

• Spatial separation—initial fixation occurs in mesophyll cells using PEP carboxylase; the 4-carbon product moves to bundle sheath cells where $CO_2$ is released to RuBisCO
• PEP carboxylase has no oxygenase activity and higher $CO_2$ affinity than RuBisCO, eliminating photorespiration at the first step
• ATP cost—C4 requires extra energy (2 ATP per $CO_2$) but gains efficiency in hot, bright conditions where photorespiration would otherwise dominate

CAM Photosynthesis

• Temporal separation—stomata open at night to fix $CO_2$ into malate (stored in vacuoles); during the day, malate releases $CO_2$ for the Calvin cycle
• Water conservation—by opening stomata only at night (when transpiration rates are lowest), CAM plants survive extreme drought
• Ecological trade-off—CAM is slower than C3 or C4, limiting growth rates; it's advantageous only where water is severely limiting

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Quick Reference Table

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Self-Check Questions

1. Trace the path of an electron from water to $NADPH$—which protein complexes does it pass through, and where does proton pumping occur?

2. Compare ATP synthesis in chloroplasts to ATP synthesis in mitochondria. What structural and functional similarities exist, and what is the energy source for each?

3. Why does photorespiration increase at high temperatures? Explain in terms of RuBisCO's dual activity and gas solubility.

4. A plant growing in a hot, dry environment keeps its stomata closed during the day. Which photosynthetic pathway (C3, C4, or CAM) would best suit this plant, and why?

5. If you blocked the electron transport chain but supplied a plant with ATP and $NADPH$, could the Calvin cycle still operate? What about photolysis—would it continue? Explain the dependencies between these processes.