Biochemistry - Lesson 16: Photosynthesis and Carbon Fixation

Photosynthesis is the process by which plants, algae, and certain bacteria capture light energy and convert it into chemical energy. In eukaryotic photoautotrophs (such as green plants), this occurs in chloroplasts, comprising the light reactions (thylakoid membranes) and carbon fixation (Calvin cycle in the stroma). Light reactions harness solar energy to generate ATP and NADPH, whereas the Calvin cycle uses these products to fix CO₂ into carbohydrates. This pathway is the counterpart to heterotrophic respiration, sustaining the global carbon cycle and fueling nearly all terrestrial and aquatic food webs.

Light Reactions

The light-dependent reactions occur within the thylakoid membranes of the chloroplast. Light absorption drives electron flow through photosystems II (PSII) and I (PSI), creating both a proton gradient and NADPH. This flow resembles mitochondrial ETC, but water rather than NADH serves as the electron source:

• Photosystem II (PSII): Oxidizes water to O₂, releasing protons into the thylakoid lumen. Excited electrons travel to plastoquinone (PQ), analogous to ubiquinone in mitochondria.
• Cytochrome b₆f Complex: Transfers electrons from PQ to plastocyanin (PC). Proton pumping into the lumen boosts the electrochemical gradient.
• Photosystem I (PSI): Further excites electrons using additional photons, passing them eventually to NADP⁺, yielding NADPH via ferredoxin–NADP⁺ reductase.
• ATP Synthase: Utilizes the proton gradient (high [H⁺] in the lumen) to drive ATP production in the stroma (photophosphorylation).

Thus, the light reactions provide ATP and NADPH essential for carbon assimilation in the Calvin cycle. They also release O₂, replenishing atmospheric oxygen.

Calvin Cycle (Carbon Fixation)

The ATP and NADPH from light reactions fuel CO₂ fixation into organic molecules in the stroma. The Calvin cycle has three main phases:

• Carboxylation: Ribulose-1,5-bisphosphate (RuBP) reacts with CO₂, catalyzed by Rubisco, forming two 3-phosphoglycerate (3-PGA) molecules.
• Reduction: 3-PGA is phosphorylated (ATP) and reduced (NADPH) to glyceraldehyde-3-phosphate (G3P). Some G3P exits for sugar synthesis.
• Regeneration: Most G3P is rearranged (via transketolase, aldolase) to regenerate RuBP, completing the cycle.

For every three molecules of CO₂ fixed, one triose phosphate (G3P) can be exported, while the remainder reconstitutes RuBP. This cycle interlinks with other metabolic pathways and can be regulated by stromal pH, Mg²⁺ levels, redox status, and intermediates.

Photorespiration and Regulation

Under lower CO₂ or high O₂ conditions, Rubisco can add O₂ to RuBP, leading to photorespiration. This process consumes energy (ATP, NADPH) and releases previously fixed CO₂, particularly in hot, bright conditions (as stomata close to limit water loss, reducing [CO₂]). Some plants (C₄ and CAM) mitigate photorespiration by concentrating CO₂ around Rubisco in specialized tissues or temporal separation, enhancing net carbon fixation.

Calvin cycle enzymes are activated in light, favored by higher stromal pH and reduced ferredoxin (leading to reductive activation of certain proteins). Conversely, dark conditions reduce NADPH levels, slowing carbon fixation.

Flowchart of Photosynthetic Electron Flow

flowchart LR W(["H₂O"]) --> PS2["Photosystem II
(Splits water)"] PS2 --> Cytb6f["Cytochrome b₆f
(H+ pumping)"] Cytb6f --> PS1["Photosystem I"] PS1 --> NADP["NADP⁺ → NADPH"] Cytb6f -->|Proton gradient| ATP["ATP Synthase
(Photophosphorylation)"]

Summary

Photosynthesis transforms light energy into chemical bonds, producing ATP and NADPH in the light reactions, then using these to fix CO₂ in the Calvin cycle. PSII extracts electrons from H₂O (releasing O₂), passes them via cytochrome b₆f to PSI, which generates NADPH, while the proton gradient supports ATP synthesis. The Calvin cycle incorporates CO₂ into carbohydrates, with Rubisco as a pivotal enzyme susceptible to O₂ competition, leading to photorespiration. Regulation ensures coordinated operation of light harvesting and carbon fixation, adjusting for environmental cues and metabolic demands. Understanding these processes highlights the fundamental energy conversion underpinning most ecosystems and global carbon flow.

Suggested Reading:
Lehninger Principles of Biochemistry (chapters on photosynthesis and carbon assimilation)
Biochemistry by Berg, Tymoczko, and Stryer (sections covering thylakoid electron transport, Calvin cycle, and photorespiration)
MIT OpenCourseWare: General Biochemistry (lectures on photosynthesis)