Photosynthesis and the Calvin Cycle Explained

Overview of Photosynthesis

• Plants convert light energy into chemical energy through photosynthesis, involving two key phases: the light reactions and the Calvin cycle.
• Photosystems: Two types, PSI and PSII, are critical for this process:
  • PSII: Leads in absorbing light and oxidizing water to produce oxygen (O₂).
  • PSI: Absorbs light to aid in the production of NADPH.

Photosystem II (PSII)

• Components:
  • Light Harvesting Complex: Contains numerous pigment molecules that absorb photons.
  • Reaction Center (P680): Special pigment that absorbs light at 680 nm, crucial for initiating electron transport.
• Process of Energy Transfer:
  • Absorption of light leads to the excitation of electrons within pigment molecules.
  • Resonance Energy Transfer: Energy (not electrons) is passed from one pigment molecule to another until it reaches P680.
  • Excitation of P680 produces P680\* (excited state).

The Calvin Cycle

• Occurs in the stroma of chloroplasts in plants and algae, and in the cytoplasm of cyanobacteria.
• Function: Converts carbon dioxide (CO₂) from the atmosphere into carbohydrates (e.g., glucose).
• Importance:
  • Carbohydrates are essential as precursors for organic molecules and primary energy storage.
• Phases of the Calvin Cycle:
  1. Carbon Fixation: CO₂ is combined with Ribulose bisphosphate (RuBP), catalyzed by the enzyme rubisco.
  2. Reduction Phase: Uses ATP and high-energy electrons from NADPH to convert 3-phosphoglycerate (3PG) into glyceraldehyde-3-phosphate (G3P).
  3. Regeneration of RuBP: Enzymatic reactions transform some G3P back to RuBP to allow the cycle to repeat, utilizing ATP.

Experimental Insights

• Melvin Calvin's Work: Utilized C-14 isotope for tracing carbon during experiments involving Chlorella algae to elucidate steps of the Calvin Cycle.
• Method:
  • Inject C-14 labeled CO₂, incubate samples, halt reaction, and analyze organic molecules through autoradiography and chromatography.
  • Identified the sequence of products during the Calvin cycle and the incorporation of carbon.

Carbon Reduction and Energy Storage

• CO₂ → Organic Molecules: The Calvin cycle transitions carbon from an inorganic state (CO₂) to an organic state (e.g., G3P), rich in potential energy.
• Bond Types: Conversion from C-O bonds (in CO₂) to C-H and C-C bonds (in organic molecules) enables the formation of energy-rich compounds.
• Carbohydrates: Formation of glucose or other polysaccharides from G3P, which can be stored as starch.

Photorespiration and Adaptations

• Photorespiration: A wasteful process where rubisco reacts with O₂ instead of CO₂, especially in conditions of water stress.
  • Causes loss of carbon fixation potential (up to 50% reduction of photosynthetic output).
  • May provide protection against damage from toxic molecules when stomata are closed (water conservation).
• C3 vs C4 vs CAM Plants:
  • C3 Plants: Majority group, directly fix CO₂ using rubisco, susceptible to photorespiration.
  • C4 Plants: Utilize PEP carboxylase to limit photorespiration by separating carbon fixation and Calvin cycle spatially through mesophyll and bundle sheath cells (e.g., sugarcane, corn).
  • CAM Plants: Separate these processes temporally (night/day) to minimize water loss (e.g., cacti).

Conclusion

• Photosynthesis is a crucial biological process allowing plants to utilize light energy, producing organic matter essential to life. Understanding its detailed mechanisms, adaptations, and energy dynamics is key to comprehending plant biology and ecology.