Photosynthesis and Carbon Fixation Pathways

Photosynthesis Overview

• Definition: The conversion of light energy into chemical energy stored in sugars and organic compounds.
• Organisms: Occurs in plants, algae, and certain prokaryotes.
• General Process: In the chloroplasts of mesophyll cells, light, water ($H_2O$), and carbon dioxide ($CO_2$) are used to produce oxygen ($O_2$) and sucrose.

The Light Reactions

• Location: Thylakoid membranes of the chloroplast.
• Light Absorption: Light excites electrons from chlorophyll, which are transferred to a primary electron acceptor in photosystem 2.
• Water Splitting: $H_2O$ is split into $2$ electrons (replacing those lost by chlorophyll), $2H^+$ ions, and an oxygen atom that forms $O_2$.
• Electron Transport Chain (ETC): Electrons travel down an ETC, pumping $4H^+$ ions into the thylakoid space, increasing the concentration gradient and decreasing $pH$.
• Chemiosmosis: The diffusion of $H^+$ from the thylakoid space back to the stroma through ATP synthase produces $ATP$.
• NADPH Formation: Low-energy electrons are ultimately transferred to $NADP^+$ to form high-energy $NADPH$, removing $H^+$ from the stroma.

The Calvin Cycle

• Location: Occurs in the stroma of the chloroplast.
• Mechanism: Uses $ATP$ and $NADPH$ to convert $CO_2$ into the sugar $G3P$.
• Phase 1: Carbon Fixation: Three molecules of $CO_2$ enter; the enzyme Rubisco attaches them to ribulose biphosphate (RuBP), forming 3-phosphoglycerate.
• Phase 2: Reduction: Uses $6\,ATP$ and $6\,NADPH$ to produce $6$ molecules of glyceraldehyde 3-phosphate ($G3P$). One $G3P$ molecule exits as a sugar product.
• Phase 3: Regeneration: The remaining $5$ molecules of $G3P$ use $3\,ATP$ to regenerate $3$ molecules of the $CO_2$ acceptor, RuBP.

Alternative Pathways: C4 and CAM

• C4 Plants (e.g., sugarcane):
  • Spatial Separation: Carbon fixation and the Calvin cycle occur in different cells.
  • Process: PEP carboxylase adds $CO_2$ to PEP in mesophyll cells to form a four-carbon compound (e.g., malate). Malate moves via plasmodesmata to bundle-sheath cells to release $CO_2$ for the Calvin cycle.
• CAM Plants (e.g., pineapple):
  • Temporal Separation: Carbon fixation and the Calvin cycle occur at different times.
  • Process: $CO_2$ is fixed at night and the Calvin cycle runs during the day.