Photosynthesis Overview

Overview

This lecture explains the two main stages of photosynthesis: the light-dependent reactions and the light-independent reactions (Calvin cycle), focusing on their processes, locations, and significance.

Light-Dependent Reactions

• Occur in the thylakoid membrane of the chloroplast.
• Involve two photosystems: Photosystem II acts first, then Photosystem I.
• Light energy excites electrons in chlorophyll a, which are transferred to a primary electron acceptor.
• Lost electrons in Photosystem II are replaced by electrons from water, producing O₂ as a byproduct.
• Electrons travel down the electron transport chain, passing through molecules like plastoquinone and the cytochrome complex.
• Electron transport pumps protons into the thylakoid lumen, creating a proton gradient.
• ATP synthase uses the proton gradient (chemiosmosis) to convert ADP and inorganic phosphate into ATP.
• Final electron acceptor is NADP+, which becomes NADPH.
• Products: ATP and NADPH, used in the Calvin cycle.

Light-Independent Reactions (Calvin Cycle)

• Occur in the stroma of the chloroplast.
• Do not directly require light but depend on ATP and NADPH from light reactions.
• Three main stages: carbon fixation (CO₂ attached to RuBP by rubisco), reduction (using NADPH to produce G3P), and regeneration of RuBP (using ATP).
• It takes three turns of the cycle (three CO₂) to produce one G3P molecule, which can form glucose, starch, or cellulose.
• Key enzyme: Rubisco, which attaches CO₂ to RuBP.

Photorespiration and Plant Adaptations

• Rubisco can bind O₂ instead of CO₂, leading to photorespiration—a wasteful reaction using energy but not making sugar.
• C3 plants undergo photorespiration under low CO₂/high O₂.
• C4 plants minimize photorespiration by separating rubisco into bundle sheath cells, first converting CO₂ to a 4-carbon molecule.
• CAM plants (e.g., pineapples) intake CO₂ at night and run the Calvin cycle during the day.

Key Terms & Definitions

• Thylakoid membrane — site of light-dependent reactions inside chloroplasts.
• Stroma — fluid inside chloroplast where Calvin cycle occurs.
• Photosystem — protein-pigment complexes absorbing light and transferring electrons.
• Electron transport chain — series of proteins that move electrons and pump protons.
• ATP synthase — enzyme that synthesizes ATP using a proton gradient.
• NADPH — high-energy electron carrier produced in light reactions.
• Rubisco — enzyme catalyzing carbon fixation in the Calvin cycle.
• Photorespiration — process where rubisco binds O₂, wasting energy and not producing sugar.
• C3 plants — plants using the Calvin cycle directly, prone to photorespiration.
• C4/ CAM plants — plants with adaptations to limit photorespiration.

Action Items / Next Steps

• Review recommended animations and videos from textbook links about photosynthesis.
• Prepare for next chapter on cell communication.