Photosynthesis in Higher Plants

Overall Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (Light energy, Chlorophyll)

Site of Photosynthesis: • Chloroplast — contains thylakoid (light reactions) and stroma (dark reactions) • Chlorophyll a (most active), Chlorophyll b, Carotenoids • Chlorophyll a absorbs red (680 nm) and blue-violet (430 nm) light

Stage 1 — Light Reactions (Thylakoid Membrane): • Also called the 'Hill Reaction' or photochemical phase • Products: ATP, NADPH, O₂ • Photosystem I (PS I): absorption at 700 nm (P700) • Photosystem II (PS II): absorption at 680 nm (P680) • Electron flow: PS II → Electron transport chain → PS I • Water splitting (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂ • Cyclic photophosphorylation: only PS I involved; only ATP produced • Non-cyclic photophosphorylation: PS I + PS II; ATP + NADPH + O₂

Stage 2 — Dark Reactions / Calvin Cycle (Stroma): • Also called carbon fixation or biosynthetic phase • Does NOT directly need light but uses ATP and NADPH from light reactions • CO₂ fixation enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) • First product: 3-phosphoglycerate (3-PGA) — 3-carbon compound → C3 plants • 3 turns of Calvin cycle fix 3 CO₂ → 1 G3P (triose phosphate) • 6 turns → 1 glucose molecule

C3 vs C4 Plants:

Photorespiration: • Occurs in C3 plants in presence of high O₂ • RuBisCO adds O₂ instead of CO₂ → wastes energy • C4 plants have CO₂ concentrating mechanism → reduces photorespiration

Factors Affecting Photosynthesis: • Light intensity (direct effect) • CO₂ concentration (major limiting factor) • Temperature (affects enzyme activity) • Water (photolysis; also affects stomata) • Blackman's Law of Limiting Factors: rate is limited by the slowest factor

Important Terms: • Action spectrum: wavelengths used in photosynthesis (red + blue peaks) • Absorption spectrum: wavelengths absorbed by chlorophyll • Emerson Enhancement Effect: using both red + far-red light gives greater photosynthesis than either alone