Chapter A: Overview and Equation

Photosynthesis converts light to chemical energy in green plants, algae, and cyanobacteria. Balanced equation: $6CO_{2}$ + $12H_{2}O$ → $C_{6}H_{12}O_{6}$ + $6H_{2}O$ + $6O_{2}$. The $12H_{2}O$ reflects that $O_{2}$ comes from water (Van Niel, confirmed by ^{18}O labelling). Occurs in chloroplasts: light reactions (thylakoid membranes, grana) and Calvin cycle (stroma).

Chapter B: Photosynthetic Pigments

Chlorophyll a is the primary pigment (reaction centre: P680 in PS II, P700 in PS I). Accessory pigments (Chl b, carotenoids, xanthophylls, phycoerythrin) absorb a wider wavelength range and transfer energy to Chl a. Carotenoids and xanthophylls also provide photoprotection. Absorption spectrum shows blue (~430 nm) and red (~662 nm) peaks for Chl a. Action spectrum (rate of photosynthesis vs. wavelength) closely mirrors Chl a absorption, confirming its primary role.

Chapter C: Light Reactions

PS II (P680) initiates electron flow by absorbing a 680 nm photon, splitting water (Hill reaction: $2H_{2}O$ → 4$H^{+}$ + 4$e^{-}$ + $O_{2}$). Electrons travel the Z-scheme: PS II → PQ → Cyt b6f → PC → PS I (P700) → Fd → NA$DP^{+}$ reductase → NADPH. ATP is produced by chemiosmosis: $H^{+}$ accumulates in the thylakoid lumen (from photolysis + PQ shuttle) and flows through $CF_{0}$-$CF_{1}$ ATP synthase. Non-cyclic photophosphorylation (PS I + PS II): ATP + NADPH + $O_{2}$. Cyclic photophosphorylation (PS I only): ATP only, in stroma lamellae, tops up ATP:NADPH ratio to 3:2.

Chapter D: Calvin Cycle (Dark Reactions)

Three stages — Carboxylation (RuBisCO: C$O_{2}$ + RuBP → 2 × 3-PGA), Reduction (3-PGA + ATP + NADPH → G3P), Regeneration (G3P + ATP → RuBP). Mnemonic: CRR (Cars Race Rapidly). 6 turns = 1 glucose, 18 ATP + 12 NADPH. 12 G3P total; 10 for regeneration, 2 for glucose. RuBisCO is the most abundant protein on Earth (but slow: ~3 C$O_{2}$/active site/second).

Chapter E: C4 and CAM Pathways

C4 plants (Kranz anatomy): PEP carboxylase in mesophyll fixes C$O_{2}$ → OAA (4C, first stable product) → malate → bundle sheath → C$O_{2}$ released → Calvin cycle (RuBisCO). No photorespiration. Extra ATP cost: ~2 ATP/C$O_{2}$ for PEP regeneration (PPDK). Examples: maize, sugarcane, sorghum. Optimum: 30–40°C. CAM plants: Temporal separation. Night = C$O_{2}$ fixed → malic acid stored in vacuoles; Day = stomata closed, malic acid decarboxylated → C$O_{2}$ for Calvin cycle. Highest WUE. Examples: cacti, Bryophyllum, pineapple.

Chapter F: Photorespiration and Limiting Factors

Photorespiration: RuBisCO oxygenase activity (high $O_{2}$/low C$O_{2}$) → phosphoglycolate → C2 cycle (3 organelles) → C$O_{2}$ released, no ATP. Absent in C4/CAM. Blackman's law: rate limited by the factor in minimum supply (light, C$O_{2}$, temperature). Rate plateaus when one factor is addressed unless the next limiting factor is also addressed.