Photosynthesis converts light energy to chemical energy (glucose) in plants, algae, and cyanobacteria, using $6CO_{2}$ + $12H_{2}O$ ($12H_{2}O$ confirmed by Van Niel) to produce glucose + $6O_{2}$ + $6H_{2}O$. It occurs in chloroplasts: light reactions on thylakoid membranes (grana) and the Calvin cycle in the stroma. Chlorophyll a is the primary pigment with reaction centres P680 (PS II) and P700 (PS I); accessory pigments broaden the spectrum and transfer energy to Chl a. PS II (P680) acts first, splitting water (Hill reaction: $2H_{2}O$ → 4$H^{+}$ + 4$e^{-}$ + $O_{2}$) and initiating the Z-scheme electron transport chain. Non-cyclic photophosphorylation (both PS I and PS II) produces ATP, NADPH, and $O_{2}$; cyclic photophosphorylation (PS I only, in stroma lamellae) produces ATP only. ATP synthesis occurs by chemiosmosis through $CF_{0}$-$CF_{1}$, driven by the $H^{+}$ gradient between the thylakoid lumen and stroma. The Calvin cycle (CRR) uses 18 ATP and 12 NADPH to fix 6 C$O_{2}$ (via RuBisCO + RuBP) into one glucose, through carboxylation (→3-PGA), reduction (→G3P), and regeneration (→RuBP) stages. C4 plants (maize, sugarcane) use Kranz anatomy and PEP carboxylase to fix C$O_{2}$ spatially (mesophyll → bundle sheath), eliminating photorespiration; CAM plants (cacti, Bryophyllum) use temporal separation (night C$O_{2}$ fixation, day Calvin cycle). Photorespiration is RuBisCO's wasteful oxygenase activity, producing phosphoglycolate and consuming ATP without gain, absent in C4/CAM plants. Blackman's law states that photosynthesis rate is limited by the least available factor — light intensity, C$O_{2}$ concentration, or temperature.