Photosynthesis is the foundational autotrophic process by which green plants, algae, and cyanobacteria convert light energy into chemical energy stored as glucose. The overall reaction — $6CO_{2}$ + $12H_{2}O$ → $C_{6}H_{12}O_{6}$ + $6H_{2}O$ + $6O_{2}$ — uses $12H_{2}O$ (not $6H_{2}O$), as demonstrated by Van Niel's experiments with photosynthetic bacteria, and later confirmed by Ruben and Kamen using ^{18}O-labelled water. This established unequivocally that all $O_{2}$ released during photosynthesis comes from water, not C$O_{2}$.

The process occurs in chloroplasts: the light reactions on thylakoid membranes and the Calvin cycle (dark reactions) in the stroma. Photosynthetic pigments are organized into photosystems. Chlorophyll a is the primary pigment, serving as the reaction centre in PS I (as P700, absorbing at 700 nm) and PS II (as P680, absorbing at 680 nm). Accessory pigments — chlorophyll b, carotenoids, and xanthophylls — broaden the absorption spectrum and transfer energy to chlorophyll a, while also providing photoprotection against photo-oxidative damage. Phycoerythrin is an additional accessory pigment in red algae, absorbing green wavelengths for deep-water photosynthesis. The absorption spectrum shows peaks in the blue (~430 nm) and red (~660–680 nm) regions. The action spectrum (photosynthesis rate versus wavelength) closely mirrors the absorption spectrum of chlorophyll a, confirming its central role in driving photochemical reactions.

The light reactions begin at PS II, where P680 absorbs a photon, becoming excited and ejecting a high-energy electron. This oxidized P680^{+} is the most powerful biological oxidant known and drives the photolysis (Hill reaction) of water: $2H_{2}O$ → 4$H^{+}$ + 4$e^{-}$ + $O_{2}$. The liberated electrons travel through the Z-scheme: PS II → plastoquinone (PQ) → cytochrome b6f complex → plastocyanin (PC) → PS I (P700) → ferredoxin (Fd) → NA$DP^{+}$ reductase → NADPH. This linear, non-cyclic electron flow produces ATP, NADPH, and $O_{2}$. PQ serves as a lipid-soluble shuttle that picks up $H^{+}$ from the stroma and deposits it into the thylakoid lumen, helping build the proton gradient. ATP synthesis occurs via chemiosmosis: $H^{+}$ accumulated in the thylakoid lumen drives proton flow back to the stroma through the $CF_{0}$-$CF_{1}$ ATP synthase complex. Cyclic photophosphorylation, involving only PS I, produces ATP exclusively (no NADPH, no $O_{2}$) by recycling electrons from ferredoxin back to the PQ pool. It occurs in stroma lamellae and supplements ATP supply to achieve the 3:2 ATP:NADPH ratio required by the Calvin cycle.

The Calvin cycle (C3 pathway) comprises three stages. Carboxylation: RuBisCO catalyses the addition of C$O_{2}$ to RuBP (5C), yielding an unstable 6C intermediate that splits into two molecules of 3-phosphoglyceric acid (3-PGA, 3C) — the first stable product. Reduction: 3-PGA is converted to glyceraldehyde-3-phosphate (G3P) using ATP (for phosphorylation) and NADPH (for reduction). G3P is the first carbohydrate product. Regeneration: 10 of 12 G3P molecules are used to regenerate 6 RuBP (using ATP), while 2 G3P exit the cycle for glucose and other biosynthesis. Producing one glucose requires 6 turns, 18 ATP, and 12 NADPH.

C4 plants (maize, sugarcane, sorghum) evolved Kranz anatomy to overcome photorespiration. PEP carboxylase in mesophyll cells fixes C$O_{2}$ to PEP (3C), forming oxaloacetate (OAA, 4C — the first stable product). OAA is reduced to malate, transported to bundle sheath cells, and decarboxylated, releasing C$O_{2}$ at high concentration for the Calvin cycle (RuBisCO). This C$O_{2}$-concentrating mechanism suppresses RuBisCO's oxygenase activity, eliminating photorespiration. PEP is regenerated from pyruvate in mesophyll cells via PPDK (pyruvate phosphate dikinase), at the cost of 2 extra ATP per C$O_{2}$ delivered. The advantage — eliminating the 25–40% carbon loss of photorespiration — outweighs this extra cost in hot, bright, water-limited tropical conditions.

CAM plants (cacti, Bryophyllum, pineapple) use temporal separation: stomata open at night for C$O_{2}$ fixation by PEP carboxylase (→ malic acid stored in vacuoles) and close during the day, when malic acid is decarboxylated to supply C$O_{2}$ to RuBisCO and the Calvin cycle. This temporal decoupling minimises transpiration, giving CAM plants the highest water use efficiency of all photosynthesis types.

Photorespiration is a wasteful process in C3 plants when RuBisCO acts as an oxygenase (fixing $O_{2}$ instead of C$O_{2}$), producing phosphoglycolate. The ensuing C2 cycle involves three organelles (chloroplast, peroxisome, mitochondrion), releasing C$O_{2}$ without producing any ATP, and wasting fixed carbon. C4 and CAM plants suppress photorespiration entirely through their C$O_{2}$ concentrating mechanisms.

Blackman's law of limiting factors states that when multiple factors control a process, the rate is determined by the factor present in the minimum (most limiting) quantity. For photosynthesis, the three principal limiting factors are light intensity, C$O_{2}$ concentration, and temperature. When one factor is not limiting, the rate plateaus until another factor is addressed. This has direct applications in greenhouse management (C$O_{2}$ enrichment), crop distribution (C3 in temperate, C4 in tropical zones), and understanding the impact of climate change on agricultural productivity.