Photosynthesis

Photosynthesis is the fundamental autotrophic process by which green plants, algae, and cyanobacteria convert light energy into chemical energy. The overall balanced equation is: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂. Note the use of 12H₂O (not 6H₂O), as demonstrated by Van Niel's experiments with photosynthetic bacteria, confirming that oxygen is released from water, not CO₂. The process occurs in chloroplasts — light reactions on thylakoid membranes and the Calvin cycle in the stroma.

Photosynthetic pigments are organised into two photosystems. Chlorophyll a is the primary pigment, functioning as the reaction centre in both PS I (P₇₀₀ — absorbs at 700 nm) and PS II (P₆₈₀ — absorbs at 680 nm). Chlorophyll b (yellow-green), carotenoids (yellow-orange), and xanthophylls (yellow) serve as accessory pigments that broaden the absorption spectrum and transfer energy to chlorophyll a. The absorption spectrum shows peaks in blue and red wavelengths, while the action spectrum (actual photosynthesis rate versus wavelength) closely mirrors chlorophyll a absorption, confirming its central role.

Light reactions begin at PS II, where P₆₈₀ absorbs light and splits water (photolysis/Hill reaction): 2H₂O → 4H⁺ + 4e⁻ + O₂. Excited electrons travel through the Z-scheme: PS II → plastoquinone (PQ) → cytochrome b6f complex → plastocyanin (PC) → PS I (P₇₀₀) → ferredoxin (Fd) → NADP⁺ reductase, producing NADPH. Non-cyclic photophosphorylation involves both PS II and PS I, producing ATP, NADPH, and O₂. Cyclic photophosphorylation involves only PS I, occurs in stroma lamellae, and produces only ATP — no NADPH and no O₂ evolution. The chemiosmotic hypothesis explains ATP synthesis: as electrons pass through the transport chain, protons accumulate inside thylakoids, creating a gradient that drives ATP synthase (CF₀-CF₁ complex).

The Calvin cycle (C₃ pathway) has three stages: Carboxylation — RuBisCO fixes CO₂ to the 5-carbon RuBP, producing two molecules of 3-phosphoglyceric acid (3-PGA, the first stable C₃ product). Reduction — 3-PGA is reduced to glyceraldehyde-3-phosphate (G₃P) using ATP and NADPH. Regeneration — G₃P molecules are rearranged to regenerate RuBP using ATP. Six turns of the cycle (fixing 6 CO₂) produce one glucose molecule, requiring 18 ATP and 12 NADPH.

C₄ plants (maize, sugarcane, sorghum) exhibit Kranz anatomy: a ring of bundle sheath cells surrounding vascular bundles, enclosed by mesophyll cells. In mesophyll cells, PEP carboxylase (with higher CO₂ affinity than RuBisCO) fixes CO₂ into oxaloacetate (OAA, 4-carbon — hence C₄), which converts to malate and is transported to bundle sheath cells, where CO₂ is released and enters the Calvin cycle. This CO₂-concentrating mechanism eliminates photorespiration.

CAM plants (cacti, Bryophyllum) use temporal separation: stomata open at night for CO₂ fixation (via PEP carboxylase → malic acid stored in vacuoles) and close during the day when the Calvin cycle operates using released CO₂, thus minimising water loss in arid environments.

Photorespiration is a wasteful process occurring in C₃ plants when RuBisCO acts as oxygenase instead of carboxylase, generating phosphoglycolate in a C₂ cycle that produces no ATP and wastes fixed carbon. It is absent in C₄ plants due to their CO₂ concentrating mechanism. According to Blackman's law of limiting factors, the rate of photosynthesis is limited by the factor present in the least amount — light intensity, CO₂ concentration, or temperature.

The key testable concept is the distinction between PS I (P₇₀₀, reduces NADP⁺) and PS II (P₆₈₀, splits water), and between non-cyclic (ATP + NADPH + O₂) and cyclic photophosphorylation (ATP only).