Photosynthesis is the biological process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored in carbohydrates. It occurs in two interconnected phases: the light-dependent reactions that capture energy and the light-independent Calvin cycle that fixes carbon dioxide.
Chloroplast Structure
Photosynthesis takes place in chloroplasts. The thylakoid membranes house the photosystems and electron transport chain components. These membranes are organized into stacked grana connected by unstacked stromal lamellae. The stroma, the aqueous space surrounding the thylakoids, contains the enzymes of the Calvin cycle, including ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco).
Light-Dependent Reactions
The light-dependent reactions begin when photons are absorbed by pigment molecules in the antenna complexes of photosystem II (PSII) and photosystem I (PSI). Chlorophyll a and b, along with accessory carotenoids, capture photons over a broad range of wavelengths. Excitation energy is transferred by resonance energy transfer to the reaction center.
In PSII, excitation of the P680 reaction center drives electron transfer to pheophytin, then through plastoquinone, the cytochrome b6f complex, and plastocyanin. Water is split at the oxygen-evolving complex, releasing O2 as a byproduct and providing electrons to reduce the oxidized P680. The plastoquinone pool transfers both electrons and protons across the thylakoid membrane, generating a proton gradient.
In PSI, excitation of the P700 reaction center passes electrons through ferredoxin to NADP+ reductase, producing NADPH. The proton gradient generated by the electron transport chain drives ATP synthesis via ATP synthase, a process called photophosphorylation. The overall products of the light-dependent reactions are ATP, NADPH, and O2.
Cyclic electron flow around PSI generates additional ATP without producing NADPH or O2, adjusting the ATP/NADPH ratio to meet metabolic demands.
The Calvin Cycle
The Calvin cycle (light-independent reactions) occurs in the stroma and uses ATP and NADPH to reduce CO2 to carbohydrates. It proceeds through three phases: carboxylation, reduction, and regeneration.
Carboxylation is catalyzed by Rubisco, which attaches CO2 to ribulose-1,5-bisphosphate (RuBP), forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). Rubisco can also catalyze a wasteful oxygenation reaction in photorespiration, where O2 competes with CO2 at the active site.
In the reduction phase, 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P). One molecule of G3P is exported per three CO2 molecules fixed to synthesize glucose and other carbohydrates.
The regeneration phase uses the remaining G3P to regenerate RuBP through a series of reactions consuming additional ATP, allowing the cycle to continue.
C4 and CAM Photosynthesis
C3 plants face the problem of photorespiration, which increases under high temperature and low CO2 conditions. C4 plants have evolved a spatial carbon-concentrating mechanism. CO2 is initially fixed in mesophyll cells by phosphoenolpyruvate carboxylase (PEPCase) into a four-carbon acid (oxaloacetate), which is transported to bundle sheath cells and decarboxylated, concentrating CO2 around Rubisco. This minimizes photorespiration and improves water and nitrogen use efficiency.
CAM (Crassulacean acid metabolism) plants separate carbon fixation temporally. They open stomata at night to take up CO2, fixing it into malate that is stored in vacuoles. During the day, the stomata close to conserve water, and the stored CO2 is released for the Calvin cycle.
Regulation and Environmental Responses
Photosynthesis is regulated by light intensity, CO2 concentration, temperature, and water availability. Non-photochemical quenching dissipates excess energy as heat to protect against photodamage. The xanthophyll cycle interconverts violaxanthin, antheraxanthin, and zeaxanthin to regulate energy dissipation. State transitions redistribute light energy between PSII and PSI.
Under stress conditions, reactive oxygen species (ROS) can accumulate and damage the photosynthetic apparatus. Plants have evolved antioxidant systems including ascorbate, glutathione, and enzymes such as superoxide dismutase and ascorbate peroxidase to mitigate oxidative damage.
