Photosynthesis works by coupling the absorption of light with electron transfer, proton movement, ATP production, reduction of carbon dioxide and synthesis of organic molecules. In a typical plant cell, the process takes place mainly inside chloroplasts. Pigments embedded in thylakoid membranes absorb photons, and the energy of those photons excites electrons within specialised reaction centres. In oxygenic photosynthesis, two major photosystems operate in sequence, photosystem II and photosystem I.
Water is oxidised at photosystem II, replacing electrons that have been excited and transferred onward. The electrons move through an electron transport chain, while the energy released during electron transfer contributes to proton accumulation inside the thylakoid lumen. ATP synthase uses the resulting proton motive force to produce ATP. Photosystem I then re energises electrons with additional light energy, ultimately enabling production of NADPH. ATP and NADPH are subsequently used in the Calvin cycle to convert carbon dioxide into organic carbon compounds.
The first major stage is light absorption. Chlorophyll molecules absorb particular wavelengths of visible light, with especially strong absorption in blue and red regions of the spectrum. Accessory pigments broaden the range of usable light and help protect the photosynthetic apparatus. Pigments are organised into antenna complexes that transfer excitation energy towards reaction centre chlorophyll molecules. At the reaction centre, light energy produces a high energy electron state capable of participating in electron transfer.
This charge separation is fundamental because it converts absorbed electromagnetic energy into a form that can drive chemical reactions. Photosystem II contains a reaction centre known as P680, while photosystem I contains a reaction centre known as P700, names based on characteristic absorption wavelengths. The precise molecular arrangement ensures that excitation energy reaches the reaction centre efficiently and that electrons are transferred to appropriate acceptor molecules rather than simply losing their energy as heat.
Photosystem II initiates the electron flow that ultimately leads to oxygen production. When light excites its reaction centre, an electron is transferred to an electron acceptor, leaving the reaction centre electron deficient. A specialised water oxidising complex replaces the lost electrons by extracting them from water molecules. This process produces molecular oxygen, protons and replacement electrons. The oxygen released during photosynthesis therefore originates from water rather than directly from carbon dioxide.
Electrons then pass through carriers including plastoquinone and the cytochrome b6f complex before reaching plastocyanin and photosystem I. As electrons pass through the cytochrome complex, proton translocation contributes to the electrochemical gradient across the thylakoid membrane. This sequence links water oxidation to the formation of a proton gradient and demonstrates how photosynthetic electron transport converts light induced electron energy into a gradient that can later power ATP synthesis.
Photosystem I receives electrons arriving from the earlier part of the chain and uses another photon to raise their energy again. Excited electrons are transferred through electron carriers to ferredoxin, after which an enzyme called ferredoxin NADP reductase facilitates reduction of NADP plus to NADPH. NADPH is an important source of reducing power for carbon fixation. ATP is generated through photophosphorylation, in which the proton gradient created across the thylakoid membrane drives ATP synthase.
In linear electron flow, both photosystem II and photosystem I contribute to the production of ATP and NADPH. Plants can also use cyclic electron flow around photosystem I, which increases ATP production without producing NADPH or oxygen. This flexibility allows the chloroplast to adjust the relative supply of ATP and reducing power to the requirements of carbon fixation and other metabolic processes.
The Calvin cycle uses ATP and NADPH to incorporate inorganic carbon into organic molecules. Carbon dioxide is initially attached to ribulose bisphosphate by RuBisCO, producing an unstable intermediate that splits into two molecules of 3 phosphoglycerate. ATP and NADPH then drive reduction reactions that convert these compounds into glyceraldehyde 3 phosphate and related triose phosphate molecules.
Some triose phosphate leaves the cycle to contribute to carbohydrate synthesis, while most is used in reactions that regenerate ribulose bisphosphate so the cycle can continue. The cycle therefore requires continual supplies of ATP, NADPH and carbon dioxide. Although often called the light independent stage, the Calvin cycle normally depends indirectly on light because ATP and NADPH are supplied by light dependent reactions and several enzymes are regulated by light associated changes in the chloroplast.
The efficiency of photosynthesis is affected by environmental conditions. Light intensity influences photon availability, but increasing light does not produce unlimited increases in photosynthetic rate. At sufficiently high intensities, photosynthetic machinery can become saturated or damaged. Carbon dioxide concentration can constrain carbon fixation, particularly when stomata close. Temperature affects enzyme activity and membrane processes, with excessive heat increasing photorespiration and potentially damaging proteins.
Water availability is also critical because water supplies electrons for oxygenic photosynthesis and drought can cause stomatal closure that reduces carbon dioxide entry. Nutrients such as nitrogen, magnesium and iron are needed to construct photosynthetic proteins, pigments and electron transport components. Plants therefore balance carbon gain against water loss and protect their photosynthetic systems against excessive excitation.
Photorespiration is an important complication of carbon fixation. RuBisCO can act not only as a carboxylase but also as an oxygenase, particularly when the ratio of oxygen to carbon dioxide around the enzyme becomes high. The resulting pathway consumes energy and releases previously fixed carbon, reducing net photosynthetic efficiency under some conditions. C4 plants have evolved mechanisms that concentrate carbon dioxide around RuBisCO, reducing the frequency of oxygenation.
CAM plants achieve a related carbon concentrating strategy through temporal separation of carbon dioxide uptake and carbon fixation. These pathways demonstrate that photosynthesis is shaped by environmental pressures and evolutionary history. The basic light reactions remain broadly conserved, but the way organisms supply carbon dioxide to the Calvin cycle can differ significantly. Such adaptations are especially important in hot, dry or high light environments where water conservation and carbon fixation present competing demands.
Photosynthesis therefore works through a chain of coupled physical and chemical events. Photons provide the initial energy, chlorophyll based reaction centres create high energy electrons, water supplies replacement electrons, electron transport creates a proton gradient, ATP synthase produces ATP, photosystem I enables production of NADPH, and the Calvin cycle uses ATP and NADPH to fix carbon dioxide into organic molecules. The process is tightly regulated and integrated with respiration, mineral nutrition, water transport and plant growth.
It is inaccurate to describe sunlight as being directly transformed into glucose because several intermediate stages intervene and the immediate products of carbon fixation are mainly triose phosphates rather than free glucose. Nevertheless, photosynthesis ultimately enables light energy to become stored in chemical compounds that can be used to build carbohydrates, lipids, amino acids, nucleotides and structural materials. This makes photosynthesis one of the most important energy conversion processes in the biosphere.
