Photosynthesis is a biological process through which photosynthetic organisms capture energy from light and use it to drive the synthesis of energy rich organic molecules from relatively low energy inorganic materials. In oxygenic photosynthesis, which occurs in plants, algae and cyanobacteria, water supplies electrons and oxygen is released as a byproduct. Carbon dioxide provides the carbon that is incorporated into carbohydrates and other organic compounds. In plants, most photosynthesis occurs in chloroplasts, specialised organelles containing chlorophyll and other pigments.
The overall process can be summarised approximately as carbon dioxide plus water plus light energy producing carbohydrate and oxygen, although this simplified equation hides a large network of individual reactions. Photosynthesis does not simply create energy. It converts electromagnetic radiation into chemical energy stored in molecular bonds. This stored chemical energy can subsequently support cellular respiration, biosynthesis, growth and food webs.
Photosynthesis consists of two broad sets of reactions traditionally called the light dependent reactions and the light independent reactions. The light dependent reactions occur in the thylakoid membranes of chloroplasts. Pigment molecules absorb photons and use their energy to excite electrons. In oxygenic photosynthesis, the water splitting reactions associated with photosystem II replace electrons lost from the photosynthetic electron transport chain and release oxygen. Electron transport contributes to the creation of a proton gradient across the thylakoid membrane.
ATP synthase then uses the proton gradient to produce ATP, while electrons are ultimately used to reduce NADP plus to NADPH. ATP and NADPH provide energy and reducing power for carbon fixation. The light dependent reactions therefore convert light energy into chemical intermediates that can be used by subsequent reactions rather than directly producing large quantities of glucose in a single step.
The light independent reactions are commonly referred to as the Calvin cycle, although this terminology should not imply that they can operate indefinitely without the products of light dependent reactions. In plants, the Calvin cycle occurs mainly in the chloroplast stroma. Carbon dioxide is incorporated into an organic molecule through the action of the enzyme RuBisCO. The resulting compounds are processed through a sequence of reactions that consume ATP and NADPH and produce triose phosphate molecules.
Some of these molecules are used to regenerate the carbon dioxide accepting compound, while others can contribute to the synthesis of carbohydrates and many other organic substances. The direct products of carbon fixation are not usually free glucose molecules. Instead, the plant uses carbon intermediates to build sugars such as sucrose and starch as well as structural compounds and metabolic precursors. Photosynthesis therefore supplies both energy rich carbon and raw material for a vast range of plant biochemistry.
Chloroplast structure is closely related to photosynthetic function. A chloroplast contains an outer and inner envelope surrounding the stroma, together with an internal membrane system made of flattened sacs called thylakoids. Thylakoids are frequently arranged in stacks called grana and are connected by membrane regions called stroma lamellae. Photosystems, electron carriers and ATP synthase are embedded in these membranes. The separation between the thylakoid lumen and stroma permits the establishment of a proton gradient needed for ATP synthesis.
Chlorophyll and accessory pigments are organised into light harvesting complexes that increase the range of wavelengths that can contribute to photosynthetic excitation. The spatial organisation of these structures allows light capture, electron transport, proton movement and chemical synthesis to be coordinated. Photosynthetic architecture therefore illustrates how biological membranes can be specialised to convert energy efficiently through coupled molecular processes.
Photosynthesis is closely connected with the carbon and oxygen cycles of Earth. By removing carbon dioxide from the atmosphere or aquatic environment and incorporating its carbon into organic compounds, photosynthetic organisms form the primary entry point for much inorganic carbon into most biological food webs. Oxygenic photosynthesis also produces a large proportion of Earth's atmospheric oxygen.
The evolution of oxygenic photosynthesis was associated with the long term oxygenation of Earth's atmosphere, which fundamentally changed the chemistry of the planet and enabled the evolution and diversification of organisms that use oxygen as a powerful terminal electron acceptor. Plants, algae and cyanobacteria therefore have ecological effects far beyond their individual cells. They support consumers by providing organic carbon, influence atmospheric composition, contribute to soil and aquatic ecosystems and participate in global carbon exchange. Photosynthesis is consequently both a cellular biochemical process and a major planetary process.
Photosynthetic organisms differ considerably in how they capture carbon and manage environmental conditions. Most plants use the C3 pathway, in which carbon dioxide enters the Calvin cycle directly. C4 plants spatially separate initial carbon fixation and the Calvin cycle, reducing the impact of photorespiration under conditions such as high temperature and strong light. CAM plants temporally separate carbon dioxide uptake and carbon fixation, often opening stomata at night to reduce water loss in dry environments.
These adaptations demonstrate that photosynthesis is not identical in every plant. The photosynthetic machinery can also respond to light intensity, temperature, carbon dioxide concentration, water availability and nutrient supply. Excessive light can damage photosynthetic components, so plants possess protective mechanisms that dissipate surplus excitation energy and repair damaged proteins. Photosynthesis is therefore regulated dynamically rather than operating at a fixed rate regardless of environmental conditions.
Photosynthesis has enormous practical importance in agriculture, forestry, biotechnology and climate science. Crop productivity depends on the ability of plants to convert light energy into biomass, although growth is also constrained by water, nutrients, temperature, genetics and other factors. Photosynthetic pigments and reaction centres have inspired research into artificial photosynthesis and solar energy conversion. Understanding carbon fixation is also important for predicting plant responses to rising atmospheric carbon dioxide and changing climate conditions.
Photosynthesis does not mean that plants obtain all of their nutrition from sunlight. Light supplies energy, while plants also require mineral nutrients such as nitrogen, phosphorus, potassium, magnesium and iron. Plants must additionally obtain water and carbon dioxide. Photosynthesis therefore provides a central energetic and carbon building function, but it operates within a larger physiological system involving mineral nutrition, respiration, transport and growth.
Photosynthesis is best understood as a coordinated energy conversion system rather than a single reaction or a simple process in which plants make food from sunlight. Light harvesting, charge separation, electron transport, water oxidation, proton gradients, ATP formation, NADPH production and carbon fixation are connected through multiple molecular complexes and regulatory pathways. Its products feed into respiration and biosynthesis, while its raw materials are linked to atmospheric and environmental cycles.
The process also varies among species and environments, with different carbon fixation strategies reflecting evolutionary adaptations. At the cellular level, photosynthesis converts light energy into chemical energy and fixes inorganic carbon into organic molecules. At the ecological level, it provides the foundation for most terrestrial and many aquatic food webs. At the planetary level, oxygenic photosynthesis has profoundly influenced atmospheric chemistry and the global carbon cycle.
