Photosynthesis is the biological process by which organisms capture light and convert its energy into chemical forms that support the synthesis of organic compounds. It occurs in plants, algae, cyanobacteria, and several other groups of bacteria. In oxygen-producing photosynthesis, light powers the conversion of carbon dioxide and water into carbohydrates, with oxygen released as a by-product. This process provides the organic matter supporting most food webs and connects biological activity with the composition of Earth’s atmosphere. (openstax.org)
Organisms and cellular structures
Plants and algae carry out photosynthesis in chloroplasts, specialized structures within their cells. A chloroplast contains an internal membrane system composed of flattened sacs called thylakoids, often arranged in stacks called grana. The surrounding fluid, the stroma, contains enzymes involved in carbon assimilation. Light-driven reactions take place in the thylakoid membranes, whereas the principal carbon-fixing reactions occur in the stroma. (openstax.org)
Cyanobacteria perform oxygen-producing photosynthesis without chloroplasts. Their photosynthetic machinery is located in cellular membranes. Chloroplasts originated through the incorporation of a cyanobacterial ancestor into a host cell, an evolutionary relationship explained by endosymbiotic theory. Chloroplasts retain their own genetic material but depend extensively on proteins encoded by the host’s nucleus. (ncbi.nlm.nih.gov)
Overall chemistry
A familiar net equation for oxygen-producing photosynthesis is:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
This equation summarizes many separate reactions rather than a single biochemical step. Glucose represents the eventual production of carbohydrates; it is not the immediate product of the carbon-fixing cycle. The oxygen released comes from water, not carbon dioxide. Water supplies electrons, while carbon dioxide supplies the carbon incorporated into organic compounds. (openstax.org)
Photosynthesis and cellular respiration have complementary roles in biological energy transformations, but they are not simply the same pathway operating in opposite directions. Photosynthesis stores energy in reduced carbon compounds; respiration releases usable energy through their oxidation. Photosynthetic organisms also respire, consuming some of the organic matter they produce. (ncbi.nlm.nih.gov)
Light-dependent reactions
Chlorophyll and accessory pigments absorb light over particular wavelength ranges. In plants, chlorophyll absorbs strongly in blue and red regions of the visible spectrum, while carotenoids broaden light absorption and help protect the photosynthetic apparatus. Pigment molecules are organized with proteins into photosystems, each containing a light-harvesting antenna and a reaction center. The antenna transfers excitation energy to the reaction center, where an electron is passed to an acceptor. (openstax.org)
In the main electron-flow pathway, photosystem II extracts electrons from water, releasing oxygen and hydrogen ions. Electrons pass through transport components to photosystem I, where further light absorption raises their energy again. They ultimately contribute to the formation of NADPH, a carrier of reducing power. Electron transport also establishes a proton gradient across the thylakoid membrane. (openstax.org)
Protons flowing back through ATP synthase drive the formation of adenosine triphosphate, or ATP. The production of ATP through this light-powered mechanism is called photophosphorylation. ATP and NADPH connect the light-dependent reactions to carbon assimilation. Cyclic electron flow around photosystem I can contribute additional ATP without directly producing NADPH or oxygen. (ncbi.nlm.nih.gov)
Carbon fixation and the Calvin cycle
Carbon fixation incorporates inorganic carbon into organic molecules. In plants, algae, and cyanobacteria, the principal pathway is the Calvin cycle. Its first stage is catalyzed by Rubisco, an enzyme that attaches carbon dioxide to the five-carbon compound ribulose-1,5-bisphosphate. The resulting unstable intermediate divides into two molecules of three-carbon 3-phosphoglycerate. (openstax.org)
ATP and NADPH support subsequent reactions that produce glyceraldehyde-3-phosphate, or G3P. Some G3P becomes available for carbohydrate synthesis, while most participates in regenerating the carbon dioxide acceptor. Producing one net G3P requires fixation of three carbon dioxide molecules and expenditure of nine ATP and six NADPH. Although often called “light-independent reactions,” these reactions depend on products of the light reactions, and several involved enzymes are activated by light. (openstax.org)
C3, C4, and CAM pathways
Most plants use C3 photosynthesis, named for the three-carbon product of initial Calvin-cycle fixation. Rubisco can also react with oxygen, initiating photorespiration, which consumes energy and releases some previously fixed carbon. This becomes especially significant when hot, dry conditions reduce the carbon dioxide available inside leaves. (ncbi.nlm.nih.gov)
C4 photosynthesis, found in maize and sugarcane, initially incorporates carbon into four-carbon compounds. In typical C4 leaves, these compounds deliver carbon dioxide to bundle-sheath cells, concentrating it around Rubisco. Crassulacean acid metabolism, or CAM, separates initial carbon uptake and Calvin-cycle activity in time: plants commonly open their stomata at night, store carbon in organic acids, and release carbon dioxide internally during daylight. Both strategies can reduce water loss, although their mechanisms and energetic costs differ. (ncbi.nlm.nih.gov)
Other forms and ecological significance
Some bacteria perform anoxygenic photosynthesis, which does not release oxygen. Green and purple sulfur bacteria, for example, can use hydrogen sulfide rather than water as an electron donor. Their pigments, reaction centers, and carbon-assimilation pathways differ from those of oxygen-producing organisms. (pmc.ncbi.nlm.nih.gov)
Photosynthesis is a central component of the carbon cycle and supports primary production in terrestrial and aquatic ecosystems. In the oceans, photosynthetic phytoplankton provide organic matter for marine food webs. Their growth depends on light and nutrient availability, while temperature and circulation influence where productive populations develop. Atmospheric carbon uptake through photosynthesis is balanced in part by respiration and decomposition; consequently, carbon fixation does not necessarily imply long-term carbon storage. (science.nasa.gov)