The basic idea
Photosynthesis is the process by which plants, algae and some bacteria use light energy to build sugar from carbon dioxide and water. Oxygen is released as a by-product. The name comes from Greek roots meaning roughly “putting together with light.”
Almost all life on Earth depends on it, directly or indirectly. It is the entry point for energy in most food webs, because producers capture sunlight and store it in chemical form. It is also the main source of the oxygen in the atmosphere, built up over a very long time in Earth’s history.
The summary equation
The overall process for the sugar glucose can be written as:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
In words: six molecules of carbon dioxide and six of water, with light energy, yield one molecule of glucose and six of oxygen. The equation is balanced. Both sides have 6 carbon atoms and 12 hydrogen atoms, and 18 oxygen atoms (12 from carbon dioxide and 6 from water on the left; 6 in glucose and 12 in oxygen on the right). Like any chemical reactions, it rearranges atoms rather than creating or destroying them.
This equation is a simplification. In reality, photosynthesis involves many steps and several intermediate molecules, and plants build many compounds besides glucose.
Where it happens
In plants, photosynthesis takes place mainly in leaves, inside small structures in cells called chloroplasts. Chloroplasts contain a green pigment, chlorophyll, which absorbs light most strongly in the blue and red parts of the spectrum and reflects more of the green. That is why most leaves look green.
Leaves are well designed for the job. Their flat shape catches light. Tiny pores called stomata let carbon dioxide in and oxygen and water vapor out. Water comes up from the roots through vessels called xylem, and the sugars produced are carried to other parts of the plant through the phloem.
Stage one: the light-dependent reactions
In the first stage, chlorophyll and related molecules in the chloroplast’s internal membranes absorb light. The captured energy is used to split water molecules. This releases oxygen, which leaves the leaf, along with electrons and hydrogen ions. A chain of molecules passes the electrons along, and the energy released is used to make two energy-carrying molecules, usually called ATP and NADPH. Think of them as small rechargeable batteries that carry usable energy to the next stage.
The oxygen released at this point comes from the water, not from the carbon dioxide. This was shown in the twentieth century by experiments using a heavy form of oxygen as a tracer, which is an application of atomic isotopes.
Stage two: the Calvin cycle
The second stage does not directly need light, though it depends on the products of the first. It takes place in the fluid part of the chloroplast. Here, carbon dioxide is attached to a five-carbon molecule by an enzyme commonly called Rubisco. The resulting compounds are converted, using the energy from ATP and NADPH, into small sugar molecules. These can be combined to make glucose, and some of the products are used to regenerate the starting molecule so the cycle can run again.
The plant uses its sugars in several ways. It can burn them in respiration to power its cells, which uses oxygen and releases carbon dioxide, just as animals do. It can link them into starch for storage or into cellulose to build cell walls, and it can use them as raw material for fats, proteins and other compounds, with minerals taken up from the soil. This is how a tree can grow large from air and water: most of its dry mass comes from carbon taken from the air, not from the soil.
What limits the rate
Several factors affect how fast photosynthesis runs. Light intensity matters, up to a point beyond which other things become limiting. So do the carbon dioxide concentration, temperature and water supply. If it is hot and dry, plants often close their stomata to save water, which also cuts off the supply of carbon dioxide. Some plants have evolved variations to cope with this. Many tropical grasses, including maize and sugarcane, use a modified pathway that concentrates carbon dioxide around Rubisco, and many desert plants take in carbon dioxide at night.
Beyond land plants
Photosynthesis is not only a land-plant activity. Algae and cyanobacteria in oceans and lakes carry out a large share of the photosynthesis on Earth, and tiny drifting algae form the base of many marine food chains. Cyanobacteria are thought to have been the first organisms to produce oxygen in this way, billions of years ago.
Why it matters for people
Photosynthesis underlies agriculture, forests and fisheries. Fossil fuels are the remains of ancient photosynthesizing life, so the energy in coal, oil and gas can be traced back to sunlight captured long ago. Forests and oceans take up part of the carbon dioxide humans emit, which is a topic in the science of the greenhouse effect, but the amount that can be absorbed is limited and varies with conditions.
Researchers are also exploring how to improve crop photosynthesis or imitate it artificially to produce fuels. These are active research areas, and it is too early to say how much they will achieve.