Photosynthesis is the process that feeds most life on Earth. Plants, algae and some bacteria use light energy to build sugars from simple ingredients, and as a by-product they release the oxygen that many living things breathe.
Key takeaways
- Photosynthesis turns carbon dioxide and water into sugar and oxygen using light energy.
- It takes place mainly in chloroplasts, which contain the green pigment chlorophyll.
- It has two broad stages: light-dependent reactions and the Calvin cycle.
- Plants also use oxygen for respiration, so they are not only oxygen producers.
- Photosynthesis underpins nearly all food chains and stores carbon in living material.
The overall idea
In simple terms, the plant takes in carbon dioxide from the air through tiny pores called stomata and water from the soil through its roots. Using light energy absorbed by chlorophyll, it converts these into glucose, a sugar, and releases oxygen. The balanced equation is often written as carbon dioxide plus water, with light, producing glucose plus oxygen.
Where it happens
Inside leaf cells are structures called chloroplasts. They contain stacks of membranes where chlorophyll and other pigments capture light. Chlorophyll absorbs mostly red and blue wavelengths and reflects green, which is why leaves appear green.
Stage one: capturing light energy
In the light-dependent reactions, light energy excites electrons in chlorophyll. The energy is used to split water molecules, which releases oxygen, and to make two energy-carrying molecules, ATP and NADPH. These molecules act like rechargeable batteries that carry energy to the next stage.
Stage two: building sugar
The second stage is the Calvin cycle, which takes place in the fluid of the chloroplast. Using the energy from ATP and NADPH, the plant attaches carbon from carbon dioxide onto existing molecules and gradually builds simple sugars. These sugars can be used immediately for energy, turned into starch for storage or used to make cellulose, which forms cell walls.
What affects the rate
Light intensity, carbon dioxide concentration, temperature and water supply all influence how fast photosynthesis runs. Too little light slows the light-dependent stage, while very high temperatures can damage the enzymes involved. Plants adjust by opening and closing their stomata, balancing the need to take in carbon dioxide against the risk of losing water.
Why it matters
Photosynthesis produces the food that supports almost every food chain, directly in plants and indirectly in the animals that eat them. It also removes carbon dioxide from the air and stores carbon in plants and soils. Plants do respire too, using some of the sugar and oxygen for their own energy, so over a full day a plant's net oxygen output depends on how much light it receives.
Different routes: C3, C4 and CAM
Most plants, including wheat, rice and trees, use the C3 pathway, named after the three-carbon molecule formed first in the Calvin cycle. In hot, dry conditions, C3 plants close their stomata to save water, and an enzyme called Rubisco sometimes grabs oxygen instead of carbon dioxide, wasting energy in a process called photorespiration. C4 plants, such as maize and sugarcane, have evolved a way to concentrate carbon dioxide around Rubisco, which makes them more efficient in hot, bright conditions. CAM plants, including cacti and many succulents, open their stomata at night to take in carbon dioxide and store it, then use it by day with the stomata closed, saving water in deserts.
Light and pigments
Chlorophyll comes in several forms. Chlorophyll a and b absorb mostly red and blue light and reflect green. Other pigments, such as carotenoids, absorb blue-green light and pass energy to chlorophyll, and also protect against damage from too much light. The pattern of which wavelengths drive photosynthesis most effectively, called the action spectrum, peaks in blue and red. This is why growers using artificial lights often use red and blue LEDs, though full-spectrum white light also works.
Photosynthesis in the sea
Land plants are only part of the story. Microscopic algae and cyanobacteria in the ocean, known as phytoplankton, carry out a large share of global photosynthesis, often estimated at roughly half of the oxygen produced each year. Phytoplankton form the base of marine food webs, and their growth depends on light, nutrients and temperature. Changes in ocean conditions can affect their abundance, which is why satellites that track ocean colour are used to monitor them.
Improving photosynthesis
Because photosynthesis converts only a small share of sunlight into stored energy, researchers are looking for ways to boost crop yields. Projects have tried to make Rubisco work better, reduce wasteful photorespiration and speed up the plant's recovery from shade. Others work on artificial photosynthesis, devices that use sunlight to split water or turn carbon dioxide into fuels. These approaches are still developing, and moving them from laboratory to field or industry is challenging.
Photosynthesis and the carbon cycle
Photosynthesis pulls enormous amounts of carbon dioxide from the air each year, with estimates of well over one hundred billion tonnes of carbon fixed by land plants annually. Much of it returns through respiration and decay, so the net effect on atmospheric carbon depends on the balance. Forests, grasslands and soils can store carbon for long periods, which is why protecting and restoring them is part of climate strategies, alongside cutting emissions.
Frequently asked questions
Do plants photosynthesise at night?
The light-dependent reactions need light, so photosynthesis largely stops in the dark, but respiration continues.
Are all plants green?
Most are, because of chlorophyll. Some leaves look red or purple because of other pigments, but they usually still contain chlorophyll.
Do algae photosynthesise?
Yes. Algae, including many ocean species, and cyanobacteria perform photosynthesis and contribute a large share of the oxygen in the atmosphere.
What is Rubisco?
Rubisco is the enzyme that captures carbon dioxide in the Calvin cycle. It is thought to be the most abundant enzyme on Earth, but it is slow and sometimes mistakes oxygen for carbon dioxide, which is one reason scientists study ways to improve it.
Do plants get their energy from the soil?
No. Plants get their energy from sunlight and build their bodies mostly from carbon dioxide and water. Soil supplies water and minerals such as nitrogen, phosphorus and potassium, which are needed in smaller amounts.
Can photosynthesis happen under artificial light?
Yes. Plants can photosynthesise under artificial light if it contains suitable wavelengths and enough intensity. Indoor farms and greenhouses use LED lighting, and the choice of spectrum and brightness depends on the crop.
Sources and further reading
Written by
FactNest Editorial Team
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