A clear photosynthesis definition is the process by which green plants, algae and some bacteria use the energy of light to turn water and carbon dioxide into sugar, giving off oxygen as a by-product. The sugar feeds the plant; the oxygen goes into the air we breathe.
Almost every food chain on land and in the sea starts here, because animals eat plants or eat animals that ate plants. That makes photosynthesis one of the most important processes for life on Earth.
Entry at a glance
Meaning
Making sugar from light, water and carbon dioxide, releasing oxygenPart of speech
Noun (uncountable)Pronunciation
foh-toh-SIN-thuh-sisWord forms
photosynthesize (US), photosynthesise (UK), photosyntheticOrigin
Greek phos, light + synthesis, putting together; in English by the 1890sWhere it happens
Chloroplasts, mainly in leavesKey pigment
Chlorophyll, which makes leaves green
What does photosynthesis mean, word by word?
To define photosynthesis through its parts, and get the photosynthesis meaning straight from the word itself, split it in two Greek pieces: photo, light, and synthesis, putting together. So photosynthesis literally means putting together with light. That is exactly what happens: the plant builds a sugar molecule using light as its power source.
The term was borrowed into English from German in the 1890s. For a short while a rival name, photosyntax, was also proposed, but photosynthesis won out among scientists.
The equation in plain words
Biology classes summarise the overall reaction like this:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Read aloud: six molecules of carbon dioxide plus six of water, using light, make one molecule of glucose (a sugar) and six of oxygen. The plant takes carbon dioxide from the air through tiny pores called stomata, draws water up from its roots, and catches light with chlorophyll.
Two stages inside the leaf
| Stage | Needs light? | What happens |
|---|---|---|
| Light-dependent reactions | Yes | Chlorophyll absorbs light; water is split, releasing oxygen; energy is stored in carrier molecules |
| Calvin cycle (light-independent reactions) | Not directly | The stored energy is used to fix carbon dioxide into sugar |
Both stages take place in chloroplasts, small green structures inside plant cells. An enzyme, acting as a catalyst, speeds up the step where carbon dioxide is captured.
Photosynthesis definition in example sentences
- Leaves turn toward the window because the plant needs light for photosynthesis.
- Tiny algae in the ocean do roughly half of all photosynthesis on the planet.
- Our science project measured how fast pondweed photosynthesizes under a lamp by counting bubbles.
- Cloudy weeks slowed photosynthesis in the greenhouse, and the tomatoes ripened late.
- Without photosynthesis, there would be very little oxygen in the air.
Photosynthesis and respiration
Students often mix these up. They are close to mirror images:
- Photosynthesis stores energy: carbon dioxide and water in, sugar and oxygen out. It happens only in cells with chlorophyll, and only with light.
- Respiration releases energy: sugar and oxygen in, carbon dioxide and water out. It happens in almost all living cells, day and night, including in plants.
So plants both photosynthesize and respire. In daylight, they usually make more oxygen than they use.
Why it matters beyond the classroom
Photosynthesis supplies the energy that flows through nearly every ecosystem. It removes carbon dioxide from the air, which links it to discussions about climate and forests. Farmers depend on it for every crop, which is why light, water and soil nutrients matter so much in agriculture. Even fossil fuels hold energy that ancient plants captured by photosynthesis millions of years ago.
Explore related terms in science and ideas.
Mistakes to watch for
- Plants do not get their food from the soil. Soil gives water and minerals; the sugar is made in the leaves.
- Not only plants photosynthesize. Algae and cyanobacteria do too.
- The oxygen released comes from splitting water, not from the carbon dioxide.
- Spelling: photosynthesis, with a y.
Frequently asked questions
What is the definition of photosynthesis for kids?
Photosynthesis is how plants make their own food. They use sunlight, water and air to make sugar, and they give out oxygen that we breathe.
Where does photosynthesis take place?
Inside chloroplasts, which are found mostly in the cells of leaves. Chlorophyll in the chloroplasts catches the light.
What are the products of photosynthesis?
Glucose, a sugar, and oxygen. The plant uses the glucose for energy and growth and releases most of the oxygen into the air.
Can photosynthesis happen at night?
The light-dependent stage cannot. The plant needs light to split water and store energy, so photosynthesis mostly stops in the dark, while respiration continues.
How do you pronounce photosynthesis?
Say foh-toh-SIN-thuh-sis, where SIN gets the beat and the rest stays light.
Why are leaves green?
Chlorophyll absorbs red and blue light well but reflects much of the green light, so green is the colour that reaches our eyes.