Definition · Plain-language
Photosynthesis
Photosynthesis is the process by which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen as a by-product.
The step most authors miss
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How plants make their own food
Photosynthesis is the way plants make food instead of eating it. Using light energy captured by the green pigment chlorophyll, a plant combines carbon dioxide from the air with water from the soil to build glucose, a sugar that stores energy. The word equation sums it up: carbon dioxide + water → glucose + oxygen, with light energy driving the reaction. The glucose can be used straight away in respiration, stored as starch, or built into other substances the plant needs, such as cellulose for cell walls.
Where it happens and what it needs
Photosynthesis takes place mainly in the leaves, inside organelles called chloroplasts, which are packed with chlorophyll. Leaves are well suited to the job: they are broad and flat to catch light, thin so gases move easily, and full of tiny pores called stomata that let carbon dioxide in. The process needs three things — light, carbon dioxide and water — plus a suitable temperature, because the reactions are controlled by enzymes. If any of these is in short supply, it becomes a limiting factor that slows the rate of photosynthesis.
Why photosynthesis matters
Photosynthesis underpins almost all life on Earth. By making glucose, plants form the base of nearly every food chain: the energy in your food traces back to light captured by plants. Photosynthesis also releases the oxygen that most living things need for respiration, and it removes carbon dioxide from the atmosphere. In effect, photosynthesis and respiration are opposite, complementary processes — one stores energy and oxygen, the other releases energy and uses oxygen — and together they keep the gases of the atmosphere in balance.
Key facts
At a glance
- Definition: plants use light to make glucose from carbon dioxide and water
- Word equation: carbon dioxide + water → glucose + oxygen (using light)
- Where: in chloroplasts, mainly in the leaves
- Pigment: chlorophyll, which traps light energy
- By-product: oxygen, released into the air
- Limiting factors: light, carbon dioxide and temperature
Common misconceptions
What people often get wrong
Often heard: Plants get their food from the soil through their roots.
Actually: Plants make their own food by photosynthesis, not by absorbing it from soil. Roots take up water and dissolved minerals, but the glucose that feeds the plant is built in the leaves from carbon dioxide and water using light energy.
Often heard: Plants photosynthesise instead of respiring.
Actually: Plants do both. They photosynthesise in the light to make glucose, and they respire all the time to release energy from it. Photosynthesis and respiration are opposite processes that go on in the same plant.
Often heard: Photosynthesis happens only in bright sunlight and stops completely otherwise.
Actually: Photosynthesis needs light, so it slows in dim conditions and stops in darkness, but it can still occur in moderate light. Light is one of several limiting factors, alongside carbon dioxide level and temperature, that set its rate.
Going deeper
