Photosynthesis: How Plants Turn Sunlight Into Food and Oxygen
Almost all life on Earth depends on plants capturing sunlight. Follow the steps of photosynthesis, from a leaf drinking water to the oxygen you are breathing.
Every meal you eat can be traced back to photosynthesis. The same process produces much of the oxygen in the air. It is the quiet engine that powers life on Earth.
The simple version
Plants, algae, and some bacteria take in carbon dioxide from the air and water from the soil. Using the energy of sunlight, they turn them into sugar for food, and release oxygen.
In words: carbon dioxide + water + light energy → sugar + oxygen.
Where it happens
Inside plant cells there are small structures called chloroplasts. They contain a green pigment called chlorophyll, which captures sunlight. This is why most leaves are green. Chlorophyll absorbs red and blue light well and reflects green.
The two stages, step by step
Light is absorbed
Chlorophyll in the chloroplasts captures sunlight energy.
Water is split
The energy splits water molecules into hydrogen and oxygen. The oxygen is released through tiny pores on the leaf called stomata.
Energy is stored
The captured energy is stored in energy-carrying molecules. This first stage is called the light-dependent reactions.
Carbon dioxide is taken in
The plant takes in carbon dioxide through its stomata.
Sugar is built
Using the stored energy, the plant joins carbon atoms into sugar molecules. This second stage is called the Calvin cycle.
The sugar is used
The plant uses it for energy right away, stores it as starch, or builds new leaves, roots, and fruit.
Why photosynthesis matters
- Food: it is the base of nearly every food chain.
- Oxygen: it released the oxygen that animals breathe. Oxygen in our atmosphere built up over billions of years thanks to early photosynthetic life.
- Carbon dioxide: it removes carbon dioxide from the air, which is why forests and oceans are called carbon sinks.
- Fuel: coal, oil, and gas are ancient plant and algae energy stored over millions of years.
What affects the speed
- Light: more light speeds it up, until a limit.
- Carbon dioxide level.
- Temperature: too cold or too hot slows it down.
- Water: shortage forces plants to close their stomata, which also stops carbon dioxide from entering.
What most people miss
- Plants breathe too. At night they use oxygen for respiration. Overall, they produce much more oxygen than they use.
- A lot of the world's oxygen comes from the ocean, from tiny algae called phytoplankton.
- Some plants are not green but still photosynthesise. Their green pigment is simply hidden by red or purple pigments.
