Photosynthesis Quiz: How Plants Eat Light
Food from light
Plants don't eat food the way animals do. They make it. In photosynthesis, a plant uses the energy of sunlight to turn carbon dioxide from the air and water from the soil into a sugar called glucose.
Oxygen is left over and released into the air. In words, the recipe is: carbon dioxide plus water, with light, gives glucose plus oxygen.
The plant uses the glucose for energy and as a building block. It links sugars together to make starch, which it stores, and cellulose, which forms its cell walls and gives wood much of its strength.
Inside the leaf
Photosynthesis happens mainly in leaves, inside tiny green structures called chloroplasts. They contain chlorophyll, a pigment that absorbs red and blue light well but reflects green, which is why most leaves look green.
Carbon dioxide gets in through tiny pores called stomata, usually on the underside of the leaf. The same pores let oxygen and water vapor out. Water travels up from the roots through tubes called xylem, while other tubes, called phloem, carry the sugars around the plant.
At night, with no light, photosynthesis stops, but plants keep breathing: like us, they use oxygen to release energy from their sugar.
Why it matters
Almost every food chain on Earth starts with photosynthesis. When you eat bread, fruit, or even meat from an animal that ate plants, you're using energy that a plant captured from sunlight.
Photosynthesis also made the oxygen in our air. Tiny bacteria called cyanobacteria began doing it billions of years ago, slowly filling the air with oxygen. Today about half of Earth's oxygen still comes from the ocean, made by tiny drifting organisms called phytoplankton.
Even coal, oil and gas hold energy from ancient sunlight: they formed from plants and other living things buried millions of years ago.