You eat breakfast, lunch and dinner. A plant never eats at all! Instead, it makes its own food, right inside its green leaves.
Key idea
Green leaves use sunlight, air and water to make sugar, the plant's food. They give off oxygen, the gas we breathe.
Why it works
Light gives the leaf energy. The leaf uses that energy to join a gas from the air, called carbon dioxide, with water from the roots. Together they become sugar. The plant uses the sugar to live and to build new leaves, stems and wood.
Worked example: A famous test
Long ago, a scientist named van Helmont grew a willow tree in a pot for five years. The tree grew from 5 pounds to 169 pounds. Did it eat the soil?
- He weighed the soil at the start and at the end.
- The soil had lost only about 2 ounces.
- The tree had gained about 164 pounds.
Answer: No. The tree was not built from soil. We know now it was built mostly from air and water.
Common mistake
Many people think a plant eats soil, so a big tree must have used up lots of dirt. The soil gives water and a few nutrients. The tree itself is built mostly from air and water, put together with the energy of sunlight.
For the grown-up. Van Helmont thought the weight came from water alone. Scientists later found that much of it comes from carbon dioxide in the air. The process is called photosynthesis, a word for later grades.