Built from air

About this video

A cow grows big by eating grass, but a tree eats nothing at all, and still an acorn can become an oak that weighs many tonnes. Where does the wood come from? About 400 years ago Jan Baptist van Helmont tested whether plants eat soil. He dried about 90 kg of soil, planted a willow of about 2 kg in it and for five years gave it nothing but water. The willow grew to about 77 kg; the soil was only about 57 grams lighter. So the tree had not eaten the soil.

In 1771 Joseph Priestley put a sprig of mint into a closed jar of air in which a candle had burned until it went out. Ten days later a new candle could burn in that air again: the plant had mended the air. Jan Ingenhousz then showed that only the green parts of a plant do this, and only in light.

Today we call it photosynthesis. In the chloroplasts of its leaves, a plant takes carbon dioxide from the air and water from the soil and uses the energy of light to make glucose, a sugar; oxygen is left over and goes out into the air. The plant uses the glucose for energy, stores it as starch in rice, wheat and potatoes, and joins it into cellulose, the stuff of wood. So the willow was built from water and from carbon dioxide taken out of the air, and almost all other life feeds on what plants make. This video goes with the lesson How Plants Make Food in Biology Level 1 on Knaph.

Transcript

1. 0:00An acorn becomes an oak that weighs tonnes. But a tree eats nothing. So where does all that wood come from?
2. 0:10About 400 years ago, van Helmont planted a two kilogram willow in ninety kilograms of dry soil.
3. 0:19For five years he gave it only water. The willow grew to about 77 kilograms, but the soil lost only 57 grams.
4. 0:32In 1771, Priestley put mint in a jar where a candle had burned out.
5. 0:38Ten days later, a new candle could burn there again. The plant had mended the air.
6. 0:48Here is the recipe. Carbon dioxide comes in from the air, and water comes up from the roots.
7. 0:56Using the energy of light, the leaf turns them into glucose, a sugar. Oxygen is left over and goes out.
8. 1:06Carbon dioxide, water and light make glucose and oxygen.
9. 1:13The plant burns some glucose for energy, stores some as starch, and joins the rest into wood.
10. 1:22So van Helmont's willow was built mostly out of air and water.
11. 1:30And almost every other living thing feeds on what plants make.