🌱 Photosynthesis
The tree that grew from thin air
Nearly all of a huge oak tree was built from the air.
Ask anyone where all that wood came from.
They’ll point at the ground.
But nearly all of it came from the air.
The puzzle
So what is a tree made of?
Where does all the wood in a tree come from? Nearly four hundred years ago, a man called Jan van Helmont decided to stop guessing and weigh it.
He planted a willow in a pot and weighed the willow and the soil. Then he gave it nothing but rain for five years and weighed both again. Whatever weight the tree gained had to come from somewhere.
Your turn. Drag the slider through the five years and watch what happens to the weight of the tree and the soil.
The willow, year 0
2.3 kg
Soil: 90.00 kg
Tree has gained: +0.0 kg
The tree only had rainwater for five years. It put on a lot of weight. Watch what happens to the soil’s weight at the same time.
The trap
So where did 75 kilogrammes come from?
After five years, the willow had put on about 75 kg, which is more than thirty times its starting weight, and the soil weighed almost exactly the same. All that new wood, leaf and root had to come from somewhere. Before you read on, pick one. No changing your mind after.
The 75 kg of new tree came mostly from…
↑ The thin brass edge on the right shows what the soil lost in five years: about 57 grammes, roughly the weight of a slice of bread.
Most of a tree is made from an invisible gas in the air. Van Helmont showed that the wood didn’t come from the soil, and he guessed it came from water, which was partly right. Scientists worked out the rest later.
Each leaf takes in gas from the air…
…and turns part of it into wood.
The why
How a tree takes in the air
The air has a gas in it called carbon dioxide, or CO₂. A leaf uses the energy in sunlight to split water and take in CO₂. The carbon goes into the wood, and the spare oxygen goes back into the air. Tap the button to feed the tree one molecule at a time.
0
carbon kept
0
oxygen let out
CO₂ + water + sunlight → sugar (the stuff a tree is built from) + O₂
Each molecule you pull in was part of the air a second ago, and now it’s part of the solid wood.
The why
The oxygen you just breathed
The oxygen the tree lets out is what you breathe in, and the CO₂ you breathe out is what the tree takes in. You and the tree are passing the same atoms back and forth. Tap the button to take a breath.
You give the tree the gas it uses to grow, and the tree gives you the oxygen you need to live.
Try the smaller version
You don’t need five years and a willow
Van Helmont’s experiment took five years. You can check it faster by growing one seed in soil and one in just water. If plants were made from soil, the one in water wouldn’t grow. Drag the slider through the days to see what happens.
Day 0: both just planted
Plants can grow in water without any soil. It’s called hydroponics, and a lot of supermarket lettuce and tomatoes are grown that way. They grow well, which shows that most of a plant doesn’t come from the ground.
But building all that takes energy.
A tree can’t plug into anything…
…so it gets all its energy from sunlight.
Where the energy goes
A tree stores sunlight
Turning gas from the air into wood takes a lot of energy. Every leaf works like a solar panel: it catches sunlight and stores that energy in the sugar it makes. So a tree is made of stored sunlight as well as air, and burning the wood lets that energy back out.
Sunlight stored in the wood
The log holds years of stored sunlight. Tap the button to let it out.
The same thing happens in other places. Tap one:
The energy in each of these started as sunlight. Pick one to see how.
A tree is air that’s been made
solid…
…using energy from the sun.
It’s built from an invisible gas, using light from a star ninety-odd million miles away.
One more thing
So if someone tells you a plant grows by “eating the soil”, tell them about the willow that gained 75 kg while the soil lost only 57 grammes. Ask them where the rest came from, and then tell them it was the air. You could also tell them that the heat from a campfire is sunlight a tree stored a few summers ago.