Biology · 25 minutes

A tree is built from air

A growing tree gains matter. That matter must come from somewhere. Decide which source supplies the carbon that becomes new wood.

Keep this question in mindWhere does the carbon in new wood come from?

Start with the mystery

A seed gained enough matter to become a giant.

The General Sherman tree began as a tiny seed. As it grew, it gained atoms. Your job is to find the main source of its carbon.

The General Sherman giant sequoia seen from the ground, with its trunk rising above the surrounding forest — General Sherman is the largest known living single-stem tree by volume. It began as a seed.Daniel G Rego · CC0 ↗

Three things to know

Follow matter. Do not guess from appearances.

A root is hidden in soil, a leaf touches the air, and sunlight reaches the whole tree. Only evidence can tell us which source supplies carbon.

Wood is matter

A tree cannot make atoms from nothing. Every atom in a new branch came from the tree's surroundings.

Leaves exchange gases

Tiny pores in a leaf are called stomata. Carbon dioxide can enter through them.

Light is energy

Sunlight gives a plant energy. Light is not matter, so it does not supply carbon atoms.

Your prediction

Which source supplies the carbon atoms in new wood?

Pick one source. Then explain why it seems possible. The next page will try to rule sources in or out.

AThe soilCarbon moves from the soil through the roots and into the wood.BThe waterCarbon arrives in the water that the roots take in.CCarbon dioxide in the airCarbon enters the leaves as a gas and becomes part of the wood.DThe sunlightCarbon arrives with the light that reaches the leaves.

Why did you choose that source? Write one clear sentence.

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