The Oxygen Came Out of the Water

You'll learnhow a chloroplast's two rooms split the light reactions from sugar-building and relay between them.

A leaf is not one machine. It is two rooms and a relay. In the thylakoid membrane, light knocks an electron out of chlorophyll, and to replace it the plant tears water apart — which is where the oxygen you are breathing right now came from, not the carbon dioxide. Next door in the stroma, an enzyme spends what that first room made, bolting carbon out of the air into sugar without ever seeing a photon. Twelve short steps; press Next for each one.

AP BIOLOGY · 12 STEPS · 6 QUICK CHECKS · O₂ FROM H₂O, NOT CO₂ · LIGHT-INDEPENDENT ≠ IN THE DARK · PLANTS RESPIRE ROUND THE CLOCK · v6

discs catch the light · fluid builds the sugar · one relay between

Before you start

What you're looking at

One chloroplast drawn as two rooms: stacked green thylakoid discs on the left, the fluid stroma on the right, and a pipe between them.

The question

Which room does each job, and what is the only thing that actually crosses from one to the other?

Step 1 / 12