Plants don't eat soil — they make their own food out of air, water and sunlight. Inside their leaves, light energy snaps apart water and carbon dioxide and rebuilds them into sugar, the plant's food. The leftover bit is oxygen, which the plant simply breathes out — and which we breathe in. Turn up the light in the simulator and the plant makes sugar faster, until something else (like CO₂) runs short and it can't speed up any more.
Most people think plants eat food through their roots from the soil. In fact they build their own sugar from light, water and carbon dioxide; roots mostly draw water, and the leftover oxygen is the air you breathe.
What's actually happening
It's easy to assume plants eat through their roots, drinking food from the soil. They don't. Roots mostly draw up water and a few minerals; a plant's actual food, sugar, is built fresh inside its leaves out of three free ingredients: sunlight, water, and the carbon dioxide it pulls from the air. This is photosynthesis, and it is the process that ultimately feeds almost every living thing, because the animals that don't photosynthesise eat the ones that do.
Inside leaf cells sit tiny green factories packed with chlorophyll, the pigment that gives plants their colour and, crucially, captures light. The captured energy is used to rip apart water molecules and rearrange them, along with carbon dioxide, into glucose — an energy-rich sugar the plant stores and burns to grow. The discarded by-product is oxygen, which the plant releases through pores in its leaves. That "waste gas" is the oxygen in every breath you take: the entire breathable atmosphere is, in effect, plant exhaust accumulated over billions of years. The simulator shows the trade live — photons rain onto the leaf, sugar accumulates, and O₂ bubbles drift away.
Speed it up and you meet a fundamental rule. Turn up the light and the leaf makes sugar faster — but only to a point. Soon the rate flattens, because making sugar also needs carbon dioxide, and once the leaf is grabbing CO₂ as fast as it can, more light doesn't help; the plant is now "CO₂-limited". Raise the CO₂ and the rate climbs again until temperature or water becomes the bottleneck instead. This is the law of limiting factors: a process runs only as fast as its scarcest ingredient allows, which is exactly why greenhouses pump in extra CO₂ and warmth and light together, and why a houseplant in a dim corner grows slowly no matter how much you water it.
A leaf is a solar-powered kitchen, and like any process it runs only as fast as its scarcest ingredient — turn up the light and CO₂ soon becomes the bottleneck.
- 1Submerge a fresh leaf (pondweed works best) in a glass of water in bright sunlight.
- 2Watch the leaf's surface: tiny bubbles form and rise. Those are oxygen — the plant exhaling as it photosynthesises.
- 3Move the glass into shade and the bubbling slows or stops. You've just dialled the light down and watched the sugar factory throttle back, exactly like the slider.
Common questions
Because making sugar also needs carbon dioxide. Once the leaf is grabbing CO₂ as fast as it can, more light does not help — it becomes "CO₂-limited". A process runs only as fast as its scarcest ingredient allows.
Chlorophyll absorbs red and blue light to power the reaction but reflects green, so the colour you see is the one wavelength the plant is not using.
Virtually all of it is plant exhaust, released by photosynthesis over billions of years. Early life had almost none until photosynthetic microbes flooded the planet in the Great Oxidation.