Trap One: a Tree Is not Built from the Soil
This is the big one, and almost everyone gets it wrong first. Where does the mass of a tree come from — all those tonnes of wood? It feels obvious that it must come up out of the ground, “eaten” through the roots. It doesn't. The roots take up water and a trace of dissolved minerals, but the actual building material — the carbon that wood is made of — comes from carbon dioxide in the air. The plant grabs carbon out of the sky and stitches it into glucose, then into wood. A four-hundred-year-old gum is, in a real sense, built mostly out of air. (This was proven centuries ago: a willow in a weighed pot grew enormously while the soil lost almost no weight at all.)
Trap Two: Plants Respire Too — All the Time
People file it as “animals breathe in oxygen, plants breathe in carbon dioxide”, as if plants are the opposite of us. Not quite. Plants respire as well — every living cell does, to release the energy locked in glucose — and respiration takes in oxygen and gives out carbon dioxide, day and night, around the clock. In bright light a plant photosynthesises faster than it respires, so on balance it takes in carbon dioxide and gives out oxygen. But the respiration never stops. A plant is doing both at once, not just the photosynthesis half we like to remember.
Trap Three: Oxygen Is the Leftover, Not the Point
It's tempting to think a plant photosynthesises in order to make oxygen — as if it's doing us a favour. It isn't. The whole point, for the plant, is the glucose — its food. Oxygen is simply the by-product, the leftover, what's tossed out when the carbon dioxide and water are rearranged. It's wonderful for us that the leftover is exactly what we breathe, but the plant isn't making it on purpose.
And the Quiet One: No Light, No Photosynthesis
Photosynthesis is powered by light energy — it's written right over the arrow in the equation. So in the dark it simply stops. A plant at night, or in a cupboard, is not photosynthesising at all; it's only respiring, slowly burning the food it banked during the day. That's why “plants make oxygen” needs a quiet asterisk: only in the light. Take the light away and the factory goes dark — literally.