Photosynthesis Myths: What Plants Really Do

Photosynthesis happens out of sight, inside cells, and turns an invisible gas into solid wood. That is hard to picture, so many people, adults and teachers included, fall back on everyday ideas: plants 'eat' through their roots, 'breathe' like animals, or get 'plant food' from a bottle. The myths on this infographic are among the most common, and each has a clear piece of evidence that sets it straight.

Plants do take things from the soil, but not food. Their roots absorb water and mineral ions such as nitrate and magnesium, which they need in small amounts to build proteins and chlorophyll. Fertiliser sold as 'plant food' is really a mix of these minerals. The food itself, glucose, is made in the leaves.

The mass of a plant is the most surprising part. Van Helmont's willow tree gained about 74 kg in five years while its soil lost only about 57 g. Most of the new mass is built from carbon dioxide taken from the air, together with water. You can check this with the equation: in a glucose molecule, the carbon and oxygen atoms come from carbon dioxide and only the hydrogen comes from water, so by mass more than nine-tenths of the sugar is built from CO₂.

Because carbon dioxide goes in and oxygen comes out, it is natural to think the plant pulls the oxygen off the carbon dioxide. It does not. In 1941, scientists gave algae water labelled with a heavy isotope of oxygen, and the heavy oxygen came out in the gas the algae released. The oxygen we breathe is split from water in the chloroplasts, and the oxygen atoms from carbon dioxide end up in the sugar.

Many people learn that 'plants photosynthesise and animals respire'. In fact every living plant cell respires all the time, day and night, just like animal cells. In bright light the plant photosynthesises faster than it respires, which hides its respiration, but in the dark respiration is all that is happening.

Plants do not breathe either. They have no lungs and do not pump air. Gases simply diffuse in and out of the leaves through the stomata.

A widely shared claim says the Amazon rainforest makes 20% of the world's oxygen. Scientists interviewed by National Geographic put its share of the world's oxygen production at roughly 6–9%. More importantly, the forest's own plants and the microbes that decompose fallen leaves use almost all of that oxygen again, so its net effect on the air is close to zero. The Amazon matters enormously for wildlife, rainfall and storing carbon, just not as the planet's 'lungs'.

When you meet a claim about plants, ask two questions. Where do the atoms come from and go to? And what is happening at night? Most myths fall apart under one of them.