Where does the mass of a plant come from?

Answer 1

In photosynthesis, the mass of the growing plant, as in the bark, wood, fruit, and leaves, comes from carbon dioxide, NOT water. I think the reason this one is common is It can be a bit hard to conceptualize plants growing from an invisible gas in the air. Water is still required for the reaction, it leaves as oxygen, and it is crucial for plant growth and maintenance, but is not where plant mass comes from. Not a perfect analogy, but think of it like water being boiled to spin a turbine, with the turbine spinning representing plant growth. Water is used and it leaves altered as steam, but it is used to allow the conversion of heat into spinning the turbine. It’s not the direct source of energy. Sunlight is the same. Saying it’s plant “food” is a good way to introduce the idea, but its real purpose is to drive chemical reactions in the plant that eventually produce glucose, and is never converted directly to plant mass.

Plants also take nutrients from the soil, but it does not use it as energy. In the 17th century, a scientist named Jan Van Helmont weighed a tree and the dry soil he would plant it in. After 5 years, he reweighed both. The tree had grown considerably, but the soil lost barely any weight. The reason for this is that the nutrition plants get from the soil is like vitamin supplements for humans. You don’t consume vitamins to directly break them down into energy, but they serve as important reagents and enzymes in biological processes that create energy. Anyone who tried to live on only supplements would get hungry very fast, but in many cases they’re a good addition to a diet. If the nutrients haven’t been moved to an organ that humans remove for food, like a fruit or a tuber, putting the dead plant back into the soil can actually return some of those nutrients for the next plant to use.