Where does the “mass” come from when potted plants grow?

Question

It’s easy conceptually to understand when animals eat physical food matter, it adds to their own body mass as they grow.

But potted plants grow 2x, 3x, etc their mass while the only (seemingly) inputs are Sun, water and occasional plant food. The soil level doesn’t seem to change much either, so where is the “material” coming from to make the plant bigger? Is it just from what I mentioned, and is there an easy way to understand how those things transforms to plant mass?

Edit: Thanks for all the answers! It unexpectedly helped me understand how plants capture carbon from the air and “store” it physically. I have a new appreciation for my houseplants 🪴

In: 8561

Answer 1

This is actually an interesting question! I will try to answer this question from an ecology/greenhouse effect angle. The short answer is that the most important chunk of it comes from CO2 in the air.

Most of the mass of most plants, like humans, is water. But plants, as carbon-based life forms, have carbon as their most important building block.

Carbon-based lifeforms (which are nearly all on earth) participate in the “carbon cycle”. You ever hear of forests as “carbon sinks”? The steps of the carbon cycle, at a basic level, are:

* Plants **use** energy from the sun to turn the carbon in CO2 into life-form building stuff
* Other things eat plants and/or other things that ate the plants to get energy and that main building block of carbon (for humans, carbs, proteins, and fats)
* All these lifeforms will “burn” some of that carbon for energy, releasing CO2. Go back to step 1.

Fun fact: all that carbon as “mass” inside lifeforms is carbon *not* in the air as CO2. So a forest is a “carbon sink” because its existence means less CO2 in the air. Furthermore, over a really, really long time, some of that life will die and that carbon, instead of being “burned”, gets buried. When humans dig it up and burn it, we call that fossil fuels.

Before life existed on earth, CO2 levels in the atmosphere were much, much, higher, around 20%. Because so much of that carbon is now either sitting in living beings or dead things deep underground, CO2 levels are much, much lower today. But humans are doing their darndest to put as much of that CO2 back into the atmosphere.

EDIT: added a word to make what I said clearer.