Photosynthesis in ecosystems | Middle school biology | Khan Academy

When you look at a rain forest ecosystem like this, one of the obvious questions may be: where do these plants come from? How do they grow? They're growing all the time, getting larger and larger and larger. Where does that mass, where does that matter come from?

Pause this video and think about that. Well, you might already be guessing where it comes from. It might have something to do with photosynthesis, which we go into a lot of depth in other videos. But this is a process where you have carbon dioxide from the air in conjunction with water that primarily is coming from the soil.

It uses energy from the sun, so I'll just draw that as these yellow squiggles coming from the sun. In order to do two things, the plant is going to be building itself using the matter and the carbon dioxide in the water while also expelling oxygen as a byproduct.

This matter that the plant is able to take from its environment with photosynthesis is used to both become the structure of the plant and a store of energy in the form of sugars. Now animals like you and me, we get our energy by then eating these plants.

And how do we unlock that energy? Well, that's where this oxygen is really useful. That's why we need to breathe oxygen. Because by breathing the oxygen, we can essentially do photosynthesis in reverse, and we can break down this matter, these sugars that we're getting from plants.

So all of this biomass is coming from essentially water from the soil and carbon dioxide, and energy from the sun is used to essentially put it together. There might be a few other nutrients that are also coming from the environment like the soil, but primarily the water and the carbon dioxide.