Photosynthesis in ecosystems | Middle school biology | Khan Academy

- [Instructor] When you look at

a rainforest 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,

they're 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,

and it uses energy from the sun

so I'll just draw that in,

these yellow squiggles
coming from the sun,

in order to do two things.

The plant is going to be building itself

using the matter in the
carbon dioxide and the water

while also expelling
oxygen as a byproduct.

And 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 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.