Matter and energy in food webs | Middle school biology | Khan Academy

In this video, we're going to talk about food webs, which is really just a way of picturing how all of the matter and how all of the energy flows inside of an ecosystem. Now, when I talk about matter, I'm talking about the atoms in an ecosystem, the molecules. When you look at your hand, it is made up of atoms, and it turns out that the matter is not created or destroyed; it's just recycled throughout an ecosystem, and we're going to see that in a second.

Then, when we talk about energy, it's the energy that your cells need to be alive—the energy you need to be alive, to not just exist but to do things, to think, to move. So, this food web essentially describes that. As we've talked about in other videos, in most ecosystems, the great majority of the energy in an ecosystem comes from the Sun.

What we have here is the Sun produces energy; it travels to Earth, and then you have organisms, which we would call producers, that are able to take that light energy from the Sun and then take atoms and molecules from its environment—things like carbon dioxide in the air, things like water, and other nutrients. It's able to construct itself using that energy from the Sun.

Now, when it constructs itself, it not only gives it structure, but it's also able to store energy. Right over here, we have several producers depicted. We have this tree here, which is able to do photosynthesis. We have the grass here that's able to do photosynthesis, and it's not just plants; you have things like algae and other microorganisms that are able to be producers.

But then we have things like this bunny, and this bunny is not able to harness energy from the Sun by itself. In order for it to get its energy and its matter, it needs to eat one of these producers, probably some of this grass. So, we would call this bunny right over here, this rabbit, a consumer.

And it is a consumer—you could think of it as both matter and energy. When it eats that plant, those atoms are then able to make up the bunny. It will poop out a little bit, so some of the matter might end up right over here, but then also, there is energy that is stored in those molecules, and that rabbit can use that energy to exist and live.

But as it does it, it does release some of the energy in the form of heat. Actually, even producers need to use energy in order to live, and as they do that, they also release heat. Now, we have this fox. The fox is not a vegetarian; it does not eat grass, it does not eat trees; it likes to eat things like squirrels and bunnies. But big picture, it's not producing its own food; it's consuming food, so it also is a consumer.

We can differentiate more in the future between things that eat plants and things that eat other animals. You can see in this food web we draw an arrow from the thing that is being consumed to the thing that is doing the consuming. So, a rabbit consumes a plant, and so the arrow goes from the plant to the rabbit; a fox consumes a rabbit or a squirrel, so an arrow goes from the rabbit or the squirrel to the fox.

Now, some of you might have noticed that we have these arrows that are pointing downward. Let me scroll down a little bit, and we see these microorganisms—the worms, the fungi, the mushroom right over here—and we call these decomposers. Now, what decomposers are really doing is what you would imagine: breaking down all of the things that might die, the poop that is falling down, and by doing that, it's continuing to recycle that matter.

When it decomposes, those atoms are released back into the soil or the atmosphere, and then that can be reused by the producers. Once again, every organism in this food web is using some of the energy in order to exist and do whatever it needs to do, and some of that energy is being released as heat in every single situation.