Flow of energy and matter through ecosystems | High school biology | Khan Academy

Let's think a little bit about how energy flows and how matter is recycled in an ecosystem. So, the whole time that we go through this video, think about these two ideas. And then, even after watching this video, look at ecosystems around yourself, even ones that you are part of, and think about how energy flows and how matter is recycled.

So, let's first think about energy. The energy for most ecosystems originally comes from the sun. There are other sources of energy you can think about, even moonlight, but that essentially comes from the sun. There’s also geothermal energy, but the sun is a source of most energy for most ecosystems we can think of.

How does the ecosystem make use of that energy? How does that get stored within the ecosystem, especially as biomass? Well, it starts with primary producers, which are usually going to be plants. They can also be bacteria that are able to photosynthesize, who are able to take that energy and create biomolecules that store energy from it. These are primary producers; these plants in this diagram sometimes you'll see them referred to as autotrophs. They are getting their own food from the sun, from this energy.

Once again, how is that energy stored? Well, it's stored in these biological molecules. If you were to zoom into the molecules in this plant, and this is a huge oversimplification, you’ll see all these bonds between these carbons. To make those bonds requires energy, and if you were to break those bonds, it could release energy. You might say, "Well, where did all these carbons come from that are in this tree?" Well, the carbon is coming from the air. Our air has carbon dioxide in it; it has carbon dioxide. So those are the carbons. Maybe let me draw some oxygens—two oxygens for every carbon.

The whole process of photosynthesis is all about fixing that carbon. Let me write that word down: we are fixing that carbon from a gaseous form when it's part of carbon dioxide into the structure of the plant, into the biological molecules of the plant. So it's storing that energy. Now, it's not a perfectly efficient process; not all of the energy from the sun is going to be able to be stored. Some of it is being reflected. Even the plant itself, as it lives, as it reproduces, as its cells divide, some of that energy is used, and eventually, that energy is released as heat.

You're going to see this trend a lot in thermodynamic systems. You are going from one energy; you're using energy to do some work, but in the process, you are going to be producing heat. But this is just the beginning of our energy flow. Now, we can think about how that energy now flows to the other actors in the ecosystem.

So, the next phase, and this is a very simplified diagram of an ecosystem that we're thinking about; most ecosystems are far more complex. Let's think about the characters that would eat the plants, the characters that would eat the primary producers. We call the folks that eat the primary producers primary consumers. So, this bunny or this squirrel right over here—they are primary consumers. They consume the primary producers.

And why do they consume them? Why does a bunny eat the grass? Well, because it gets energy from those bonds between the biological molecules, from those carbon bonds and other bonds. It's able to use that energy to grow itself, to reproduce, to live, to run around. It also stores some of that energy in its own body biomass. Once again, this process is not very efficient; going from one layer of trophic to another layer of trophic, you only have about 10% of the energy getting transferred or stored in the next layer.

Why only 10%? Well, because not all of the plants get eaten. The whole process of eating plants, digesting plants, some of the energy gets pooped out because the primary consumer here, or the consumer, isn't able to get all of it out of the actual biological molecules. So, overall, it's an inefficient process. Now, we're not done yet. We still have energy stored in the biological molecules of this primary consumer that someone might be interested in.

We know that in many ecosystems, there are things that like to eat rabbits or even squirrels. In this drawing, it will be this fox. Because it eats primary consumers, we would call it a secondary consumer—a secondary consumer. You could keep going on with this. If there was some character out here, let’s say there’s some guy who likes to eat foxes—that's a knife in his hand that he uses to go after the foxes with. Well, the fox could go to him.

Once again, why is he eating foxes? Well, he wants that energy in that fox, and actually, some of them—it's not just about molecules; we'll talk about matter in a second. He wants the energy and the matter from the fox to grow and live himself. So this character would be called a tertiary consumer—a tertiary consumer. If there’s no one who wants to eat him, well then, he would be considered an apex consumer or an apex predator.

These characters that eat other animals—we've talked about it before—they're called carnivores. But let me just say, he’s the apex. An apex, we're really thinking about the top of the food chain. That’s why they're called an apex consumer or an apex predator. But we're not done yet because at some point, all of these characters—whether we're talking about the trees, the bunnies, the fox, this character who likes to eat foxes—they’re going to die.

And that energy just doesn’t disappear. In general, you're going to see energy is conserved, and it flows from one place to another. That energy is then going to be used by these characters right over here, which we call decomposers. They can take all that leftover energy in that dead carcass or even in the poop, and they can make use of it once again for them to live, for them to reproduce.

By breaking that down, they can release a lot of those nutrients and the matter that’s used. Once again, the matter is recycled, to be used by the plant. So it creates this really nice cycle. The important thing to realize is it comes in as light; that energy gets transferred as we go through the different layers of trophic. It’s not a completely efficient process, and a lot of that energy, especially as these organisms live and reproduce and run around, gets released as heat.

Now, we focus a lot on the energy; let's think a little bit about the matter. I’ve already touched on it, but the matter is recycled. There isn’t, at least in the way we’ve set this up, new matter that is entering or leaving these ecosystems or being magically created or magically destroyed. As I mentioned, when you look at a leaf on a plant growing, or a tree growing, or a leaf of grass growing, that matter isn’t just coming out of nowhere. It’s coming out of the—it’s just a different form.

Or maybe the best way to put it is that matter was always there in the form of carbon dioxide. The plant is just using that energy from the sun to fix that carbon from a gas form into a solid form. It’s able to use that energy to form bonds between the carbons in these biological molecules that actually store energy, and the plant can use that energy to grow.

As we've talked about, things that eat the plants or things that eat the things that eat the plants can use that energy. As we talked before, the carbon dioxide comes in these plants. Maybe this arrow might be a little bit misleading, so let me erase that. Let me erase that for now. But we release oxygen, O2. That oxygen, and we've seen that as part of the photosynthesis process, that oxygen is used by the animals to metabolize these biological molecules.

We studied that in biology and in respiration. The matter itself, as we say, we have this carbon right over here. When it gets eaten, well then, that becomes part of the biomolecules inside this bunny. When the bunny uses any of these biological molecules as a source of energy, it's able to break those bonds through respiration. Well then, that carbon gets released in the form of carbon dioxide.

So, maybe this is a better way, and actually it was already drawn right over here. The important thing to realize is that energy is flowing; light from the sun comes in. All this action goes on, and then it gets released as heat at almost every step. But the matter itself has always been there. All of the atoms in our body on Earth are just constantly being recycled.

It was actually generated inside of stars many, many billions of years ago, and we just keep reusing it over and over and over again. It gets recycled from one form to another. Even after all of these biological actors die, the decomposers break them down into simple inorganic molecules, as we talked about before, that can then be used by the plants in conjunction with the carbon from the air and the light from the sun and water that it gets through its roots to start that process all over again.