Biogeochemical cycles | Ecology | Khan Academy

Talk a little bit about biogeochemical cycles. The term "biogeochemical" sounds very fancy, but really these are just cycles that involve different molecules that are essential for life and how they circulate through an ecosystem. And really, how they circulate through an entire biosphere.

The molecules that we care about could consist of one element or multiple elements. They are things like water molecules (H2O). Oxygen and hydrogen make up a lot of living creatures, a lot of biomass. Water is just an essential element that is involved in life as we know it.

We're also going to be talking about carbon, which takes on many forms. When we think about biogeochemical cycles, there's carbon dioxide in the air. There's a lot of carbon in organic molecules that form up most of the mass of life as we know it. Then there are factors that may not get as much attention, like nitrogen. And of course, you have phosphorus.

You might say, "Okay, I get that most organic molecules are made up of a bunch of carbons and hydrogens and every now and then oxygens." But what about nitrogens and what about phosphorus? Remember your DNA, deoxyribonucleic acid. We're talking about nitrogenous bases—your adenine and guanine. All these things that we talk about in DNA involve nitrogen.

There are other biomolecules as well, like amino acids. Proteins, which make up amino acids, involve nitrogen. Phosphorus is also important, especially in ATP—the adenosine triphosphate. It's essential in that core biomolecule and in the backbone of DNA.

These are all essential elements for life. The key thing is that they all get recycled through biogeochemical cycles. They are all recycled. We talked about how energy flows in an ecosystem. It might start with light energy from the Sun. Over time, as it's transferred from one form to another, as it flows from one form to another, it gets dissipated as heat. But the matter, the elements, the molecules—they are recycled.

Most of the matter around us was here at the dawn of when the Earth was first formed. It was first created inside stars due to fusion reactions billions of years ago. So all of life, everything that we've seen so far in the history of Earth, for the most part, is just the same elements and the same molecules that have been recycled over and over again.

When we think about biogeochemical cycles, we will consider things like the water cycle. In short, water can be stored in multiple different ways. Some of it can evaporate as water vapor. Eventually, it condenses in the form of clouds, and then it can rain back down. Along the way, animals can access especially fresh water and make it part of the living organisms.

We go into some depth in another video. You'll also hear people talk about the carbon cycle. Just as a very high-level overview of the carbon cycle, let's say this is ground and this is a plant. It's a primary producer, an autotroph.

Let me draw a leaf here. This plant is growing. What it's doing is that it takes light energy from the Sun, which helps it fix carbon. The carbon that makes up this plant isn’t emerging out of nowhere; it's being recycled from carbon molecules in the air. The carbon in the air is in molecular form—it’s carbon dioxide (CO2).

So the plant uses that light energy and that gaseous CO2, and it's able to fix the carbon to construct itself. It looks like that plant is just growing, but literally, it's coming from the air. It might also get a few nutrients—especially things like nitrogen and phosphorus—from the ground.

Now we could go into the whole nitrogen and phosphorus cycles. Once these plants fix all this carbon, some of it gets stored in biomass. Sometimes the plant dies and gets buried. With enough pressure, it can turn into hydrocarbons. So, hydrocarbons are what you use to fuel your car. It’s really energy stored by dead plants a long time ago that got buried and transformed.

But it was, essentially, plants through photosynthesis that first stored that energy. Of course, we've talked about how animals can eat those plants. Let me draw an animal that is eating it.

That might look like an alligator or chicken-looking thing. Well, you get the idea. When it eats that plant, it's using some of that carbon that was originally fixed by that plant for its own biomass. However, as it metabolizes, it will use that energy stored in those chemical bonds to live and grow.

As it does that, it will exhale CO2, sending it back into the environment. This is a huge oversimplification of the carbon cycle, but this is the general idea. Eventually, this thing might die or get eaten by other consumers. Then you have decomposers who can further make use of those chemical bonds that the primary producer first created using that light energy.

Let me put decomposers right down here. They might also release more CO2, which can then be fixed again by a primary producer. There are similar cycles for nitrogen and phosphorus, often involving bacteria to fix nitrogen and phosphorus from the air to make it available as nutrients in the soil for primary producers.

Nitrogen doesn’t get a lot of attention, but this is actually the most common gas in our atmosphere. It’s all around you right now. Even while we’re breathing, we are inhaling a lot of nitrogen, although it doesn't play as strong of a role as oxygen.

The oxygen in the air is molecular oxygen, so I could write O2 right here. As we breathe out, we produce carbon dioxide, which I could write as CO2. Hopefully, this gives you a sense of how matter is recycled as energy is flowing through ecosystems, including the largest ecosystem on our Earth, which is Earth as a biosphere.

You have matter being constantly recycled, and the cycle of that matter is what we call biogeochemical cycles. I think I just said the word "cycle" a lot!