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Biogeochemical cycles overview

Biogeochemical cycles are nature's way of recycling essential elements for life, like water, carbon, nitrogen, and phosphorus. These cycles circulate these molecules through ecosystems and the entire biosphere. From the water cycle to the carbon cycle, these processes ensure the constant reuse of matter, while energy flows through the ecosystem. Created by Sal Khan.

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Video transcript

- [Instructor] Talk a little bit about biogeochemical cycles. And 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 and an ecosystem and really how they circulate through the entire, through an entire biosphere. And the molecules that we care about, and molecules that could consist of one element or multiple elements. They are things like well, water molecules. H2O. Oxygen and hydrogen make up a lot of living, a lot of living creatures, a lot of biomass. And water is just an essential element that is involved in life as we know it. We're also gonna be talking about carbon. And carbon takes on may 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. And there is actors that maybe don't get as much attention. There are things like nitrogen and of course, you have characters like, characters like phosphorus. And 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?" And remember, your DNA, deoxyribonucleic acid. We're talking about nitrogenous bases. Your adenines and guanines and all these things that we talk about in DNA, they involve nitrogen. And there's other biomolecules. Amino acids, proteins, amino acids which make up proteins that involve nitrogen. Phosphorus, ATP. The adenosine triphosphate. It's essential. It's in that core biomolecule. It's also in the backbone of DNA. So these are all essential elements for life. And the key thing is, is that they all get recycled through biogeochemical cycles. So they are all recycled. We talked about how an ecosystem energy flows. It might start with light energy from the sun and then 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 here. This is recycled. It was a originally formed. I mean there's a few meteorites that hit every now and then. But the most part, most of the matter around us was here at the dawn of, when the earth was first formed. It was first created in the inside due to fusion reactions of stars billions and billions and billions of years ago. So all of life, everything that we've seen so far in the history of earth, for the most part, it's the same elements and the same molecules that have been recycled over and over and over again. And so when we think about biogeochemical cycles, we will think about things like, if we're talking about H20, we're gonna think about the water cycle. Water cycle. And we have a whole video on that but in (stutters) the short version of it is you could have water actually stored in a 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 and along the way, you could have animals that get access especially to the fresh water and make that part of the actual living organisms. You actually, I also have living organisms that view the water as part of their ecosystem but we go into some depth in another video. You'll also hear people talk about, if we're talking about carbon, the carbon cycle. The carbon cycle. And just as a very high level of overview of the carbon cycle. So let's say that's the ground. Let's say that this is a plant right over here. Take primary producer and autotroph. We talk about that in our ecosystem video. So let me draw a leaf here. This is a plant that is growing. What it's doing while it takes light energy, so it gets light energy from the sun. So that's the light energy right over there and it uses that light energy to fix carbon. The carbon that is making up this plant. It's just not emerging out of nowhere, it's being recycled. It's being recycled from carbon molecules that are in the air. So the carbon molecules in the air, they're going to be, or the carbon in the air, it's gonna be in molecular form in terms it's going to be carbon dioxide. So that over there is CO2. And so the plant uses that light energy and that gaseous, that CO2 gas, and it's able to fix the carbon to construct it. So it's looks like that plant is just growing. It's not clear where all that mass is coming from. But it literally is coming from the air. And it might be able to get, it might get a few of the nutrients. Especially things like nitrogen phosphorus from the ground and that's where we could go into the whole nitrogen and phosphorus cycles. But then once these plants fix all of this carbon, some of it gets stored in the biomass, sometimes the plant dies and then it gets buried and if there was enough pressure, it can turn into a hydrocarbons. So a hydrocarbons, when you fuel your car, it's a really it's really energy stored by dead plants a long long time ago that got buried and put into that form but it was essentially plants through photosynthesis that first stored that energy. But then of course, and we've talked about this, you can have animals that eat those plants. So let me draw an animal that is eating it. So that looks like an animal. Well that one looks like something that likes to eat more than just plants. So this is, you draw it like it's like an alligator chicken looking thing. Well, that's close enough. Well you get the idea. When it eats that plant, it's using some of that carbon that was fixed originally by that plant for it's own biomass but it'll also as it metabolizes the some of it (stutters) it will use that the bond, the energy stored in those chemical bonds, to live and grow and move about. And as it does that, it will exhale the CO2. So that the CO2 goes back into the environment. Now this is a huge oversimplification of the carbon cycle but this is the general idea. And eventually this thing might die or might get eaten by other consumers. And then you have the decomposers down here who could further make use of those chemical bonds that the primary producer first created using that light energy. Decom, decomposers right over here. And they might also, as they consume that, release more CO2 which will eventually then get fixed again by a primary producer. And there's similar cycles for nitrogen and phosphorus. These often involving bacteria to fix the nitrogen and the phosphorus from the air to make it available as nutrients in the soil for some of the primary producers in say, the carbon cycle. And nitrogen doesn't get a lot of attention, but this is actually the most gas in our atmosphere. So it's all around you right now even while we're breathing, we are breathing a lot of nitrogen. Although it doesn't play as strong of a role as say oxygen and the oxygen in the air is going to be molecular oxygen. So I could also write O2 right over here. Or as we breathe out, carbon dioxide. So I could write C CO2. CO2 right over there. But hopefully this gives you a sense of how matter is recycled. Energy is flowing through ecosystems including the largest ecosystems on earth which is earth is biosphere. You have matter being constantly recycled. And the cycle of that matter, we call biogeochemical cycles. I think I just said the word cycle a lot.