Phosphorous cycle | Ecology | Khan Academy

So let's talk a little bit about the element phosphorus and its importance to life and how it cycles through living systems.

We're going to talk about the phosphorus cycle. So first, it's important to appreciate that phosphorus is a very reactive element. So it's seldom found by itself; normally, we find it in phosphate form.

So this right over here, the orange, is a phosphorus atom, and it's bound to four oxygens right over here. At least the way that it's set up right over here, it would have a negative three charge. This would often form ionic bonds with other things, or one of the oxygens can have a covalent bond. You can form phosphoric acid, where the oxygen is bound to hydrogens, but this is the general form that you typically find it.

So that's the phosphorus that I'm pointing to, but this whole thing we would call this a phosphate. We would call this a phosphate, and you see this showing up in very important macromolecules in biology. This is DNA, and both DNA and RNA have phosphate backbones. You can see the sides of the ladder, I guess you could view it as that way, or the backbone of our DNA molecules; you can see these phosphates there, and in the center of the phosphate, you have that phosphorus atom.

Also, over here, you have ATP. In biology, we study that; it's the powerhouse, it's the energy currency of biological systems. It stands for adenosine triphosphate. You have three phosphate groups right over here, and when you pluck one of those phosphate groups off, it can power reactions, it can change the conformation of enzymes, it can do all sorts of interesting things.

So hopefully, you can appreciate that phosphate is essential for life, because life as we know it involves ATP and involves DNA and many, many other things that phosphates are involved with.

Now, phosphate, or phosphorus, I should say in particular, is a little bit different than carbon or nitrogen in that it's not found in the atmosphere. It's not typically found as a gas. Instead, phosphate is going to be formed and is going to be found in rocks, and it's going to be in phosphate-based rocks. There will be other elements in there; it could be chlorine. There could be, well there's a bunch of different types of phosphate rocks.

But when they're in their sedimentary form, so let me create some soil here. So let's say this soil has some phosphates in it; well then it allows things like plants to grow in that soil. So this is a plant growing in that soil, and the plant, and we've talked about it before, it could be taking its carbon from the atmosphere using light energy, but its phosphate is going to come from the soil. That phosphate was already there, and that helps that plant grow because that plant needs that phosphate for its ATP, its DNA, its RNA, and for other things.

Phosphate is often considered a rate-limiting factor for the growth of things like plants, and that's why a lot of fertilizers will have phosphorus in them or phosphates. Nitrogen's another one. So next time you think about fertilizer, you're fertilizing things in your garden, look at the ingredients. You will see phosphate there because that might be the scarce resource, or the thing that is limiting the actual growth of the plant.

And then you might say, "Well, I have ATP and DNA in my body. How do I get phosphates?" Well, you get it by eating plants. So this is you eating a plant. The plant goes back there, and how does this form a cycle?

Well, when any of this living matter dies—and I’ve said it in a previous video—I’ll show the dead plant because a dead animal is like, it's just a little darker. So whenever you see, so this is a plant; now it's dead, let's say it's got buried somehow in some soil. So this is the dead plant right over there. Well, the phosphates in that plant can then go back into the soil.

So you could view this as a very tight cycle, and the same thing would be once you die or I die, the phosphates, if we're buried, would go into the soil. But there could be other ways that the phosphate and the corresponding phosphorus get recycled. You think on a bigger scale where you could have a—let's say this, let's say there's a river. There's a river right over here; this is either a very small river or a very big plant that I drew.

And so that river can take phosphates from that soil and could put it into the ocean. Then those phosphates could be used by sea life, and then when that sea life dies, it goes to the base of the ocean floor. At some point in the future, that base of the ocean floor could be pushed up, and a plant grows on it. So you can imagine all sorts of these cycles, and we're actually seeing more and more of this.

As human beings have said, "Hey, if phosphate is the rate-limiting factor for the growth of plants, and we need to grow a lot of plants in order to feed ourselves," well, we've started mining it and we've started adding a lot more to the soil. But it also allows a lot of that to get washed away into our rivers and streams and eventually end up in the ocean.

As we'll see, this can actually have very negative, negative consequences for our biosphere.