Phosphorous cycle | Ecology | Khan Academy


– [Instructor] Let’s
talk a little bit about the element, phosphorous,
and its importance to life and how it cycles through living systems. So we’re gonna talk about
the phosphorous cycle. First, it’s important to
appreciate that phosphorous 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 phosphorous atom, and it’s bound to four
oxygens right over here, and at least the way that
it’s set up right over here, it would have a -3 charge. This would often form ionic
bonds with other things, or one of the oxygens
can have a covalent bond, or you can form phosphoric
acid where the oxygen’s bound to hydrogens, but
this is the general form that you typically find it. That’s the phosphorous
that I’m pointing to, but this whole thing, we
would call this a phosphate. We would call this a phosphate. 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 word, the
backbone of our DNA molecules. You can see these phosphates
there and in the center of the phosphate, you
have the phosphorous atom. Also over here, you have ATP. In biology, we study that. It’s the powerhouse. It’s the energy currency
of biological systems, 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 confirmation of enzymes. It can do all sorts of interesting things, so hopefully you can
appreciate that phosphate is essential for life,
’cause life as we know it involves ATP, and involves DNA, and many, many other
things that phosphates are involved with. Phosphate, or phosphorous
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 found in rocks, and it’s going to be
phosphate-based rocks. There will be other elements in there. It could be chlorine. There’s a bunch of different
types of phosphate rocks, but when they’re in
their sedimentary form, let me create some soil here. Let’s say this soil has
some phosphates in it. Then, it allows things like
plants to grow in that soil. This is a plant growing in that soil, and the plant, we’ve
talked about it before, it could be fixing 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 phosphorous
in them, or phosphates, or nitrogen’s another one. 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. Then you might say, “I have ATP and DNA in my body. “How do I get phosphates?” Well, you get it by eating plants. This is you eating a plant. The plant goes back there. How does this form a cycle? When any of this living matter dies, and I’ve said it in previous video, I’ll show the dead plant
’cause a dead animal, it’s just a little darker. So whenever you see the
plant now, it’s dead. Let’s say it got buried
somehow, some soil, so it’s all just a dead
plant right over there. The phosphates in that plant
can then go back into the soil. 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
phosphorous gets recycled. You could think on a bigger
scale where you could have, 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. That river can take
phosphates from that soil, and it could put it into the ocean, and then those phosphates
could be used by sea life. Then when that sea life
dies, it goes to the base of the ocean floor, and 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. We’re actually seeing
more and more of this as human beings have said, “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,
we’ve started mining it.” We’ve started adding a
lot more to the soil, but it also allows a lot
of that to be washed away into our rivers and streams,
and eventually end up in the ocean. As we’ll see, this can
actually have very negative consequences for our biosphere.