Importance of Natural Resources

Nitrogen and phosphorus cycles: Always recycle! | Crash Course ecology | Khan Academy


– There’s nothing quite so terrible as needing something that’s
sitting right in front of you, but not being able to get it. Say you’re on a lifeboat in the ocean, and you’re super thirsty, and there’s 300 million
cubic miles of water sitting right in front of you. But you can’t drink any of it. Or having to sit next to Megan
Kell every day in math class, but knowing she’s really
dramatically out of your league. A lot of organisms on Earth find themselves in this situation. Pretty much constantly. Except that the thing that’s everywhere that they can’t have, isn’t water or physical closeness. It’s nutrients, specifically
nitrogen and phosphorus. Of course there are tons of elements that cycle around the Earth, hanging out in one place
or form for a while, before moving on to the next. And, as you know, living
things need a bunch of stuff. Animals, for instance, need
oxygen and carbon and hydrogen. These elements basically
cover the water cycle and the carbon cycle, that
I talked about last time. But we’re also about 3%
nitrogen and 1% phosphorus. Those numbers might not
sound super significant. But even though we’ve just got teensy bits of the stuff in our bodies, we need nitrogen to make like amino acids, which make proteins, which
make our whole bodies up. DNA and RNA too. DNA and RNA also require phosphorus. Not to mention that
phosphorus is the P in ATP, and the phospho in phospholipid bilayer. So we might not need to tend to the stuff, but it is important, and, it’s hanging out everywhere. The air we breathe is mostly nitrogen. The water and rocks all around us are jam-packed full of phosphorus. Like I said, they’re rarely in a form that’s biologically available. As per usual, the organisms
that solve this problem are the plants. Anything else that needs these nutrients are just going to have to eat some plants. Or eat something that ate some plants. But how do plants solve this problem? And why is it a problem
in the first place? Well, give me a few minutes, I’ll explain. (upbeat music) So, let’s talk about the
nitrogen cycle first, since nitrogen really is
actually all around us. I can feel it right now. There it is in the air. So why is it so hard to get this stuff that’s constantly
surrounding us in the air into our actual bodies
to be useful for us? Because even though nitrogen gas makes up about 78% of the atmosphere, you’ll notice here that nitrogen gas is made up of two nitrogen
atoms stuck together with a triple bond. It’s one thing to break
apart a single covalent bond But three! So, as you can imagine, those two nitrogen atoms are
a total pain to pry apart. But that molecule has to be split in order for a plant to get at the pieces. In fact, plants can assimilate a bunch of different forms of nitrogen, nitrates, nitrites, to a lesser
extent, and even ammonium, which is what you get when
you mix ammonia with water. But all that darn nitrogen
gas in the atmosphere is beyond their powers of assimilation. So plants need help taking advantage of this ocean of nitrogen that we’re all swimming in, which is why they need to
have that nitrogen fixed; so that they can use it. Even though plants aren’t wily enough to wrangle those two nitrogen atoms apart, and certain nitrogen fixing bacteria are. These bacteria hang out in soil or water, or even form symbiotic relationships with the root nodules of some plants, most of which are legumes. That’s a pretty big family of plants. Soybeans, clover, peanuts,
kudzu are legumes. So these bacteria just sit around converting atmospheric
nitrogen into ammonia, which then becomes ammonium
when it’s mixed with water, which can be used by plants. They do this with a special
enzyme called nitrogenase, which is the only biological enzyme that can break that crazy triple bond. Ammonia can also be made by
decomposers, fungi, prodists, other kinds of bacteria that munch on your proteins
and DNA after you die. But they’re not picky. They like poop and urine too. Once this has happened, other bacteria, known
as nitrifying bacteria can take this ammonia and
convert it into nitrates. Three oxygen atoms attached
to a single nitrogen atom, and nitrites, two oxygen
atoms attached to a nitrogen. And those are even easier than ammonium for plants to assimilate. So that take home, here is, if
it wasn’t for these bacteria, there’d be a whole lot less of biologically available
nitrogen hanging around. And as a result, there’d be a lot fewer
living things on the planet. As usual, thanks bacteria, we owe you one. But I should mention that
it’s not just bacteria who can wrangle those
two nitrogen atoms apart. Lightning, of all things, has enough energy to break
the bonds between nitrogens, which is obviously awesome, and
therefore, worth mentioning. And in the 20th century,
smarty pants humans also figured out various ways to synthetically fix a ton
of nitrogen all at once, which is why we have
synthetic fertilizers now, and so much food growing
all over the place. Once the atmospheric nitrogen
is converted into a form that plants can use to make
DNA and RNA and amino acids, Organic nitrogen takes
off up the food chain. Animals eat the plants and use all that sweet,
sweet bio-available nitrogen to make our own amino acids. Then we pee or poop it out, or die. The decomposers go to town on it, breaking it down into ammonia. And it just keeps going. Until one day, that organic nitrogen finds itself in denitrifying bacteria, whose job it is to metabolize
the nitrogen oxides and turn them back into nitrogen gas, using a special enzyme
called nitrate reductase. These guys do their business, then release the N2 back
into the atmosphere. And that my friends is the nitrogen cycle. If you remember nothing
else, remember that A, you owe bacteria a solid, because they were smart
enough to make an enzyme that could burst open the
triple bonds of nitrogen gas. B, you owe plants a solid for wrestling nitrogen into their bodies, so that you could just eat a carrot and not have to think about it. And C, nitrogen is awesome and everywhere and yet also elusive and
deserving of your respect. So, moving on to the phosphorus cycle. The interesting thing about phosphorus is that it’s the only element
that we’re going to talk about that doesn’t involve the atmosphere. Phosphorus wants nothing
to do with your air. However, the lithosphere, fancy
word for the Earth’s crust, is amply supplied with phosphorus rocks containing inorganic phosphate, especially sedimentary rocks that originated in old
ocean floors and lake beds, where living things died,
and sank to the bottom, where their phosphorus-rich
bodies piled up and made phosphorus-rich rocks over time. Unfortunately, there aren’t a lot of rock
eating organisms on Earth. Just a couple of bacteria, which are called lithotrophs, by the way. However, when these rocks are re-exposed and water erodes them, some of the phosphates are
dissolved into the water. These dissolved phosphates
are immediately available to and assimilated by plants, which are then eaten by animals. From here, the same thing
goes for the decomposers as with the nitrogen cycle. When a leaf drops or
something poops or dies, The decomposers break it down, and release the phosphate
back into the soil or water. Phosphates get about as
much downtime in the soil as a $20 bill on the sidewalk. Decomposed phosphate is
immediately re-assimilated back into plants. And this little cycle just
keeps going and going. Plants, to the animal, to
the decomposers, to the soil, and back into a plant. That is, until that atom of phosphorus makes its way into some
kind of body of water. Because aquatic and marine ecosystems need phosphorus like crazy. Once a phosphorus atom makes its way into a deep lake or ocean, it cycles around among
the organisms there, algae, plankton, fish. And this cycling can
go on for a long time. I mean, not as long as a
phosphorus atom trapped in a rock, that can be millions of years, but, by some estimates,
a single phosphorus atom can be caught in a biological
cycle for 100,000 years. Eventually, it’s in something that dies and falls into a hole so deep, that decomposers can’t survive there. Then sedimentation builds
up and turns into rock, which are eventually uplifted
into mountains and exposed. And the phosphates are weathered back out. And, it’s a cycle. So yeah, that’s the deal
with nitrogen and phosphorus. Living things need them. But even though they’re
all over the place, they’re at a premium
in biological systems, because they’re hard to get at. Either because they have
to be converted into a form that organisms can use, or they’re locked away underground. But do you know who the
smartest monkeys are? Us. Yeah, you can bet your face that we figured out how to unleash all kinds of nitrogen and phosphorus onto this big green planet. Mostly, in an effort to help feed our children and each other. We usually mean well, but we can be a bit overbearing sometimes. It’s just the human way; to see something in nature that seems to be lacking or imperfect, and try to make it the best thing ever. So with the phosphorus
and nitrogen cycles, we have introduced fertilizers, lots and lots of fertilizers, the main ingredients of which are, you guessed it, nitrogen and phosphorus. The story of how we learnt to synthesize nitrogen into ammonia for fertilizers and chemical weapons is a very, very interesting one, involving an evil lunatic. I suggest as soon as this is over, you watch this video on Fritz Haber, the guy who made all of this
happen during World War One. You’ve heard of too much
of a good thing, right? Well, through the miracle
of synthetic fertilizers, we were able to grow much, much, more food than we ever have before. As a result, ecosystems all over the world are being bombarded by these incredible amounts
of nitrogen and phosphorus. This takes us into the next chapter in our exploration of ecology. The Human Impacts On The Biosphere. Sometimes, out of our desire
to make nature better, sometimes out of stupid human selfishness, and most often, both, we’ve ended up really
messing up the environment in more ways than we can count.


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