Your body has all sorts of complicated processes going on inside it, and a lot of these processes are carried on by incredibly powerful, and often incredibly large, molecules. Some of these molecules that you're using right now are so big that they challenge the definition of what a molecule is.

Technically, a molecule is the smallest group of atoms that performs a function. But sometimes, they're really complicated groups of atoms that work together. They're definitely more complex than, say, molecules of water or carbon dioxide, but they're still just one unit. 

Many of your body's most important molecules fall under this category.

Biologists refer to these huge, life-giving arrangements of atoms as macro-molecules. Some of them you've possibly heard of, others you might never have known were that important. But, together, they are what's keeping you alive.

And aside from water, and glucose, and other nutrients you have to ingest to keep going, these are some of the most important molecules in your body. 

And, hey, I love and appreciate all of them equally. No playing favorites here. But if I had to pick five of the most important molecules that keep your body ticking, I have to include these five.

And we'll start with the first one I mentioned. Deoxyribonucleic acid, or DNA. It's probably your body's most famous molecule, and rightfully so. It's the one that tells yourselves what proteins to make, as well as how and where.

Now, you usually hear people talk about DNA in the context of genetics. Like they'll say that their hair color, or their allergy to cucumbers, or whether they have wet or dry earwax, is because of their DNA.

And that's true, your DNA contains your genetic blueprints, the instructions of how to make you you.

But what's important to understand is that you use your DNA every single time your body creates a new cell. And it does this two trillion times every day. 

It's made up of pairs of interlocking molecules called nucleotides, building blocks that are arranged in a very particular order. And each of your cells is able to read these sequences of nucleotides as a kind of code. Interpreting different segments, which are your genes, as instructions of how to build proteins.

And when it comes to your body protein are the boss. If your don't count water, they make up 75% of your body weight.there are more than 100,000 different proteins in you right now. And your DNA knows how to make them all.

That is a lot of instructions. So it should be no surprise that DNA is by far the largest molecule you have. If you stretched out the DNA in just one of your cells it would be about two meters long.

And this enormous code is readable thanks to another macro-molecule known as messenger RNA.

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it collects the recipe from whatever segment of DNA and delivers it to your cell's  protein assembly lines, called ribosomes.

So each of your cells has the instructions to make proteins, but they still need the ingredients to make them, those are the amino acids. And those hundred thousand different proteins that you have, all are made from different combinations from the same 20 amino acids.

In a pinch, your body can make about half of these amino acids all by itself, but nine of them, the essential amino acids, have to come from food.

And that's where another one of your bodies most important molecules comes in, pepsin.

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Pepsin is an enzyme and it is a really big complicated macro-molecule that helps digest proteins in food, breaking them down into amino acids. It hunts down the bonds between certain amino acids, and just mercilessly just rips them apart.

And pepsin is just really good at its job. It's so powerful that your stomach cells have to produce it in a special, de-activated form, called pepsinogen.

If it didn't the pepsin would set to work digesting your stomach's own cells. But once the pepsinogen is released into the stomach it becomes activated by hydrochloric acid in your stomach. And then it is free to reek havoc.

But just how strong is this stuff?

Well, if you have ever had the misfortune of having your stomach contents come out of you you felt it. the reason that things like vomiting and acid reflux are so unpleasant is because they allow pepsin to come into contact with your unprotected tissues.

So that burning that you feel is basically pepsin digesting the cells in your esophagus. But normally, and thankfully, pepsin limits its work to your stomach releasing those essential amino acids that your cells use to build the proteins that you are made of.

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But all this protein building takes energy, and to get energy you first need oxygen. you breath it in, but then oxygen needs to get from your lungs to the rest of your body somehow.

Enter hemoglobin
hemoglobin is a protein found in red blood cells, and it is so big that those red blood cells get rid of most of the other stuff inside them, like the nucleus, to make room for just one macro-molecule of hemoglobin.

And even then, red blood cells can get so big that they get stuck in your capillaries that have to dilate to let your hemoglobin embedded in red blood cells pass through.

hemoglobin's main function is to transport oxygen and carbon-dioxide. Each molecule is made up of for groups, called hemes, that have one iron atom a piece. these iron atoms have a positive charge, they are really good at bonding with electrons in oxygen. And when just one of the iron atoms bond with an oxygen molecule the whole shape of the hemoglobin changes so that three more oxygen molecules can just pop right into the other hemes, which are just empty and waiting.

Then the hemoglobin travels through your circulatory system to unload the oxygen to the cells that need it. Once the oxygen if safely delivered the iron atoms pick up carbon dioxide. the hemoglobin takes that to your lungs, and the whole process starts all over again.

Meanwhile the cells use this oxygen to crank out another one of your bodies most crucial molecules, ATP.

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ATP, or adenosine triphosphate, is your bodies fuel. Your cells manufacture it in organelles that are like power stations, the mitochondria. they usually make it with the help of that oxygen that the hemoglobin has supplied.

Of the many important molecules that we are talking about today, ATP is the only one that is not considered a true macro-molecule because it is smaller. But it still has plenty of moving parts.

The most useful thing about ATP is that it had three phosphorus atoms, each of which is surrounded by oxygen atoms. Cramming these combinations if atom, called phosphate groups, together forces those oxygens closer to one another which serves to store allot of energy.

And breaking one of the phosphates off releases that energy. Your body is constantly going through loads of ATP to power everything that requires energy. That includes big things, like making your muscles contract, and small things, like dilating tiny capillaries so that a single red blood cell can get though.

So, your cells get their instructions from DNA. Some of the ingredients from the amino acids are kindly provided by the pepsin and then the cells use oxygen, delivered by the hemoglobin, to make ATP for energy. The digestive, respiratory, circulatory, and nervous systems are all working together here. But your body still needs to know when to do what.

That is when your endocrine system come in and it owes allot to cholesterol.

Now hole up, if you've ever seen a commercial for heart medicine, or diet supplements, or so called health food, you have probably been subjected to some pretty negative press about cholesterol. It has a bad rap as an artery clogger, but it also has a really important job, making hormones.

Hormones are chemical messengers that regulate processes that don't need to happen all the time. when the villain in the movie suddenly pops up on the screen brandishing a chain-saw, it is your hormones that tell your blood vesicles to constrict and your heart to start pumping faster preparing you to fight that monster, or run away.

And cholesterol, is the first step in making some of these hormones. Even though you hear people talking about cholesterol and their diet, most of the cholesterol that your body uses doesn't come from food.

Your liver makes up a gram of the stuff everyday. Some of that cholesterol stays in your liver, where it is used to make vitamin D. But the rest is carted through your blood stream by molecules, called lipoproteins, which are sort of their personal chauffeur. And it is these reckless drivers that have been giving cholesterol a bad name. When lipoproteins have too much fat in them, and not enough protein, they can get stuck to the walls of arteries and form plaque that can impede the flow of blood.

That's a big problem, but it is not caused by the  cholesterol directly, it's the fault of it's chauffeur. Meanwhile, the cholesterol molecules that have good healthier lipoproteins toting then around have better things to do then just hang out on the side of your arteries. These cholesterols are incorporated into cell membranes, keeping them fluid, and functioning, while others are turned into steroid hormones like cortisol, estrogen, and testosterone.

So cholesterol can be pretty cool, just like you. Looking at these five molecules gives allot of insight into how awesome the human organism is. If you want to learn more about what is in you, and how it works, head on over to crash course, where you can take your brain for a nice long soak in biology, and anatomy, and physiology, and lots of other disciplines.

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