I weigh about 80 kilograms. Most of that, let’s say 64 percent, is water — though you can’t tell by looking. I mean, as organisms go, I like to think that I look fairly solid. After water, the next largest proportion of me is protein, about 16% — not just in my muscles, but also in things like the tiny sodium-potassium pumps in my neurons, and the hemoglobin in my blood, and the enzymes driving the chemical reactions in every one of my 37 trillion cells. Then another 16% of me is fat, which I’m totally OK with; Four percent of me is minerals, like the calcium and phosphorus in my bones, and the iron in my blood; and 1 percent is carbohydrates, most of which is either being consumed as I talk to you, or is sitting around as glycogen waiting to be used.

But here’s the thing: It’s not like I just ate 80 kilograms of food and then all this happened. Instead, my body, like yours, is constantly acquiring stuff, extracting some of it to keep, burning some of it for energy, and getting rid of the rest. But even the stuff that my body does hold onto doesn’t last forever. Some of the chemicals that I absorb in my food eventually become a part of me. But enzymes wear out, and membranes break down, and DNA gets oxidized.

So, they get discarded. And then I need more of those chemicals to reconstruct the material that I’ve lost. As a result, over the course of my lifetime, my cells will synthesize somewhere between 225 and 450 kilograms of protein … That’s like 3, or 4, or 5 separate me’s — just made of protein.

And all of the protein and fat and carbohydrates nucleic acids that make up me, of course, come from food. Every organism has to keep taking in and breaking down food, to keep resupplying itself with the raw materials it needs to survive.

And all that activity requires energy, which we also gain from food. So, how do our bodies actually convert what we eat into energy and raw materials? The answer is a neverending series of reactions that are dedicated to doing two vital, and totally contradictory, things: One set of chemical reactions destroys the reactants that you give them, reducing big, complex substances into molecular rubble. And the other set reassembles that rubble into new and bigger products that are put together again to make you.

So our bodies are constantly reinventing themselves — in a perpetual state of loss, but also always rebuilding.

And even though all of this is happening at the cellular level, its consequences could hardly be larger. These two sets of reactions are where everything that we’ve learned so far — about the digestive, endocrine, circulatory, and respiratory systems — really starts to come together. Together, these processes make up your metabolism. Now the sciencey word metabolism has come to have a meaning in popular speech, but metabolism isn’t just one thing. People talk about metabolism as meaning, like, how fast your body burns the fuel in your food, or how high your personal energy level is.

And that’s fine for use by personal trainers and fitness magazines. But physiologically, metabolism really describes every single biochemical reaction that goes on in your body. And maybe more importantly, it reconciles two conflicting chemical processes that are always, simultaneously underway inside of you. One of those chemical forces is anabolism. Anabolic reactions construct things and consume energy.

These are the processes that take the small monomer building blocks in your food — like monosaccharides and fatty and amino acids — and build them into bigger, more complex polymers like carbs, and fats, and proteins that are used in your cells. Then, when you need new building blocks, or you need to release some energy, those polymers in your body, or new ones in your food, get broken up — by catabolic reactions.

The processes of catabolism break down bigger molecules, and in breaking their bonds, release the energy you need to stay warm, and move around, and provide your cells with fuel … to build the polymers back up again. To be honest, your metabolism is a lot like Sisyphus. It works really hard.

But it is never finished. And the boulder that your inner Sisyphus is always pushing uphill and watching fall back down? That’s nutrients — the molecules that your body is forever breaking up, and then rebuilding, only to have them break apart again. And these nutrients — the materials your body needs to build, maintain and repair itself — come in six major groups. By volume, the majority of what we consume — and what makes up our bodies — is water, so that’s maybe the most vital nutrient.

Then there are vitamins, compounds that come in either fat-soluble or water soluble forms. They aren’t used as building blocks or for energy, but they’re essential in helping the body make use of other nutrients that do do those things.

Vitamin C, for example, helps improve iron absorption, while vitamin K is crucial to blood clotting, and some B vitamins are important in the production of ATP from glucose. Minerals, like vitamins, they don’t provide fuel, but they have all sorts of other functions. Calcium, magnesium, and phosphorus harden bones and teeth, while iron is, of course, crucial in hemoglobin.

Plus, potassium, sodium, and chlorine help maintain your body’s pH balance and are used in action potentials. So water, vitamins, and minerals are all … necessary. But the three major nutrients that everyone always talks about — the ones you find on food labels, from oatmeal to Pop-Tarts — are carbohydrates, lipids, and proteins. Most of the carbohydrates you’ve ever eaten — with the exception of lactose in milk — originally came from plants.

Mono- and disaccharides come from fruits, honey, sugar beets and sugar cane, while polysaccharide starches come from veggies and grains.

The main thing you need to know is that the monosaccharide glucose is the be-all-end-all molecular fuel that your cells need to make ATP. ATP being the molecule that your cells use to drive anabolic reactions, when they need to make new polymers or get anything else done — whether that’s operating a sodium-potassium pump, or detaching the head of a myosin filament to contract a muscle. But ATP is too unstable to store, so cells often store energy in the form of glucose, which they can then catabolize and convert to ATP when they need it.

Now, some of your cells can get their energy from fats. But many of the most important ones, like your neurons and red blood cells, feed exclusively on glucose.

So most of the carbs that your intestines absorb are converted to glucose for that reason. But, if it’s not needed right away, that energy can also get stored as glycogen in your liver and muscles, or converted to glycerol and fatty acids to make triglyceride fats. And even though there seems to be a marketing war going on against dietary fats, we most definitely need them. The fats in your adipose tissue store energy, of course, but they also store fat-soluble vitamins, and cushion your organs. Lipids also form the myelin that insulates the neurons in your brain and throughout your body, as well as the oil in your skin, and they provide the vital calorie content found in breast milk.

But there are other important lipids, like cholesterol, which is the precursor to things like testosterone and estrogen… ..

.and, of course, phospholipids, which form the cell membrane in every single one of the three-dozen-or-so-trillion cells you have. Now, if you’re into eating meat, a lot of the fat that you ingest might come from that. But guess what: Plants have fat too. Plants use lipids for energy storage just like we do, except they do it in fruits, and nuts, and seeds.

Which, when you think of it, are kind of like plant breast milk — it’s food for their growing babies. Either way, though, when you eat lipids, your body breaks down triglycerides into glycerol and fatty acids. Those molecules can then be processed and used in the making of ATP.

Or they might be converted into other kinds of fatty acids, which your cells can then re-assemble into your very own triglycerides or phospholipids. And your liver happens to be great at converting one fatty acid into another, but there are some it just can’t synthesize.

For example, omega 6 and 3 fatty acids are called essential fatty acids, because your body can’t make them, so they have to be ingested. They get turned into all kinds of useful molecules, like the ones used for synapse formation in the brain, and for signalling inflammation during the healing process. But — if carbohydrates provide energy, and fats insulate and store energy, then just about everything else is done with proteins. They form the bulk of your muscle and connective tissue, but they’re also what the ion channels and pumps are made of in your neurons and muscle cells, and they make up your enzymes, which are responsible for pretty much every chemical reaction in your body.

In other words, your body runs on protein, and pretty much is protein.

Nutritionally speaking, meats, dairy products, eggs, legumes, nuts, cereals are particularly high in protein. But because everything we eat was once alive, and every cell of every living thing contains protein, as long as you’re eating whole foods, you’re at least partially re-stocking your protein supplies. Now it might seem like you’d have eat muscle to make muscle, or eat enzymes to make enzymes, but that’s not how it works. Since all of your proteins are made up of just 20 amino acids, the differences between the thousands of unique proteins are simply in the sequence of those amino acids. And, of course, you have a specialized molecule that knows just which amino acids to put together in what order to make a certain protein.

It’s called DNA. When you consume some hamburger, for example, the protein actin in the meat gets catabolized into its component amino acids, which gets mixed up with all the amino acids from the other proteins in the meat — like the collagen and elastin and titin and myosin — as well as all the protein from the bun and the tomato and the mayonnaise.

Those amino acids then get reassembled using anabolic reactions into your very own, but somewhat different, proteins, as defined by your DNA. Each cell is like a picky little Gordon Ramsay and it has to have every amino acid needed — every ingredient present — before it will even think about starting to make a protein. And just like with your lipids, your cells can improvise, and convert some amino acids to others if they’re missing an ingredient.

However, there are nine essential amino acids that you cannot make from others, and have to eat. Now lots of foods don’t provide every essential amino acid, but when you combine foods, like beans and rice, or pasta and cheese, you do get all of the essential amino acids. Which is important because, remember: after water, you are mostly made of protein. On the order of 16% But what about the one percent of you? The carbohydrates?

How that tiniest fraction of you ends up creating all of the energy, is what we’ll discover next time.

But for now, you’ve learned all about the vital nutrients — including water, vitamins, minerals, carbs, fats, and proteins — as well as how anabolic reactions build structures and require energy, while catabolic reactions tear things apart and release energy. And together, these competing forces form the wonderfully conflicted process known as metabolism. Thank you to our Headmaster of Learning, Linnea Boyev, and thanks to all of our Patreon patrons whose monthly contributions help make Crash Course possible, not only for themselves, but for everyone, everywhere. If you like Crash Course and want to help us keep making videos like this, you can go to patreon.

com/crashcourse This episode was filmed in the Doctor Cheryl C.

Kinney Crash Course Studio, it was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney; our sound designer is Michael Aranda, and the Graphics team is Thought Cafe..

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