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 thatI look fairly solid. After water, the next largest proportion ofme is protein, about 16% -- not just in my muscles, but also in things like the tinysodium-potassium pumps in my neurons, and the hemoglobin in my blood, and the enzymesdriving the chemical reactions in every one of my 37 trillion cells.
Then another 16% of me is fat, which I’mtotally OK with; Four percent of me is minerals, like the calciumand phosphorus in my bones, and the iron in my blood; and 1 percent is carbohydrates, most of whichis either being consumed as I talk to you, or is sitting around as glycogen waiting tobe used. But here’s the thing: It’s not like I just ate 80kilograms of food and then all this happened. Instead,
my body, like yours, is constantlyacquiring stuff, extracting some of it to keep, burning some of it for energy, and gettingrid of the rest. But even the stuff that my body does holdonto doesn’t last forever. Some of the chemicals that I absorb in my food eventually becomea part of me. But enzymes wear out, and membranes break down, and DNA gets oxidized. So, theyget discarded.
And then I need more of those chemicals toreconstruct 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 breakingdown food, to keep resupplying itself with the raw materials it needs to survive. And all that activity requires energy, whichwe also gain from food. So, how do our bodies actually convert whatwe eat into energy and raw materials? The answer is a neverending series of reactionsthat are dedicated to doing two vital,
and totally contradictory, things: One set of chemical reactions destroys thereactants that you give them, reducing big, complex substances into molecular rubble. And the other set reassembles that rubbleinto 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 thecellular level, its consequences could hardly be larger. These two sets of reactions are where everythingthat 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 hascome 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 levelis. And that’s fine for use by personal trainersand fitness magazines. But physiologically,
metabolism really describes everysingle biochemical reaction that goes on in your body. And maybe more importantly, it reconcilestwo conflicting chemical processes that are always, simultaneously underway inside ofyou. One of those chemical forces is anabolism. Anabolic reactions construct things and consumeenergy. These are the processes that take the smallmonomer 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 proteinsthat are used in your cells. Then, when you need new building blocks, oryou need to release some energy, those polymers in your body, or new ones in your food, getbroken up -- by catabolic reactions. The processes of catabolism break down biggermolecules, and in breaking their bonds, release the energy you need to stay warm, and movearound, and provide your cells with fuel …
to build the polymers back up again. To be honest, your metabolism is a lot likeSisyphus. It works really hard. But it is never finished. And the boulder that your inner Sisyphus isalways pushing uphill and watching fall back down? That’s nutrients -- the moleculesthat your body is forever breaking up, and then rebuilding, only to have them break apartagain. And these nutrients --
the materials yourbody 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 comein either fat-soluble or water soluble forms. They aren’t used as building blocks or forenergy, but they’re essential in helping the body make use of other nutrients thatdo do those things. Vitamin C, for example, helps
improve ironabsorption, while vitamin K is crucial to blood clotting, and some B vitamins are importantin the production of ATP from glucose. Minerals, like vitamins, they don’t providefuel, but they have all sorts of other functions. Calcium, magnesium, and phosphorus hardenbones 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 everyonealways talks about -- the ones you find on food labels, from oatmeal to Pop-Tarts -- arecarbohydrates, 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 themonosaccharide glucose is the be-all-end-all molecular fuel that your cells need to makeATP. ATP being the molecule that your cells useto drive anabolic reactions, when they need to make new polymers or get anything elsedone -- whether that’s operating a sodium-potassium pump, or detaching the head of a myosin filamentto contract a muscle. But ATP is too unstable to store, so cellsoften store energy in the form of glucose, which they can then
catabolize and convertto ATP when they need it. Now, some of your cells can get their energyfrom 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 convertedto glucose for that reason. But, if it’s not needed right away, thatenergy can also get stored as glycogen in your liver and muscles, or converted to glyceroland fatty acids to make triglyceride fats. And even
though there seems to be a marketingwar 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 insulatesthe neurons in your brain and throughout your body, as well as the oil in your skin, and theyprovide the vital calorie content found in breast milk. But there are other important lipids, likecholesterol, which is the precursor to things like testosterone and estrogen... ...and, of course, phospholipids, which formthe 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 ofthe fat that you ingest might come from that. But guess what: Plants have fat too. Plants use lipids for energy storage justlike we do, except they do it in fruits, and nuts, and seeds. Which, when you think ofit, are kind of like plant breast milk -- it’s food for their growing babies. Either way, though, when you eat lipids, yourbody breaks down triglycerides into glycerol and fatty acids. Those molecules can then be processed
andused 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 convertingone fatty acid into another, but there are some it just can’t synthesize. For example, omega 6 and 3 fatty acids arecalled essential fatty acids, because your body can’t make them, so they have to beingested. They get turned into all kinds of useful molecules,
like the ones used for synapse formation in the brain, and for signalling inflammationduring the healing process. But -- if carbohydrates provide energy, andfats insulate and store energy, then just about everything else is done with proteins. They form the bulk of your muscle and connectivetissue, but they’re also what the ion channels and pumps are made of in your neurons andmuscle cells, and they make up your enzymes, which are responsible for
pretty much everychemical 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 weeat was once alive, and every cell of every living thing contains protein, as long asyou’re eating whole foods, you’re at least partially re-stocking your protein supplies. Now it might seem like you’d have eat muscleto make muscle, or eat enzymes to make enzymes, but
that’s not how it works. Since all of your proteins are made up ofjust 20 amino acids, the differences between the thousands of unique proteins are simplyin the sequence of those amino acids. And, of course, you have a specialized moleculethat 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 getsmixed up with all the amino acids from the other proteins in the meat -- like the collagenand elastin and titin and myosin -- as well as all the protein from the bun and the tomatoand the mayonnaise. Those amino acids then get reassembled
usinganabolic reactions into your very own, but somewhat different, proteins, as defined byyour DNA. Each cell is like a picky little Gordon Ramsayand it has to have every amino acid needed -- every ingredient present -- before it willeven think about starting to make a protein. And just like with your lipids, your cellscan improvise, and convert some amino acids to others if they’re missing an ingredient. However, there are nine essential amino acidsthat you cannot make from others, and have to eat. Now lots of foods don’t provide every essentialamino acid, but when you combine foods, like beans and rice, or pasta and cheese, you doget 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?
Thecarbohydrates? How that tiniest fraction of you ends up creatingall of the energy, is what we’ll discover next time. But for now, you’ve learned all about thevital nutrients -- including water, vitamins, minerals, carbs, fats, and proteins -- aswell as how anabolic reactions build structures and require energy, while catabolic reactionstear things apart and release energy. And together, these competing forces form thewonderfully conflicted process known as metabolism. Thank you to our Headmaster of Learning,
LinneaBoyev, and thanks to all of our Patreon patrons whose monthly contributions help make CrashCourse possible, not only for themselves, but for everyone, everywhere. If you likeCrash 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 CherylC. 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, editedby Nicole Sweeney; our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.
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