Name: 新陳代謝與營養，第一部分。速成班 A&P #36 (Metabolism & Nutrition, part 1: Crash Course A&P #36)
Uploaded: 2021-01-14T06:19:50.000Z
Duration: 10 min 33 s
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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

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

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

Every organism has to keep taking in and breaking down food, to keep resupplying itself with

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

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

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,

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

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

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

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,

The fats in your adipose tissue store energy, of course, but they also store fat-soluble

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