00:00:00
I weigh about 80 kilograms.
00:00:01
Most of that, let’s say 64 percent, is water
-- though you can’t tell by looking.
00:00:05
I mean, as organisms go, I like to think that
I look fairly solid.
00:00:09
After water, the next largest proportion of
me is protein, about 16% -- not just in my
00:00:13
muscles, but also in things like the tiny
sodium-potassium pumps in my neurons, and
00:00:17
the hemoglobin in my blood, and the enzymes
driving the chemical reactions in every one
00:00:21
of my 37 trillion cells.
00:00:23
Then another 16% of me is fat, which I’m
totally OK with;
00:00:26
Four percent of me is minerals, like the calcium
and phosphorus in my bones, and the iron in my blood;
00:00:31
and 1 percent is carbohydrates, most of which
is either being consumed as I talk to you,
00:00:36
or is sitting around as glycogen waiting to
be used.
00:00:38
But here’s the thing: It’s not like I just ate 80
kilograms of food and then all this happened.
00:00:43
Instead, my body, like yours, is constantly
acquiring stuff, extracting some of it to
00:00:48
keep, burning some of it for energy, and getting
rid of the rest.
00:00:50
But even the stuff that my body does hold
onto doesn’t last forever. Some of the chemicals
00:00:54
that I absorb in my food eventually become
a part of me. But enzymes wear out, and membranes
00:00:58
break down, and DNA gets oxidized. So, they
get discarded.
00:01:02
And then I need more of those chemicals to
reconstruct the material that I’ve lost.
00:01:06
As a result, over the course of my lifetime,
my cells will synthesize somewhere between
00:01:10
225 and 450 kilograms of protein …
00:01:14
That’s like 3, or 4, or 5 separate me’s
-- just made of protein.
00:01:18
And all of the protein and fat and
carbohydrates nucleic acids that
00:01:21
make up me, of course, come from food.
00:01:23
Every organism has to keep taking in and breaking
down food, to keep resupplying itself with
00:01:28
the raw materials it needs to survive.
00:01:30
And all that activity requires energy, which
we also gain from food.
00:01:33
So, how do our bodies actually convert what
we eat into energy and raw materials?
00:01:37
The answer is a neverending series of reactions
that are dedicated to doing two vital, and
00:01:42
totally contradictory, things:
00:01:44
One set of chemical reactions destroys the
reactants that you give them, reducing big,
00:01:49
complex substances into molecular rubble.
00:01:51
And the other set reassembles that rubble
into new and bigger products that are put
00:01:55
together again to make you.
00:01:57
So our bodies are constantly reinventing themselves --
in a perpetual state of loss, but also always rebuilding.
00:02:03
And even though all of this is happening at the
cellular level, its consequences could hardly be larger.
00:02:08
These two sets of reactions are where everything
that we’ve learned so far -- about the digestive,
00:02:11
endocrine, circulatory, and respiratory systems
-- really starts to come together.
00:02:16
Together, these processes make up your metabolism.
00:02:30
Now the sciencey word metabolism has
come to have a meaning in popular speech,
00:02:34
but metabolism isn’t just one thing.
00:02:37
People talk about metabolism as meaning, like,
how fast your body burns the fuel in your
00:02:41
food, or how high your personal energy level
is.
00:02:44
And that’s fine for use by personal trainers
and fitness magazines.
00:02:46
But physiologically, metabolism really describes every
single biochemical reaction that goes on in your body.
00:02:53
And maybe more importantly, it reconciles
two conflicting chemical processes that are
00:02:56
always, simultaneously underway inside of
you.
00:02:59
One of those chemical forces is anabolism.
00:03:02
Anabolic reactions construct things and consume
energy.
00:03:05
These are the processes that take the small
monomer building blocks in your food -- like
00:03:08
monosaccharides and fatty and amino acids
-- and build them into bigger, more complex
00:03:13
polymers like carbs, and fats, and proteins
that are used in your cells.
00:03:16
Then, when you need new building blocks, or
you need to release some energy, those polymers
00:03:21
in your body, or new ones in your food, get
broken up -- by catabolic reactions.
00:03:26
The processes of catabolism break down bigger
molecules, and in breaking their bonds, release
00:03:30
the energy you need to stay warm, and move
around, and provide your cells with fuel … to
00:03:35
build the polymers back up again.
00:03:37
To be honest, your metabolism is a lot like
Sisyphus. It works really hard. But it is never finished.
00:03:42
And the boulder that your inner Sisyphus is
always pushing uphill and watching fall back
00:03:46
down? That’s nutrients -- the molecules
that your body is forever breaking up, and
00:03:51
then rebuilding, only to have them break apart
again.
00:03:53
And these nutrients -- the materials your
body needs to build, maintain and repair itself
00:03:57
-- come in six major groups.
00:03:59
By volume, the majority of what we consume
-- and what makes up our bodies -- is water,
00:04:04
so that’s maybe the most vital nutrient.
00:04:06
Then there are vitamins, compounds that come
in either fat-soluble or water soluble forms.
00:04:10
They aren’t used as building blocks or for
energy, but they’re essential in helping
00:04:14
the body make use of other nutrients that
do do those things.
00:04:17
Vitamin C, for example, helps improve iron
absorption, while vitamin K is crucial to
00:04:21
blood clotting, and some B vitamins are important
in the production of ATP from glucose.
00:04:26
Minerals, like vitamins, they don’t provide
fuel, but they have all sorts of other functions.
00:04:29
Calcium, magnesium, and phosphorus harden
bones and teeth, while iron is, of course,
00:04:34
crucial in hemoglobin. Plus, potassium, sodium,
and chlorine help maintain your body’s pH
00:04:39
balance and are used in action potentials.
00:04:41
So water, vitamins, and minerals are all … necessary.
00:04:43
But the three major nutrients that everyone
always talks about -- the ones you find on
00:04:47
food labels, from oatmeal to Pop-Tarts -- are
carbohydrates, lipids, and proteins.
00:04:52
Most of the carbohydrates you’ve ever eaten
00:04:54
-- with the exception of lactose in milk --
originally came from plants.
00:04:57
Mono- and disaccharides come from fruits, honey,
sugar beets and sugar cane, while polysaccharide
00:05:02
starches come from veggies and grains.
00:05:04
The main thing you need to know is that the
monosaccharide glucose is the be-all-end-all
00:05:08
molecular fuel that your cells need to make
ATP.
00:05:11
ATP being the molecule that your cells use
to drive anabolic reactions, when they need
00:05:16
to make new polymers or get anything else
done -- whether that’s operating a sodium-potassium
00:05:21
pump, or detaching the head of a myosin filament
to contract a muscle.
00:05:24
But ATP is too unstable to store, so cells
often store energy in the form of glucose,
00:05:29
which they can then catabolize and convert
to ATP when they need it.
00:05:32
Now, some of your cells can get their energy
from fats. But many of the most important
00:05:36
ones, like your neurons and red blood cells,
feed exclusively on glucose. So most of the
00:05:40
carbs that your intestines absorb are converted
to glucose for that reason.
00:05:44
But, if it’s not needed right away, that
energy can also get stored as glycogen in
00:05:47
your liver and muscles, or converted to glycerol
and fatty acids to make triglyceride fats.
00:05:52
And even though there seems to be a marketing
war going on against dietary fats,
00:05:56
we most definitely need them.
00:05:57
The fats in your adipose tissue store energy,
of course, but they also store fat-soluble
00:06:02
vitamins, and cushion your organs.
00:06:03
Lipids also form the myelin that insulates
the neurons in your brain and throughout your
00:06:07
body, as well as the oil in your skin, and they
provide the vital calorie content found in breast milk.
00:06:12
But there are other important lipids, like
cholesterol, which is the precursor to things
00:06:15
like testosterone and estrogen...
00:06:18
...and, of course, phospholipids, which form
the cell membrane in every single one of the
00:06:21
three-dozen-or-so-trillion cells you have.
00:06:24
Now, if you’re into eating meat, a lot of
the fat that you ingest might come from that.
00:06:27
But guess what: Plants have fat too.
00:06:29
Plants use lipids for energy storage just
like we do, except they do it in fruits, and
00:06:32
nuts, and seeds. Which, when you think of
it, are kind of like plant breast milk -- it’s
00:06:37
food for their growing babies.
00:06:39
Either way, though, when you eat lipids, your
body breaks down triglycerides into glycerol
00:06:43
and fatty acids.
00:06:44
Those molecules can then be processed and
used in the making of ATP. Or they might be
00:06:47
converted into other kinds of fatty acids,
which your cells can then re-assemble into
00:06:51
your very own triglycerides or phospholipids.
00:06:53
And your liver happens to be great at converting
one fatty acid into another, but there are
00:06:58
some it just can’t synthesize.
00:07:00
For example, omega 6 and 3 fatty acids are
called essential fatty acids, because your
00:07:04
body can’t make them, so they have to be
ingested.
00:07:07
They get turned into all kinds of useful molecules,
like the ones used for synapse formation in
00:07:11
the brain, and for signalling inflammation
during the healing process.
00:07:15
But -- if carbohydrates provide energy, and
fats insulate and store energy, then just
00:07:19
about everything else is done with proteins.
00:07:22
They form the bulk of your muscle and connective
tissue, but they’re also what the ion channels
00:07:26
and pumps are made of in your neurons and
muscle cells, and they make up your enzymes,
00:07:30
which are responsible for pretty much every
chemical reaction in your body.
00:07:33
In other words, your body runs on protein,
and pretty much is protein.
00:07:38
Nutritionally speaking, meats, dairy products,
eggs, legumes, nuts, cereals are particularly
00:07:43
high in protein. But because everything we
eat was once alive, and every cell of every
00:07:48
living thing contains protein, as long as
you’re eating whole foods, you’re at least
00:07:52
partially re-stocking your protein supplies.
00:07:54
Now it might seem like you’d have eat muscle
to make muscle, or eat enzymes to make enzymes,
00:07:59
but that’s not how it works.
00:08:00
Since all of your proteins are made up of
just 20 amino acids, the differences between
00:08:04
the thousands of unique proteins are simply
in the sequence of those amino acids.
00:08:08
And, of course, you have a specialized molecule
that knows just which amino acids to put together
00:08:12
in what order to make a certain protein.
00:08:14
It’s called DNA.
00:08:15
When you consume some hamburger, for example,
the protein actin in the meat gets catabolized
00:08:19
into its component amino acids, which gets
mixed up with all the amino acids from the
00:08:24
other proteins in the meat -- like the collagen
and elastin and titin and myosin -- as well
00:08:29
as all the protein from the bun and the tomato
and the mayonnaise.
00:08:32
Those amino acids then get reassembled using
anabolic reactions into your very own, but
00:08:36
somewhat different, proteins, as defined by
your DNA.
00:08:39
Each cell is like a picky little Gordon Ramsay
and it has to have every amino acid needed
00:08:44
-- every ingredient present -- before it will
even think about starting to make a protein.
00:08:49
And just like with your lipids, your cells
can improvise, and convert some amino acids
00:08:53
to others if they’re missing an ingredient.
00:08:55
However, there are nine essential amino acids
that you cannot make from others, and have to eat.
00:09:00
Now lots of foods don’t provide every essential
amino acid, but when you combine foods, like
00:09:05
beans and rice, or pasta and cheese, you do
get all of the essential amino acids. Which
00:09:09
is important because, remember: after water,
you are mostly made of protein. On the order of 16%
00:09:14
But what about the one percent of you? The
carbohydrates?
00:09:17
How that tiniest fraction of you ends up creating
all of the energy, is what we’ll discover next time.
00:09:22
But for now, you’ve learned all about the
vital nutrients -- including water, vitamins,
00:09:26
minerals, carbs, fats, and proteins -- as
well as how anabolic reactions build structures
00:09:31
and require energy, while catabolic reactions
tear things apart and release energy. And
00:09:35
together, these competing forces form the
wonderfully conflicted process known as metabolism.
00:09:41
Thank you to our Headmaster of Learning, Linnea
Boyev, and thanks to all of our Patreon patrons
00:09:45
whose monthly contributions help make Crash
Course possible, not only for themselves,
00:09:48
but for everyone, everywhere. If you like
Crash Course and want to help us keep making
00:09:52
videos like this, you can go to patreon.com/crashcourse
00:09:55
This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written
00:09:58
by Kathleen Yale, edited by Blake de Pastino,
and our consultant is Dr. Brandon Jackson.
00:10:03
It was directed by Nicholas Jenkins, edited
by Nicole Sweeney; our sound designer is Michael
00:10:07
Aranda, and the Graphics team is Thought Cafe.
