Metabolism & Nutrition, Part 1: Crash Course Anatomy & Physiology #36

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.