Food and energy in organisms | Middle school biology | Khan Academy

Hey, quick question for you. You ever look at a person's baby pictures and wonder how people go from being small to, well, big? I mean, yes, I get it; people grow up, but here I'm thinking more on the level of the atoms and molecules that make up the body.

Because, A, I'm a scientist, and that's kind of what we do; and then, also B, because, you know, after all, all the changes we see on these larger scales are just reflections of very many changes occurring on the molecular scale, right? And so if you think about it that way, then it's likely reasonable to assume that, because the adult's body is bigger, it has more molecules and/or larger molecules than the baby's body.

And so this means that there are some molecules in this baby, and those molecules are interacting and combining with some other source of molecules in the world in order to become this adult.

Generally speaking, the source of those molecules is going to be our diet, aka our food. To kind of understand this, we can actually make a comparison here between food and wood.

So what I mean by that is that the molecules in food interact with the molecules in our bodies in two primary ways. The first is as a source of molecular building blocks, or in other words, molecules the body can use to build new structures. The second is going to be as a source of molecules for fuel, much like a campfire or firewood, right? Like what we have here.

But in order to understand how and why this can happen, we need to briefly discuss the molecular basis of food, right? So what are the molecules in food? Let me introduce you.

Because generally speaking, food is made up of these three classes of molecules. First up are fats, then we've got sugars, and finally, some protein. I'd imagine some of these sound reasonably familiar. These molecules are made basically from just a few elements, right?

So this is color-coded, and the gray balls are carbon, the white ones are hydrogen. Those are the big ones, right? So most of life's molecules are made mostly of carbon and hydrogen, but we've also got oxygen here in red and nitrogen in blue. Those are the other two big ones, and every now and then we'll also find small amounts of some other elements, like this sulfur here in yellow.

But the super interesting part here is that these are actually the same exact elements that make up most of the cells in your body, right? So you may notice on this right-hand side, the same colors is what we have on the left, right? So this is where the whole molecular lumber thing comes in.

Our cells can take food and break it down into small building blocks and then use those building blocks in order to build new structures in our body, right? So this is how we grow. This is how babies become adults. Turns out we actually are what we eat; it's pretty amazing!

All right, but before we get too excited, let's bring it in, right? So let's switch gears for a second here to talk about the other side of things. Because I told you before that food is like wood, and we can use wood to build new structures, sure, but when we need to, we can also burn wood to get energy, right?

So we can start a campfire, and if we do, we know that we're going to get some energy in the form of light, right? We'll get some energy in the form of heat, and maybe some other things like sound. Food is actually the same way, right?

So we can take those same food molecules that we broke down before to get building blocks and instead use them as fuel to generate cellular energy. Then we can use that cellular energy to fuel all of the building that we just talked about. And likewise, if our cells work together, which they do, they can use this energy to do things like running, thinking, and all the other things that we humans do.

All right, let's step back just one more time, because what we'll see is that these two processes that we've just described, right? So food is lumber, and food is fuel, actually explains so much of life.

Right? So this is not just us; every living thing on this planet is doing this, right? So you feed your dog to fuel his or her molecular construction projects as well.

So right there and then, we've already kind of answered our question from before, which reminder was how do people go from small to big?

Right? So what's happening is that this baby becomes an adult by eating food: milk, formula, gushy stuff, I don't know, whatever babies eat. And some of the molecules in that food will be used as fuel to generate energy, and then some of that energy will be used to combine food molecules with molecules in the baby to build up a larger baby, aka an adult.

All right, I hope that helps. Let's wrap it here. See you in the next one!