Introduction to carbohydrates | High school biology | Khan Academy

What we're going to do in this video is give ourselves a quick introduction to carbohydrates. You might already be familiar with the notion if you look at some packaged food. There's usually a nutritional label, and it'll say carbohydrates; it'll tell you a certain number of grams per serving. Not all carbohydrates are edible, but many of the things that we eat or many carbohydrates are edible, and many of the foods we eat have some carbohydrate component to it.

But what's actually what is it actually? Well, we could look at the word, and we see carbo, so maybe it has something to do with carbons. And it says hydrates, so maybe it has something to do with water. If you said that, you'd be pretty close because carbohydrates do involve carbons. In fact, this is a very typical carbohydrate, a very simple one right over here. This is a glucose molecule, and in gray, you see that it has six carbons. The hydrate part refers to that carbohydrates typically have oxygen to hydrogens in the same ratio as you would expect in water. So for every one oxygen, two hydrogens.

And you see that right over here where in glucose you have one, two, three, four, five, six oxygens, and you have twelve hydrogens. And so this, that's where this word comes from. Now, another word that is often used interchangeably with carbohydrates is the term saccharide. Saccharide comes from Greek for sweet, and that makes sense because if you were to taste glucose, it would taste sweet to you.

Now, what's interesting about something like glucose is glucose can be a standalone molecule, a very simple sugar in this case, or you can build up larger molecules with really glucose as a building block. So for example, right over here, we have a part of a glycogen molecule, and you can see it's just a repeating sequence of glucose molecules. And so something like this we would call glucose a monosaccharide; it's one simple sugar right over here, monosaccharide.

We would call this glycogen a polysaccharide. Poly; polysaccharide. Another way to think about it is glucose is the building block for the glycogen. Another term you might see is monomer and polymer. Those are the general terms of if I'm building a large molecule out of a chain of smaller ones, the building blocks we would consider to be monomers, and then the thing that we build out of those monomers could be our polymer.

As we'll see, this monomer-polymer phenomenon is not limited to carbohydrates or saccharides. We're going to see that same relationship, for example, between amino acids and proteins. Now, what role do carbohydrates play inside of biological systems? Well, saccharides or carbohydrates are often associated with a source of energy. Glucose can be converted very quickly to energy in biological cells.

Glycogen is also a store of energy in your liver and your muscles, and once again, it can be broken down into the glucose molecules, which once again is a very readily available source of energy. Now in plants, especially, some of these polysaccharides could also play a structural role if we're talking about things like cellulose, which is another polysaccharide.

So, there's also a structural role. Now I will leave you there. We have focused only on one type of monosaccharide in glucose and only on one type of polysaccharide in glycogen. As we will see, glucose does show up a lot, but there are many other types of monosaccharides, and there are many other types of polysaccharides.

Polysaccharides, in particular, are part of a broader group of molecules known as macromolecules. As you can imagine from the macro prefix, it's referring to large molecules oftentimes that have thousands of atoms in them. But don't get the wrong idea. They're very large at an atomic level, but each of these circles are still atoms, so you would still need a very, very, very, very powerful microscope to even take a look at even some of the largest macromolecules, including polysaccharides.