Structure and Function of Carbohydrates

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  • 0:02 Carbohydrates
  • 1:41 Counting Carbons
  • 2:37 Sugars
  • 4:33 Fiber
  • 5:25 Starches
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Lesson Transcript
Instructor: Meg Desko

Meg has taught college-level science. She holds a Ph.D. in biochemistry.

Carbohydrates are found in many foods that we eat and may be found as sugars, starches, or fiber. Learn more about these three distinct types of carbohydrates, and how they are distinguished through their chemical structures in this lesson.

Meet the Carbohydrates

In this lesson, we'll be talking about carbohydrates, which are also known as sugars. The word 'carbohydrate' comes from the atom carbon and hydrate, or water, because the first carbohydrates that were discovered consisted of carbon, oxygen, and hydrogen atoms. To give you an idea of what these look like, well, first we'll introduce four sugars that you come across often in biology, and then we'll move on to discuss how they can come together to form larger polysaccharide, or big sugar, molecules.

The first is glucose. Glucose is the sugar that serves as fuel for our bodies. Next to it is fructose, which is the sugar found in high-fructose corn syrup. The next sugar is ribose, which plays an important role in holding together our genetic material. Next is deoxyribose, which is very similar to ribose except that it lacks an oxygen or hydroxyl group on one of its carbons, hence 'deoxy'ribose. This is also an important sugar that helps hold our genetic material together.

Glucose, fructose, ribose, and deoxyribose all have ether groups.
Ether group

Now, if you look closely at these, what you'll see is that one thing all of them have in common is that they all contain an ether group; so they contain an oxygen that is single-bonded to two different carbon atoms. They also contain several hydroxyl groups, or OH groups.

You'll also notice that in each of these molecules there's one carbon atom that is single-bonded to two different oxygen atoms, and this carbon is special - this helps us decide where we're going to start counting the carbon atoms on our sugar. The order of the carbon atoms is very important in sugars because this can tell us about how sugars are linked to one another when they form bonds.

Counting Carbons

Using glucose as an example, I'll show you how to count the carbons. First, we locate the carbon that is bonded to two different oxygen atoms. Then, we see how many carbons this carbon is attached to. If this carbon is at the end of a carbon chain and is only attached to one carbon, then this carbon becomes carbon number one. If it isn't, then you move out a little bit further, and sometimes there's one other carbon atom there, and that can become carbon one in that case.

Then, we go around the ring - so moving on, we have carbon one with two oxygen atoms bonded to it. The carbon next to that is carbon two; carbon three is the one next to that; carbon four, the one next to that; carbon five, the one next to that; and since glucose has six carbons, carbon number six is the last carbon that we have for glucose.


Now, sugars are super-cool because not only can they exist on their own and have their own functionality, like glucose is a source of fuel for our bodies, they can also form bonds with other sugars and do other really cool things. Don't take it from me just because I'm a sugar chemist. Let's go and look at a nutrition label and look at the carbohydrates section and see how it breaks down.

Sucrose, or table sugar, is formed by the bond between two sugars and is known as a disaccharide.

This label is for one large apple; it contains 130 calories and by all means is pretty healthy since it has no calories from any kind of fat. If you look down the label, though, you'll see that there are 34 grams of total carbohydrates, and this includes five grams of dietary fiber and five grams of sugars. So, some of the sugars in this may be those monosaccharides, or single sugars, that we saw before, but some of these might be disaccharides, such as sucrose, also known as table sugar.

Sucrose is a disaccharide formed by the linkage of glucose from carbon one to carbon two of a fructose molecule. This glycosidic linkage, or bond between two sugars, is what holds the disaccharide, or two monosaccharide sugars, together.

So one really cool thing about glycosidic linkages, which are the bonds that holds sugars together, is that they're formed by dehydration. And that dehydration comes when two hydroxyl groups come together, leaving a carbon bonded to an oxygen bonded to another carbon (or ether), and water is a byproduct from that hydroxyl group and the other hydroxyl group.

Now, carbohydrates can do things other than form disaccharides: they can form trisaccharides, or large sugars made from three different monosaccharide units; or, they can form polysaccharides, which are sugars made from many, many sugar units.


Now we've already explained where a lot of the sugar in fruit comes from, so why don't we look at some of those other things on the label? What we call dietary fiber is also known as cellulose, and it is an important structural material in plants - it's what helps blades of grass stand up straight, so you've seen it before. Now one really interesting thing about cellulose is that it's a very long strand of glucoses all linked together - thousands and thousands of glucoses in one really long strand!

Cellulose is dietary fiber made up of thousands of glucoses linked together.

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