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Structure and Function of Carbohydrates

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  1. 0:06 Carbohydrates
  2. 1:49 Counting Carbons
  3. 3:06 Sugars
  4. 5:06 Fiber
  5. 6:01 Starches
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Taught by

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.

Meet the Carbohydrates

In this lesson we'll be talking about carbohydrates, which are also known as sugars. The word carbohydrates comes from the atom carbon and hydrate, or water, because the first carbohydrates that were discovered consisted of carbon, hydrogen and oxygen atoms. To give an idea of what these look like, we'll first take a look at four common sugars in biology. From there, we'll move on to show how they can come together to form polysaccharides, or big sugar molecules.

The first is glucose. Glucose is an important sugar that serves as the fuel for our bodies. Next to it is fructose, which can be 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 similar to ribose except it lacks an oxygen or hydroxyl group on one of its carbons, hence deoxyribose. This is also an important sugar that helps hold our genetic material together.

Counting Carbons

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

Look closely at these, and you'll see that they all have one thing in common. They all have an ether group, which consists of oxygen single bonded to two carbon atoms. They also contain several hydroxyl, or OH, groups. You'll also notice that in each sugar there is also a carbon group that's single bonded to two different oxygen atoms. This carbon is special because it helps us to decide from 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 they can tell us about how sugars are going to link to one another when they form bonds.

Using glucose as an example, I'll show you how to count the carbons. First, we locate the carbon that's bonded to two oxygen atoms. Then, we see how many carbons this carbon is attached to. If this carbon is at the end of the carbon chain and attached to only one carbon, then this carbon becomes carbon #1. If it isn't, then you move out a little further, and that carbon can become carbon #1 in that case. Then we move around the ring, counting carbons. Carbon #2 is the one next to that; carbon #3, the one next to that; carbon #4, then one next to that; and carbon #5, the one next to that. And since glucose has six carbons, carbon #6 is the last carbon that we have for glucose.

Sugars

Sugars are super-cool, because not only can they exist on their own, like glucose can serve as fuel for our bodies, but they can also form bonds with other sugars and do really cool things. Don't take it from me just because I'm a sugar chemist. Let's look at a nutrition label 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
Disaccharide

This label is for one large apple; it contains 130 calories and, by all means, is pretty healthy since it contains no calories from fat. If you look farther down the label though, you'll see that there are 34 total grams of carbohydrates. This includes five grams of fiber and five grams of sugars. So, some of those sugars in there may be those same monosaccharides, or single sugars, that we've seen before. But, some of those may be disaccharides, like sucrose, which is table sugar. Sucrose is a disaccharide, which is formed by the linkage of carbon #1 in glucose to carbon #2 in fructose. This glycosidic linkage, or bond between two sugars, is what holds the molecule together.

One really cool thing about glycosidic bonds is that they're formed by dehydration, which is the loss of a water molecule. That dehydration occurs when two hydroxyl groups come together, leaving a carbon bonded to an oxygen bonded to another carbon, or ether. And water is a byproduct of those two hydroxyl groups.

If you look at the picture, you can see that the fructose is below the glucose sugar; this means this is an alpha linkage. Now, you might say to yourself 'it's bonded below, so isn't that the opposite of alpha?' Well, to remember this, I chant to myself Beta ABOVE, Alpha BELOW. Got it?

Okay, well carbohydrates can do other things than just form disaccharides. They can form trisaccharides, or large sugars made from three monosaccharide units. Or, they can form even bigger sugars, known as polysaccharides, which are sugars consisting of more than three sugar units.

Fiber

We've already explained a lot of the sugars in fruit on the nutrition label, so why don't we explain where some of these other things come from? Let's look at dietary fiber, which is also known as cellulose. It's an important structural material in plants - it's what helps grass stand up straight, so you've seen it before. One very interesting thing about cellulose is it's a very long strand of glucoses linked together - thousands and thousands of glucoses linked together in a single strand! And these sugars are linked with a beta 1,4 linkage between the different glucose molecules.

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

Now, one interesting fact about cellulose, which my grandmother referred to as 'roughage', is it's really something that cows like to eat. This is great, because cows can break down cellulose, while humans can't. Cows have little bacteria in their stomachs that allow them to break down the cellulose and get energy from the glucose in it.

Starches

If we go back to the nutrition label, we can see that we've looked at sugar and fibers, but that only accounts for ten grams of the 34 total grams. So, what about that gap of 24 grams of carbohydrates that've not been accounted for? It turns out that most of the rest of those carbohydrates are found in starch, which consists of two complex sugars: amylpectin and amylose, shown here as amylose. You'll see here that amylose consists of 1,4 alpha linkages between the glucoses, and it's built up monosaccharide by monosaccharide into a large sugar. Starch is actually where we get a lot of energy from.

So, if you think of an athlete, who's doing a carbo-load before a big race, usually you don't think of them sitting there eating pixie sticks and chocolate. Usually, you think of them as sitting down to eat a big dinner, with something like pasta. That's because something like pasta actually contains a lot of starch. And starch is really great because our bodies can digest it, unlike starch. But because, these are long strands of sugars consisting of thousands of units, it takes our body a while to break it down, so you get an extended release formula of sugars with starch.

There's a lot of starch and cellulose in plants because that's how they store their energy for future use. To store our carbohydrates, we mammals use something known as glycogen. It turns out that the chemical formula of glycogen is almost identical to that of amylopectin. It's a big, long strand of glucose, and this sugar can be millions of sugars long, so it's a little bit bigger than what you'd find in plants, but it serves the same function of storing glucose as energy for future use.

I hope you've enjoyed meeting some monosaccharides today, seeing how the can come together to form a disaccharide, or two sugars, and also the polysaccharides that consist of many thousands, or even millions, of sugars that we can use as an energy storage.

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