Lesson Video: Carbohydrates Biology

In this video, we will learn how to describe the structure of different carbohydrates, and outline the roles of carbohydrates in organisms.


Video Transcript

In this video, we will learn about the structure and the function of different carbohydrates and about the roles they play in the cells of organisms. We’ll also learn how simple sugars bond together to form polysaccharides and how the structure and function of carbohydrates are related.

A carbohydrate is a molecule which contains carbon, hydrogen, and oxygen atoms in a particular ratio. Carbo- is a word part which means carbon and hydrate refers to water, which we know contains hydrogen and oxygen. The ratio that most carbohydrates follow is C𝑚H2O𝑛, meaning that there are 𝑚-number of carbon atoms and 𝑛-number of water molecules arranged in a particular carbohydrate. Certain carbohydrates called complex carbohydrates are macromolecules, which are also referred to as polymers. A polymer is a large molecule made of several smaller repeating units called monomers. The monomers that make up complex carbohydrates are called simple carbohydrates.

Different types of carbohydrates play different roles in living things. Carbohydrates have many functions. In animal cells, carbohydrates are used for energy transfer and energy storage. Also, carbohydrates are attached to proteins embedded in the cell membrane. These are called glycoproteins and they are an important part of how cells communicate with each other. In addition to these roles, in plant cells, carbohydrates are also responsible for providing rigidity and structure to the cell wall. In biology, we know that structure is always directly related to function. So different types of carbohydrates that are made of different parts in different arrangements have different properties.

The simplest carbohydrates are simple sugars. These are also called monosaccharides. Mono- is a word part that means one, and saccharide is another word for sugar. Monosaccharides usually follow the ratio CH2O𝑛, where 𝑛 is a number between three and seven. Monosaccharides can also be classified based on the number of carbon atoms that they possess. Examples of monosaccharides include glucose, galactose, and fructose. These are all hexose sugars, since they have six carbons each. All three of these simple sugars are also isomers of each other. They have the same chemical formula, which means that they possess the same atoms just in slightly different arrangements. The ribose and the deoxyribose sugars that you find in nucleic acids are also monosaccharides. And these are pentose sugars, since they each have five carbon atoms.

At this point, you’ve probably noticed that many of these words seem to rhyme. The suffix -ose is a word part that refers to sugar. Glucose is particularly important to cells, since it’s the sugar that’s a reactant in cellular respiration. During cellular respiration, the energy in the bonds between the atoms in the glucose molecule is released and transferred to ATP, which powers our life processes. Monosaccharides, like glucose, are also soluble in water, which means that their concentration within the cell must be carefully monitored and controlled.

Next, let’s take a look at the polymerization of carbohydrates. Monosaccharides combine together during a chemical process known as a condensation reaction. A condensation reaction occurs when the hydroxyl groups on two monosaccharides rearrange to share just one oxygen atom in what’s called a glycosidic bond. To form this glycosidic bond, two hydrogen atoms and one oxygen atom must be removed. They bond together to form one water molecule, which is released. The release of this water molecule is what gives the condensation reaction its name. You can remember by thinking about how water is released from the air when it condenses into droplets. When just two monosaccharides bond together, they form a disaccharide, which is another type of simple sugar. Di- is a prefix that means two.

When glucose bonds with glucose, the disaccharide that is formed is known as maltose. When galactose bonds with glucose, the disaccharide that is formed is known as lactose, which you may recall is a sugar found in milk. And when glucose bonds with fructose, the disaccharide that’s formed is known as sucrose, which you may be familiar with as common table sugar. Disaccharides usually follow the chemical formula C𝑚H2O𝑛, where 𝑛 is equal to 𝑚 minus one, since one water molecule had to be released during the condensation reaction. All three of these disaccharides have the chemical formula C12H22O11.

More than two monosaccharides can also bond together into large macromolecules called polysaccharides. Poly- is a word part that means many. Polysaccharides are insoluble in water, so they’re ideal for functions like structure and support and long-term energy storage. Polysaccharides can exist in higher concentrations, since they won’t affect the water balance of the cell. When a cell needs to release some of this energy from storage, the polysaccharide must first be broken apart into monosaccharides using a chemical process known as a hydrolysis reaction.

Hydro- is a word part that refers to water and lysis means to break apart. A hydrolysis reaction consumes a water molecule and breaks the glycosidic bond apart. It’s basically the reverse of the condensation reaction that we learned about a little bit earlier. Once they’re broken down again, the monosaccharides can be used in processes that transfer energy such as cellular respiration. Examples of polysaccharides include starch, glycogen, and cellulose, and these three macromolecules differ in both structure and in function. The monomers in starch and glycogen are a different isomer of glucose than the monomers in cellulose.

Recall that an isomer is a molecule with the same chemical formula but a different arrangement of the atoms. These two isomers are called alpha-glucose and beta-glucose. And the difference is here in the atoms attached to the first carbon. In alpha-glucose, the hydroxyl group attached to the first carbon is on the opposite side of the molecule from this CH2OH group that you see here. And in beta-glucose, that hydroxyl is on the same side as the CH2OH group. So I’ve added a little bit more detail to the diagrams of each of these monomers to help us to tell them apart.

Starch and glycogen are made of alpha-glucose monomers. They both function as energy storage molecules. But starches are only made by plant cells, and glycogen is made by animal cells. Starch can occur in long branched or unbranched chains, while glycogen is always a highly branched molecule. Cellulose is made of beta-glucose monomers. Because of their structure, beta-glucose molecules bond together in an alternating orientation, forming long rigid unbranched chains. And these chains easily form hydrogen bonds between them, allowing them to stack together into larger structures. For this reason, cellulose is used for the structure of plant cell walls and not for energy storage.

Now that we’ve learned about monosaccharides, disaccharides, and polysaccharides and we familiarized ourselves with their structure and their function, let’s try a practice question.

The structure of monosaccharides and disaccharides can be determined using general formulas. What is the general molecular formula of a monosaccharide? A monosaccharide has the formula C6H12O6. Two of these monosaccharides join by a condensation reaction. What is the molecular formula of the resulting disaccharide?

This question is asking us to recall what we know about the chemical formulas of monosaccharides and disaccharides, which are both types of carbohydrates. Let’s just focus on the first part of this question for now. Here, we’re interested in the general molecular formula of a monosaccharide. And since it’s the general molecular formula, it should be able to apply to more than one type of monosaccharide. Let’s also take a moment to recall that mono- is a word part that means one and saccharide means sugar. So we’re looking for the general molecular formula of one sugar.

We know that sugars are a type of carbohydrate, and a great clue to their chemical formula is present right here in the name. Carbo- is a word part that refers to carbon and hydrate means water. So we know that these molecules consist of some carbon and some H2O. A monosaccharide that you’re likely familiar with already is glucose. Glucose is the simple sugar used by ourselves in cellular respiration. One molecule of glucose looks something like this and has the chemical formula C6H12O6 because glucose possesses six atoms of carbon, 12 atoms of hydrogen, and six atoms of oxygen.

So, like our carbohydrate ratio, there are twice as many atoms of hydrogen as there are carbon and oxygen. But glucose has six times the atoms of our base ratio. So in order to complete our general formula, we must multiply the entire ratio by a variable, which we’ll call 𝑛. Continuing our example, when 𝑛 is equal to six, that means we have six times the carbon, hydrogen, and oxygen atoms in our molecule, which comes out to C6H12O6. And this is the correct chemical formula for our example monosaccharide, glucose. So the general molecular formula for a monosaccharide is CH2O𝑛.

Now let’s take a look at the second part of this question. It’s asking us about the resulting disaccharide molecule that forms when two monosaccharides join. Di- is a prefix that means two. So let’s take a look at what happens when two monosaccharides bond together to form a disaccharide. We’ll use glucose as the monosaccharides in our example, since they have the chemical formula C6H12O6 and we’re already familiar with their structure.

When glucose bonds with glucose, the disaccharide that’s formed is called maltose. During the condensation reaction, two of the hydroxyl groups are rearranged so that both molecules are now sharing one oxygen atom. This process releases two hydrogen and one oxygen atoms, and they bond to form one molecule of water. The fact that a molecule of water is released is how the condensation reaction gets its name. So to figure out the chemical formula, we could count up the number of atoms in our maltose molecule. Or we can recall that that maltose molecule was made by joining two molecules of glucose together and releasing a water molecule in the process. Either way, you’ll come up with the same solution. The molecular formula for the resulting disaccharide is C12H22O11.

Let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned about the structure of carbohydrates and their importance in living organisms. We learned how bonds form between monosaccharides through condensation reactions, forming disaccharides and polysaccharides. We learned about three examples of polysaccharides — glycogen, starch, and cellulose — and how their structure is related to their function.

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