Carbohydrates - Online Tutor, Practice Problems & Exam Prep

Carbohydrates, or saccharides, are carbon-based molecules characterized by hydroxyl groups. They are classified into three size classes: monosaccharides (single units like glucose), oligosaccharides (2-20 units), and polysaccharides (more than 20 units). Monosaccharides link through dehydration synthesis to form glycosidic bonds, while hydrolysis breaks them down. Carbohydrates serve two main functions: structural support (e.g., cellulose in plants, chitin in animals) and energy storage (e.g., starch in plants, glycogen in animals). Understanding these concepts is crucial for grasping cellular functions and metabolic pathways.

Carbohydrates

In this video, we're going to begin our lesson on carbohydrates. Now, carbohydrates can be defined as carbon-based molecules that are hydrated with many hydroxyl groups, which recall are just a functional group that looks like this with an oxygen and a hydrogen atom. And so when we take a look at our image down below of carbohydrates, one thing that you'll notice is that there are plenty of these hydroxyl groups throughout their structures and so that is definitely a characteristic feature of carbohydrates. Now carbohydrates are also referred to as saccharides, and saccharides is really the Greek word that means sugars, and so sugars are carbohydrates. Now when the term carbohydrates was originally coined way back in the 1800s, it was actually referring to compounds that had the exact chemical formula of Cn(H2O)n where you had some number of carbon atoms being hydrated by some number of water molecules, and that's exactly where it got its name from. The carbo for the carbons and the hydro or the hydrates for the water molecules that are hydrating the carbon atoms. Now, it turns out that simple carbohydrates are carbohydrates that fit this chemical formula exactly. For example, glucose is a carbohydrate that fits this chemical formula exactly. And glucose is the most abundant carbohydrate, and it's the one that you guys should all be familiar with. And we'll be able to see an example of glucose down below in our image. But not all carbohydrates fit this chemical formula exactly, and so there are some complex carbohydrates, and complex carbohydrates are going to be carbohydrates that can slightly differ from this chemical formula here, and they can also have other types of atoms such as phosphorus, nitrogen, or sulfur atoms too. So let's take a look at our example down below to distinguish between the simple and complex carbohydrates. So notice on the left-hand side over here, when we take a look at its chemical formula and we count up the total number of carbon atoms, the total number of hydrogen atoms, throughout, and, the total number of oxygen atoms throughout, what we'll see is that there are a total of 6 carbon atoms, a total of 12 hydrogen atoms, and a total of 6 oxygen atoms. And so what you'll notice is that there are 6 water molecules that we can make out of the C6H12O6, and those water molecules are hydrating the 6 carbon atoms. And so this is going to be a molecule that fits the chemical formula up above exactly. And so this is going to be an example of a simple carbohydrate. More specifically, this is the chemical structure of glucose. And once again, glucose is the most abundant carbohydrate, and you should start to recognize its chemical formula of C6HO6H12 because at some point in your course, you will need to know this chemical formula. Now over here on the right-hand side, what we're showing you is a complex carbohydrate, and we can tell that it's a complex carbohydrate because its chemical formula does not match the one that we have up above. And so notice that it has a total of 6 carbon atoms, but when you count the hydrogen atoms there are 11 of them, and when you count the oxygen atoms there are actually 9 of them, and there's also 1 phosphorus atom as well, which you can see up above right here. And so this is a complex carbohydrate not because it has a circular shape, but because its chemical formula does not fit the one up above exactly. And so, moving forward, we're mainly going to be focusing on simple carbohydrates but it's good for you to also know that complex carbohydrates also do exist. And so this here concludes our introduction to carbohydrates and we'll get to talk more and more about carbohydrates as we move forward in our lesson. So, I'll see you all in our next video.

Which of the following chemical formulas represents that of a simple carbohydrate?

a) C₂H₂O₂.

b) C₆H₁₂O₆.

c) C₅H₄O₃.

d) C₃H₆O₉.

• A. C₂H₂O₂.
• B. C₆H₁₂O₆.
• C. C₅HO₃.
• D. C₃H₆O₉.

3 Size Classes of Carbohydrates

In this video, we're going to introduce three different size classes of carbohydrates that we have numbered down below. And the numbers here correspond with the numbers that we have throughout our image. Now, what you'll notice is that for all three of these size classes, the word saccharide here is present, which recall from our last lesson video means sugar. And so, saccharide is present in all three of these size classes, which means sugar, and that's referring to carbohydrates. And so, really these three different size classes, they differ in the root word, their prefix. And so the very first size class is going to be the monosaccharide. Now, recall that mono is a prefix that means just 1 or singular. And so monosaccharides are going to be a single carbohydrate unit, or in other words, monosaccharides are the monomers of carbohydrates, and that is very very important for you all to note. Now, an example of a monosaccharide is glucose, which once again is the most abundant monosaccharide. And so, it's one that you all should start to familiarize yourself with a little bit. And so when we take a look at our image down below, notice number 1 is the monosaccharide, which is just a single carbohydrate unit or just a single hexagon here, if you will.

Now the second size class of carbohydrate is going to be the oligosaccharide. And so oligo is a prefix that means a few. And so oligosaccharide, when you put it together, means a few sugars. So somewhere between 2 and 20 covalently linked monosaccharides would be classified as oligosaccharides. So when we take a look at our example down below at number 2, the oligosaccharides, we're showing you two types. We're showing you, one here that has, 2 sugar units linked together. So this would be more specifically a disaccharide since di is a prefix meaning 2. And then, here what we have is a trisaccharide since, tri is a prefix meaning 3 and there are three sugar units linked together. But once again, anywhere between 2 to 20 would be considered oligosaccharides, and so these are some oligosaccharides.

Now the third size class of carbohydrate that you all should be familiar with are the polysaccharides. And once again, poly is a prefix that means many, and so these are going to have greater than 20 covalently linked monosaccharides together. And, polysaccharides are, of course, going to be the polymer form of the carbohydrate. And so when we take a look at our image down below at the polysaccharides here, notice that it has, more than 20 covalently linked monosaccharide units together, and so we start to form the polysaccharide. Moving forward, we're going to talk about some specific examples of polysaccharides and their functions, so keep that in mind. But for now, this here concludes our introduction to the three size classes of carbohydrates, monosaccharides, oligosaccharides, and polysaccharides, and we'll be able to get some practice moving forward in our course. So I'll see you all in our next video.

Formation & Breakdown of Polysaccharides

In this video, we're going to talk about the formation and the breakdown of polysaccharides. Recall from our previous lesson videos that dehydration synthesis reactions are needed to link individual monosaccharides together to build polysaccharides. The synthesis part of dehydration lets us know that we're going to be building or synthesizing polysaccharides. The covalent bonds that link individual monosaccharides together are referred to as glycosidic bonds. Glycosidic bonds are the covalent bonds that link monosaccharides together. Lastly, recall that the hydrolysis reaction is what's needed to break down polysaccharides into individual monosaccharides.

When we take a look at our image below, we can see the formation of a sugar called maltose from 2 glucose molecules. Notice on the far left, we have these two separate monosaccharides, one here and one there, which are two separate glucose monosaccharides. If we wanted to join these two separate glucose monosaccharides together, as shown here, we would need a dehydration synthesis reaction, which dehydrates the molecule, releasing a water molecule. A covalent bond forms, linking the two separate monosaccharides. When these two are linked together, the covalent bond that links them is referred to as a glycosidic bond, as indicated above. As soon as these two glucose monosaccharides are joined together, it becomes a new sugar called maltose, and this is a maltose disaccharide starting to build our polysaccharide.

If we wanted to break down this maltose disaccharide into its individual monosaccharides, we would need the hydrolysis reaction. Recall "lysis" is the root that means to break down or cleave, and so it will break down or cleave the glycosidic bond here to release the two separate monosaccharides. There is a lot to review here, but one of the main takeaways is that the bond between individual monosaccharides is referred to as a glycosidic bond. This concludes our introduction to the formation and breakdown of polysaccharides, and I'll see you all in our next video.

Monosaccharides are linked together via a ______________ reaction, forming a _____________bond.

a) Hydrolysis ; Glycosidic.

b) Dehydration synthesis ; Hydrogen.

c) Hydrolysis ; Peptide.

d) Dehydration synthesis ; Glycosidic.

e) Hydrolysis ; Hydrogen.

• A. Hydrolysis ; Glycosidic.
• B. Dehydration synthesis ; Hydrogen.
• C. Hydrolysis ; Peptide.
• D. Dehydration synthesis ; Glycosidic.
• E. Hydrolysis ; Hydrogen.

Which of the following chemical reactions results in energy release when glycosidic bonds are broken?

a) Condensation reaction.

b) Dehydration synthesis reaction.

c) Hydrolysis reaction.

d) Hydrogen bonding.

• A. Condensation reaction.
• B. Dehydration synthesis reaction.
• C. Hydrolysis reaction.
• D. Hydrogen bonding.

Carbohydrate Functions

In this video, we're going to talk about carbohydrate functions. Carbohydrates are structurally and functionally diverse, meaning that they can do so many different things for the cell because they can take on so many different types of structures. However, there are just 2 main functions that you all should be aware of when it comes to carbohydrate functions.

The first function that you all should be aware of is structural support. Some carbohydrates are used specifically to build the structures of cells, either within cells or on the outside of cells. For example, cellulose and chitin are two classic examples of some polysaccharides that are used for structural support. There are some others as well, for example, peptidoglycan might be one that you may or may not have heard of, but we're going to focus specifically on cellulose and chitin.

The second primary function that you should know carbohydrates have is energy storage. Carbohydrates are specifically used for short-term energy storage and can provide energy to living cells. For example, starch and glycogen are both examples of carbohydrates that are used for short-term energy storage.

Let's take a look at our example down below to focus on the polysaccharides that are in plants and animals and the functions they have. Notice that we have this image of a grid where the first column has the function, whether it is structural support like this Bob the Builder guy, or energy storage like these batteries. Then we have the polysaccharides found specifically in plants here in this column, and the polysaccharides found specifically in animals over here in this column. We are showing you 2 structural support carbohydrates, one in plants and one in animals, and also 2 energy storage carbohydrates, once again one in plants and one in animals. It's a nice little grid.

When we're talking about structural support, a classic example of a polysaccharide in plants that's used for structural support is cellulose. Cellulose is actually the most abundant carbohydrate found in plant cell walls. When we take a look at plants like this leaf right here and we zoom in on its structure, you'll notice that the cell walls of these plants contain cellulose. It is used to build cell walls and that's why it is structural support.

A structural support carbohydrate in animals is going to be what we see over here, which is chitin. Chitin is a structural support carbohydrate found in the exoskeletons of insects and crustaceans such as lobsters. If we were to zoom into the exoskeleton shell here of this lobster, we would see that chitin would be found within its structure. Chitin's structure is complex, and we don't really need to worry about its particular structure so much. Just knowing that chitin is an example of a structural support carbohydrate that's used to build the structures of exoskeletons.

In terms of energy storage, we're showing you 2 as well over here in plants. Plants mainly store their energy in the form of starch. Starch is going to be the storage form of glucose specifically in plants, and you'll find lots and lots of starch inside of potatoes like this one over here. And then over here we have an animal liver, and in animals, they store their energy in the form of glycogen. Glycogen is a polysaccharide or carbohydrate whose main function is to store glucose in animal cells.

This here concludes our introduction to carbohydrate functions, and we'll be able to get some practice applying these concepts in our next few videos. So, I'll see you all there.

Animal cells store energy in the form of _________, and plant cells store energy in the form of ___________.

a) Sucrose ; glucose.

b) Disaccharides ; monosaccharides.

c) Starch ; glycogen.

d) Cellulose ; chitin.

e) Glycogen ; starch.

• A. Sucrose ; glucose.
• B. Disaccharides ; monosaccharides.
• C. Starch ; glycogen.
• D. Cellulose ; chitin.
• E. Glycogen ; starch.

Which polysaccharide is an important component in the structure of lobsters and insects?

a) Chitin.

b) Cellulose.

c) Starch.

d) Glycogen.

e) Polypeptides.

• A. Chitin.
• B. Cellulose.
• C. Starch.
• D. Glycogen.
• E. Polypeptides.

Here’s what students ask on this topic:

What are the main functions of carbohydrates in living organisms?

Carbohydrates serve two primary functions in living organisms: structural support and energy storage. Structurally, carbohydrates like cellulose and chitin provide rigidity and strength. Cellulose is found in plant cell walls, while chitin is present in the exoskeletons of insects and crustaceans. For energy storage, carbohydrates like starch and glycogen are crucial. Starch stores glucose in plants, commonly found in potatoes, while glycogen stores glucose in animal cells, particularly in the liver. These functions are essential for maintaining cellular integrity and providing energy for metabolic processes.

What is the difference between simple and complex carbohydrates?

Simple carbohydrates, such as glucose, have the chemical formula C_nH_2nO_n and consist of single or few sugar units. They are easily digestible and provide quick energy. Complex carbohydrates, on the other hand, have more intricate structures and may include additional elements like phosphorus, nitrogen, or sulfur. They consist of long chains of sugar units, making them harder to break down. Examples include starch and glycogen. Complex carbohydrates provide sustained energy and are often found in whole grains and vegetables.

How are polysaccharides formed and broken down?

Polysaccharides are formed through dehydration synthesis reactions, where individual monosaccharides are linked by glycosidic bonds, releasing water molecules. For example, two glucose molecules can form maltose through this process. To break down polysaccharides, hydrolysis reactions are used. These reactions add water to cleave the glycosidic bonds, separating the polysaccharide into individual monosaccharides. This process is essential for digesting complex carbohydrates into simpler forms that can be utilized by the body for energy.

What are the different size classes of carbohydrates?

Carbohydrates are classified into three size classes: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are single sugar units, like glucose, and serve as the building blocks for larger carbohydrates. Oligosaccharides consist of 2 to 20 covalently linked monosaccharides, examples include disaccharides like sucrose and trisaccharides. Polysaccharides are composed of more than 20 monosaccharide units and include complex carbohydrates like starch, glycogen, and cellulose. These size classes help in understanding the structure and function of different carbohydrates in biological systems.

What is the role of glycosidic bonds in carbohydrates?

Glycosidic bonds are covalent bonds that link individual monosaccharides to form larger carbohydrate structures like oligosaccharides and polysaccharides. These bonds are formed through dehydration synthesis reactions, which release water molecules. Glycosidic bonds are crucial for the structural integrity and function of carbohydrates. For example, they link glucose units in starch and glycogen, enabling these molecules to store energy efficiently. Breaking these bonds through hydrolysis releases monosaccharides, which can then be used for energy or other metabolic processes.