Carbohydrates - Online Tutor, Practice Problems & Exam Prep

Carbohydrates, or saccharides, are organic compounds made of carbon, hydrogen, and oxygen, typically in the ratio C_nH_2nO_n. Monosaccharides, like glucose (C₆H₁₂O₆), are the building blocks of carbohydrates and can form polysaccharides through glycosidic bonds. Key polysaccharides include starch and glycogen for energy storage, and cellulose and chitin for structural support. Carbohydrates also link to proteins and lipids, forming glycoproteins and glycolipids, essential for various biological functions.

Carbohydrates

In this video, we're going to do a quick review on carbohydrates. So recall that carbohydrates are also known as saccharides, and carbohydrates are sugars that are made up of carbon, hydrogen, and oxygen atoms, in a ratio of CnH2On, where there are a certain number of water molecules hydrating a certain number of carbon atoms. We can see that in the root of carbohydrates where "carbo" refers to the carbons and "hydrates" refers to the water molecules hydrating those carbons. If we mathematically distribute this little n over here, we can rewrite this chemical formula as CnH2nOn, where there are double the amount of hydrogen atoms than there are either carbon or oxygen atoms. This is relevant because the chemical formulas of carbohydrates can be written in either format, and it's important to be able to recognize this.

Recall that monosaccharides are the monomers of carbohydrates. Monosaccharides tend to be water-soluble, white crystalline solids with a sweet taste, which is exactly how we know our sugars to be. Carbohydrates can be used for lots of different functions, but they serve as a primary short-term energy source in pretty much every living organism. Monosaccharides can be repetitively linked together to create a polysaccharide polymer. In our example below, we have these monosaccharides listed on the left, and these are monosaccharide monomers. Through a process, we can link these monosaccharide monomers together covalently to create a polysaccharide polymer. Unlike nucleic acids and proteins, polysaccharides do not have directionality. Notice that the ending ose on a word is indicative that it is a sugar. Examples include glucose, fructose, sucrose, etc., all ending in ose, and they are all sugars and carbohydrates. In the next video, we're going to talk about the monosaccharide glucose. See you guys in that video.

Monosaccharides

So now that we've refreshed our memories that monosaccharides are the monomers of carbohydrates and can be linked together to form polysaccharides, we can talk about the monosaccharide itself. Most monosaccharides contain several hydroxyl groups. Recall that a hydroxyl group is a functional group that consists of an oxygen atom bonded to a hydrogen atom. Because monosaccharides contain several hydroxyl groups, they are known as polyalcohols, where an alcohol is a molecule that contains a hydroxyl group. Recall that glucose has a chemical formula of C₆H₁₂O₆, and glucose is a monosaccharide. It is actually the most abundant hexose, where hexose ends in 'ose', so it's definitely a carbohydrate, and the 'hex' part refers to the fact that it has 6 carbons. Glucose can be used by pretty much every living organism, either as a source of fuel or to build structures. Glucose monomers can be linked together via what are known as glycosidic bonds. Let's take a look at an example.

Down here, what we'll see is that we have a glucose molecule with the chemical formula C₆H₁₂O₆. Glucose can take two different forms; it can take a linear form or a cyclic form. In both forms, what we'll see is that we have all these hydroxyl groups, and therefore, it is a polyalcohol. Glucose monomers can be linked together via glycosidic bonds, and we can see that over here, where each of these pink bonds represents a glycosidic bond that links one glucose monosaccharide to the next. When we link monosaccharides in this fashion, we create a polysaccharide. That transitions us perfectly into our next video where we're going to talk about different types of polysaccharides.

Polysaccharides

So examples of polysaccharides include starch, glycogen, cellulose, peptidoglycan, and chitin. All of these polysaccharides vary in their functions, and we'll discuss their functions in the chart below. Carbohydrates are quite interesting because they can be covalently linked to other macromolecules such as proteins to create glycoproteins, lipids to create glycolipids, and nucleic acids to create nucleotides. In the chart below, we're going to fill in the function of each polysaccharide listed in the left-hand column.

Recall that starch is used as energy storage in plants, whereas glycogen is used as energy storage in animals. You may have learned that cellulose is a predominant component of plant cell walls, while peptidoglycan forms hard shells of insects as well as crabs, and it's a predominant component in the cell wall of fungi. As you can see, there are different structural polysaccharides that have similar functions but in different organisms. The first two are energy storage polysaccharides, whereas the bottom three are more so structural polysaccharides.

Over here in the right-side image, we have a glycoprotein. The glycoprotein consists of a carbohydrate portion shown in gray and a protein portion shown in light blue. This carbohydrate portion has a chain and a secondary chain that branches off from the main chain, and carbohydrates tend to make these complex structures with lots of branching, though they do not have to.

Similarly, we have a glycolipid which contains a carbohydrate portion in gray, and it also includes the phospholipid itself. In the image below, we have a DNA nucleotide. As we can see, it features a pentose sugar, specifically a deoxyribose pentose sugar. These pentose sugars are an important component of all nucleotides, and therefore, a crucial component of all nucleic acids.

We'll talk a lot more about carbohydrates as we move forward in our biochemistry course. I'll see you guys in the practice problem videos.

Which of the following expresses the correct chemical formula of a carbohydrate?

• A. C₆H₁₄O₅
• B. C₅H₁₀O₅
• C. C₄H₁₀O₆
• D. C₆H₁₂O₅

Here’s what students ask on this topic:

What are the main functions of carbohydrates in living organisms?

Carbohydrates serve several essential functions in living organisms. Primarily, they act as a short-term energy source. Monosaccharides like glucose are quickly metabolized to provide energy. Polysaccharides such as starch and glycogen store energy in plants and animals, respectively. Structurally, carbohydrates like cellulose and chitin provide support in plant cell walls and the exoskeletons of insects and crustaceans. Additionally, carbohydrates can form glycoproteins and glycolipids by linking with proteins and lipids, playing crucial roles in cell recognition, signaling, and immune responses.

What is the chemical formula for glucose, and why is it significant?

The chemical formula for glucose is C₆H₁₂O₆. This monosaccharide is significant because it is the most abundant hexose sugar and serves as a primary energy source for nearly all living organisms. Glucose can exist in both linear and cyclic forms, and its monomers can link via glycosidic bonds to form polysaccharides like starch, glycogen, and cellulose. Its versatility and abundance make it a fundamental molecule in biochemistry and cellular metabolism.

How do monosaccharides form polysaccharides?

Monosaccharides form polysaccharides through a process called glycosidic bonding. In this process, hydroxyl groups from two monosaccharides react, releasing a water molecule and forming a covalent bond known as a glycosidic bond. This reaction can repeat, linking multiple monosaccharides into long chains or branched structures, resulting in polysaccharides. Examples include starch, glycogen, and cellulose, each serving different biological functions such as energy storage and structural support.

What are glycoproteins and glycolipids, and what roles do they play in cells?

Glycoproteins and glycolipids are molecules where carbohydrates are covalently linked to proteins and lipids, respectively. Glycoproteins consist of a carbohydrate portion attached to a protein, while glycolipids have a carbohydrate portion linked to a lipid. These molecules are crucial for cell recognition, signaling, and immune responses. They are found on cell surfaces and play roles in cell-cell interactions, communication, and the structural integrity of cell membranes.

What are the differences between starch, glycogen, and cellulose?

Starch, glycogen, and cellulose are all polysaccharides but differ in their structure and function. Starch is used for energy storage in plants and consists of amylose and amylopectin. Glycogen is the energy storage polysaccharide in animals, highly branched for rapid energy release. Cellulose, on the other hand, is a structural polysaccharide found in plant cell walls, providing rigidity and strength. Unlike starch and glycogen, cellulose has β(1→4) glycosidic bonds, making it indigestible for most animals.