Four Classes of Macromolecules: Study with Video Lessons, Practice Problems & Examples

Macromolecules are essential for cellular functions, categorized into four classes: polysaccharides, nucleic acids, proteins, and lipids. Polysaccharides, like starch and glycogen, store energy and provide structure through glycosidic bonds. Nucleic acids, composed of nucleotides, store genetic information and energy (e.g., ATP). Proteins, formed from amino acids linked by peptide bonds, perform various cellular activities. Lipids, including phospholipids and triglycerides, create cellular barriers and store energy. Understanding these macromolecules is crucial for grasping cellular biology and metabolism.

Polysaccharides

Hi. In this video, we're going to be talking about the 4 classes of macromolecules. So the first macromolecule I want to talk about is that of polysaccharides, which are otherwise known as sugars or carbohydrates, and they are responsible for cellular energy storage and support. So monosaccharides are the building blocks that make up larger polysaccharides. Monosaccharides are these small subunits that repeatedly added together to create these larger polysaccharides. Now, how are these monosaccharides attached? They're attached by these bonds called glycosidic bonds, which happen between a carbon and hydroxyl group on each monosaccharide to attach them together. Polysaccharides can form linear structures, so just sort of a line, or they can form ring structures. But essentially, they are just repeating units of this chemical formula. C, which is a carbon, 2 hydrogens, 10, that can be 200 10, that can be 200, repeating units to make up larger polysaccharides. Now, there are 2 main classes, these consist of aldose and ketose, are the 2 main classes. And the class is determined based on the position of a carbonyl group. So just looking at this example right here, you can see here there's a carbonyl group on the end, and so this is actually an aldose. And then here, you have the carbonyl group in the middle, which is the ketose. So in cell biology, we think of polysaccharides in terms of these larger complex molecules that are responsible for storage and structure. So, in plant cells, these are starch and cellulose. So, energy is stored as starch, and support is provided by cellulose. But in animal cells, it's actually glycogen. There aren't animal cells that don't have cellulose or starch, so they have glycogen, and that is responsible for storing chemical energy. Now, typically, polysaccharides are named by the number of units, subunits they contain. So these are a number of monosaccharides. So, the first one is a monosaccharide, and that's going to be 1 subunit. You can have a disaccharide, which is 2 subunits. You can have an oligosaccharide, which is up to 10. And you can have a polysaccharide, which is more than 11. And so the common sugars you may be familiar with are things like glucose, which have one subunit and therefore is a monosaccharide. We have sucrose, that's a disaccharide with 2 subunits. Lactose is also disaccharide. And then we have another one called amylose, which you may have not heard of, but we'll talk about in future topics. But that's just a polysaccharide. We haven't talked about lipids or proteins yet, and we will. You may be familiar with them from your intro bio class, but there are a lot of polysaccharides that attach to lipids and to proteins, that have really important functions, in cell biology that we're going to talk about a lot more. So just sort of know there are these things, that polysaccharides can attach to different molecules like lipids and proteins, and we'll talk about what those are, in future videos. So if we look at this example here, we have amylose, which again, if you remember is a polysaccharide. And it's made up of these bonds called \<msub>α1-4\<\/math\><p\>. So this is just sort of a fancy term that, doesn't necessarily that you don't necessarily need to understand the molecular complexity of it. Just know that these bonds refer to which carbon is bound. And so here, you can see that the bond is occurring between, these 2, the, oxygen occurring, between these two carbons. So these are the 1 and the 4, carbons. And therefore, that's how this name. Now, you so you may, just in case you see this in your textbook, you kind of understand what it means. And so, here we have these repeating subunits. Here's one subunit here. There's the n, these repeating units that make up, amylose. And there's going to be more on this end, there's going to be more down here. And this is what makes up the polysaccharide amylose, and this is kind of what polysaccharides look like. So now let's move on.<\/p>

Nucleic Acid

Okay, so now we're going to talk about the second class of micromolecules, and that is nucleic acids. We’re familiar with nucleic acids already from intro classes, but let's just do another quick review. Make sure everyone's on the same page moving forward. So nucleic acids are the subunits of DNA and RNA, which are responsible for storing and transmitting genetic information. And so each nucleotide contains a base, which you're familiar with. But we'll review in just a second a five-carbon sugar which is either deoxyribonucleic ribose and a phosphate group. Now there are two classes of bases. These are purine rings or pyrimidine things, and that has to do with their actual shape. So you can see here that purine rings have a pair of fused rings, which you can see right here, while pyrimidine things only have (let me do a different color), only have one. So these are my teens and, wrong color, purine rings. Now, the bases we are familiar with are A, G, C, T, or U: adenine, guanine, cytosine, thymine, or uracil. Uracil is found only in RNA. Okay, enjoy that there. But adenine and guanine are the purine rings, which have a pair of fused rings. While cytosine, thymine, and uracil are pyrimidine things. Now, you probably have already learned this, but these are just good classifications. You're going to have to know them if you don't remember them. And we'll review and do some practice in just a second.

So, what are the bonds that link different of these bases and these nucleotides together? The bonds that do that are phosphodiester bonds. And they are responsible for linking nucleic acids together. And it's important the order that nucleic acids are linked because the linear sequence actually encodes genetic information that makes up the genes that eventually become protein. So they can't just link any nucleic acid with another one. It has to be in a specific order. And we'll talk more about how that's done in different topics coming up.

So, there is a function of nucleotides that we really don't think of that often. We think, okay nucleotides store genetic information, but we don’t necessarily think that they can also store energy. But they do because ATP, which you've heard over and over again in your intro class, is actually adenosine triphosphate. And so that's a nucleotide that has a very specific function, not in genetic information but in supplying energy for numerous cellular reactions. So, ATP is going to be really important in the future. And we don't think of it as a nucleotide, but it is.

So here, we're just going to look at a very short DNA molecule here, and you can see you have your four bases: guanine, cytosine, adenine, and thymine. They're bound together here. And you can see that the purine rings with the two fused rings always bind to a peptide which has the one ring throughout the whole thing. And you can also see, which we didn’t go over here, but you should know from your intro class and also from chapter one, that if you remember A binds to T and G binds to C. And you can see this happening here throughout A and T, G, and C. So now, let's move on.

Proteins

So the next macromolecule that we're going to talk about are proteins. Proteins are extremely important in cell biology because they are responsible for carrying out cellular activities. So, what are the building blocks of proteins? The building blocks are amino acids, and they are linked together to make these things called polypeptide chains that then can form proteins, which carry out cellular activities. So, amino acids have an alpha carbon, a carboxyl group, and an amino group, and then an R group. And so, if we look at the amino acid structure, you can see all of these groups here. You have the amino group, you have the R group, you have the carboxyl group, and you have the alpha carbon. Sometimes amino acids may be called residues. That's another term, specifically when talking about, you know, usually specific amino acids in a protein. Amino acids are linked together through bonds called peptide bonds between the carboxyl group, which would be here, and then the amino group of an adjacent amino acid. So that would be if this carboxyl group binds to this amino group, this would be a peptide bond. And that's what links amino acids together. Proteins have a variety of different functions and their function is really determined by their structural properties. There are 20 amino acids, which, you should be familiar with some of them from your introductory class, but they are arranged in very specific formations for every protein to provide its function. Generally, the way that it has these unique functions is because of the side chain, which we just learned as the R group. The R group for each amino acid is different, and so that allows the amino acids to have different properties. These R groups can be classified as polar or charged or nonpolar, and there's even this group called the other group of amino acids that don't fit into any of these other classifications. But all of these different properties allow for the amino acids to give the protein that it is making up unique properties. Now, there are also these things called disulfide bridges, which we will talk about more in the future, but they are bonds between the sulfhydryl groups on the amino acid cysteine. They only occur on this amino acid, and they're very strong and stabilizing. So if you really want to stabilize the protein structure, you're going to have these disulfide bridges or disulfide bonds to stabilize that structure. So if we look at this polypeptide chain, so each one of these circles is an amino acid, and if we zoom in on one of them, you can see that it contains an amino group, a carboxyl group, this alpha carbon, and also this R group, which has the unique properties that allow the protein to fold into specific structures that provide a specific function. So now let's move on.

Lipids

Okay. So another macromolecule that we're going to talk about now are lipids, and they are responsible for the formation of cellular barriers. We are probably most familiar with the lipids that form the plasma membrane, which is a barrier of the cell in its external environment. We also know from our intro class that lipids are typically non-polar, which means that they do not dissolve in water. There are many different types of lipids, so we're going to go through each one individually. The first one that you're probably most familiar with is phospholipids. Phospholipids are the lipids that make up bilayer membranes, such as the plasma membrane or the mitochondrial membrane. They are composed of fatty acids.

What is a fatty acid? It is a long unbranched hydrocarbon chain, as you can see here. Here is the hydrocarbon chain, and it also has a polar group on the end, which you can see there. Phospholipids are composed of two of them; here is your phospholipid, right here, so you have your first fatty acid and your second, and you have your polar group. They are amphipathic, which means that they contain both hydrophilic and hydrophobic parts. Here, you have your hydrophobic and hydrophilic parts. That's a phospholipid.

Another class of lipids is fats, and they are really responsible for energy storage. One of the most common fats is triglycerides, and they are composed of three fatty acids. You can see 1, 2, 3, and they are linked by an ester bond to a glycerol molecule, which you can see here on the end. These fatty acids can be saturated if they do not contain double bonds or unsaturated if they do. For this first fatty acid, it does not contain any double bonds, so this one is going to be saturated. Whereas the second one has a double bond right here, so this will be unsaturated. And then the third one has no double bonds, so this one will be saturated. The saturation of molecules or fats plays a big role in the rigidity and flexibility of the molecule, and we're going to talk about this a lot more in future lessons. Fats are extremely important because they store energy; one gram of fat actually stores twice the energy of 1 gram of carbohydrates or polysaccharides. They're big energy storage molecules, and it's very hard to lose fat when we're exercising because it stores so much energy.

The final class of lipids that we're going to talk about are steroids. Steroids are a class of lipids responsible for hormone signaling and they play a major role in membrane structure. We're not going to talk about their roles today, but know that we will talk about them in the future. Steroids look different than the other lipids that we talked about because they actually have these ring formations. Down here, cholesterol, which is a really common example in cell biology, that's found in cell membranes, has a very distinct ring structure compared to something like a triglyceride, which looks like a traditional fatty acid with long hydrocarbon chains. Whereas steroids are much more in these ring formations.

Now that we've talked about lipids, let's move on.

Which of the following is false?

• A. Polysaccharides are responsible for energy storage and cellular support
• B. Nucleic acids are linked via Phosphodiester bonds
• C. Amino acid sequence and structure provide proteins with unique physical attributes
• D. Fatty acids are composed of branched hydrocarbon chains

Which of the following macromolecules does the cell use for structure and support?

• A. Polysaccharides
• B. Nucleic Acids
• C. Lipids
• D. Proteins

Animal cells store excess sugar in the form of what?

• A. Starch
• B. Cellulose
• C. Amylose
• D. Glycogen

Polysaccharides are the macromolecules that act as a source of energy storage.

• A. True
• B. False

Here’s what students ask on this topic:

What are the four classes of macromolecules and their primary functions?

The four classes of macromolecules are polysaccharides, nucleic acids, proteins, and lipids. Polysaccharides, such as starch and glycogen, primarily store energy and provide structural support. Nucleic acids, including DNA and RNA, store and transmit genetic information, while ATP (a nucleotide) stores energy. Proteins, composed of amino acids linked by peptide bonds, perform a wide range of cellular activities, including catalysis, transport, and structural support. Lipids, such as phospholipids and triglycerides, form cellular barriers like the plasma membrane and store energy. Understanding these macromolecules is crucial for grasping cellular biology and metabolism.

How are polysaccharides formed and what are their main types?

Polysaccharides are formed by linking monosaccharides through glycosidic bonds, which occur between a carbon and a hydroxyl group on each monosaccharide. The main types of polysaccharides include starch and cellulose in plants, and glycogen in animals. Starch stores energy in plants, while cellulose provides structural support. Glycogen stores energy in animal cells. Polysaccharides can be linear or branched and are classified based on the number of monosaccharide units: monosaccharides (1 unit), disaccharides (2 units), oligosaccharides (up to 10 units), and polysaccharides (more than 11 units).

What are the components of nucleic acids and their roles?

Nucleic acids, such as DNA and RNA, are composed of nucleotides. Each nucleotide contains a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), a five-carbon sugar (deoxyribose in DNA or ribose in RNA), and a phosphate group. Nucleic acids store and transmit genetic information. DNA encodes the genetic blueprint for the synthesis of proteins, while RNA translates this information into proteins. Additionally, nucleotides like ATP (adenosine triphosphate) store and provide energy for various cellular processes.

What are the building blocks of proteins and how are they linked together?

Proteins are composed of amino acids, which are linked together by peptide bonds. Each amino acid has an alpha carbon, a carboxyl group, an amino group, and a unique side chain (R group). The peptide bond forms between the carboxyl group of one amino acid and the amino group of an adjacent amino acid. The sequence and properties of the amino acids determine the protein's structure and function. Proteins perform various cellular activities, including catalysis, transport, and structural support.

What are the different types of lipids and their functions?

Lipids are a diverse group of macromolecules that include phospholipids, fats, and steroids. Phospholipids form the bilayer of cellular membranes, providing a barrier between the cell and its environment. They are amphipathic, containing both hydrophilic and hydrophobic parts. Fats, such as triglycerides, store energy and are composed of three fatty acids linked to a glycerol molecule. They can be saturated (no double bonds) or unsaturated (one or more double bonds). Steroids, like cholesterol, play roles in hormone signaling and membrane structure. Lipids are crucial for energy storage, membrane formation, and signaling.