In this video, we will learn about the composition, structure, and function of different types of lipids. We will examine triglycerides and phospholipids and how their structure is related to their function. And we’ll learn how lipids are classified based on their structures.
Lipids are molecules generally composed of carbon, hydrogen, and few oxygen atoms. They’re characterized by their possession of hydrocarbon chains and also by the fact that lipids are insoluble in water but soluble in organic solvents such as alcohols. In organisms, lipids are a dietary source of energy and a method of long-term energy storage. They provide waterproof coverings, and lipids make up the membranes of cells and most organelles. They serve as chemical messengers, and they provide thermal insulation for the bodies and organs of animals. And like everything else in biology, the structure of lipids is what allows them to carry out their specific functions.
In order to understand the basic structure of lipids, we’ll start by taking a look at a molecule called a fatty acid. A fatty acid is composed of what’s called a carboxyl group bonded to a long chain of hydrogen and carbon atoms. Each carbon atom is able to form a bond with four other atoms. In the hydrocarbon chain, each carbon atom bonds with two other carbon atoms, leaving two spaces free to bond with hydrogen atoms. Carbon–hydrogen bonds store lots of energy that organisms can transfer to other processes later. When every carbon atom is bonded with two hydrogen atoms, the chain consists of only single bonds. This is called saturated because the molecule has as many hydrogen atoms as it can possibly hold.
Sometimes, two carbon atoms will form a double bond instead. This means that each of these carbon atoms can only bond to one hydrogen atom instead of two. Since they’re less than the maximum number of hydrogen atoms in this molecule, it’s called unsaturated. You can see that saturated fatty acid molecules have a straight shape, and this shape allows them to naturally pack closer together. For this reason, saturated fatty acids tend to be solid at room temperature. In contrast, unsaturated fatty acids generally have a bent shape. This causes them to naturally possess more space between their molecules. And for this reason, unsaturated fatty acids tend to be liquids at room temperature.
This is the difference between a food like butter, which contains over 60 percent saturated fat and is solid at room temperature, and a food like olive oil, which contains over 80 percent unsaturated fat and is liquid at room temperature. These long chains of carbon and hydrogen atoms bonded together are also what give lipids their nonpolar qualities and make them insoluble in water.
Next, let’s take a look at two more complex lipids, phospholipids and triglycerides. Triglycerides are composed of three fatty acids and a glycerol molecule. The fatty acids bond with the glycerol in a chemical process called a condensation reaction. A condensation reaction occurs when the hydroxyl groups of two different molecules join. The atoms rearrange so that, at the end, both molecules are sharing one oxygen atom. The two hydrogen and one oxygen atoms that are released in the process bond together to form one molecule of water. Each of the three fatty acids bonds with one of the hydroxyl groups on the glycerol molecule. The bond that’s formed as a result of these condensation reactions is called an ester bond.
Triglycerides can eventually be broken apart again. This process, which is the opposite of a condensation reaction, is called a hydrolysis reaction. In a hydrolysis reaction, one molecule of water is consumed, and instead of sharing an atom of oxygen, the molecules break apart and form their own hydroxyl groups. Once the triglyceride has been separated into its components, the glycerol and fatty acids can be used as substrates for cellular respiration, just like glucose. The energy stored in the many high-energy carbon–hydrogen bonds is transferred and used to make ATP. This is what makes triglycerides extremely efficient molecules for energy storage.
Phospholipids have a structure similar to that of triglycerides. The main difference is that the glycerol alcohol is bonded to a phosphate group, which means that there’s only space for two fatty acid chains instead of three. This structural difference gives phospholipids some important properties. We’re already aware that hydrocarbon chains are nonpolar. Another way that we describe this part of the phospholipid is hydrophobic. Hydro- is a word part that refers to water and phobic means fearing. In contrast, the part of the phospholipid that includes the phosphate group and the glycerol molecule is polar in nature. Another way to describe it is hydrophilic or water loving.
Here, we have a simplified diagram of a phospholipid that you’re likely familiar with. It shows the hydrophilic head region and the hydrophobic tail. When phospholipids are placed in an aqueous solution, they have an interesting reaction. The hydrophobic tails orient themselves towards each other to avoid interacting with the water, while the hydrophilic heads attracted to the water and to each other form an outer layer. This spontaneous arrangement is known as a phospholipid bilayer and it’s why phospholipids make up the membranes of cells and most organelles. The specific structures of phospholipids and triglycerides are what make them specially adapted to their functions.
Next, let’s take a look at how lipids are classified. Lipids can be divided into three main categories: simple lipids, compound lipids, and derived lipids. Simple lipids yield fatty acids and alcohols when hydrolyzed or broken apart. One example of a simple lipid is a triglyceride, which we learned about a little bit earlier. Another example of a simple lipid is wax. Waxes are made of a fatty acid bonded to a long chain alcohol. They form the waterproof outer layer known as the cuticle on most leaves, and they’re also made by some animals, such as honey bees. Compound lipids yield fatty acids, alcohols, and at least one other molecule when hydrolyzed.
When you break apart a phospholipid by hydrolysis, you’ll get a phosphate group, a glycerol molecule, and fatty acids. Derived lipids are compounds with the properties of lipids that are derived from the hydrolysis products of simple and compound lipids. One example that we’re already familiar with is a fatty acid. Derived lipids also include a group of compounds known as steroids, an example of which is cholesterol. Cholesterol functions to provide strength and structure to the cell membranes of animal cells. Now that we’ve learned about the structure of lipids, their functions, and how they’re classified, let’s try a practice question.
Examples of biological molecules are provided: triglycerides, nucleic acids, phospholipids, cholesterol, cellulose. Which molecules from the examples given are classified as types of lipids?
This question provides us with a list of biological molecules. Then it asks us to determine which of these molecules will be considered types of lipids. So first, we’ll recall some facts about lipids. And then we’ll look at the structures of the molecules in the list provided, and we’ll use what we know to determine which of these are lipids. Let’s recall that lipids are molecules that are insoluble in water. They’re generally composed of carbon atoms, hydrogen atoms, and few oxygen atoms. And they’re characterized by their possession of what’s known as a hydrocarbon chain. Here, I’ve drawn simplified diagrams of each of the biological molecules listed. Can you identify them based on their structure?
If you said triglyceride, nucleic acid, phospholipids, cholesterol, and cellulose, then you know your biological molecules. In order to answer our question, let’s take note of two things here. First, we’re looking for lipids, which is a type of biological macromolecule. Well, nucleic acids are their own category of macromolecule. And cellulose is an example of what we call a carbohydrate. Our four types of biological macromolecules are proteins, lipids, nucleic acids, and carbohydrates.
Since nucleic acids and cellulose belong to other categories, we know that they’re not lipids. The second thing to note here is that lipids are characterized by the possession of a hydrocarbon chain. We can see hydrocarbon chains in the triglyceride molecule, the phospholipid molecule, and the cholesterol molecule.
Nucleic acids are composed of a phosphate group, a sugar, and a nitrogen-containing base. They do not possess hydrocarbon chains. And cellulose is made of a repeating pattern of sugars, also without hydrocarbon chains. Since triglycerides, phospholipids, and cholesterol possess hydrocarbon chains, we know that they’re classified as types of lipids.
Let’s wrap up our lesson by reviewing what we’ve learned. In this video, we learned about the properties and the functions of lipids. We learned about saturated and unsaturated fatty acids and how their structures give them specific properties. We learned about triglycerides and phospholipids and how their structures are directly related to their function. And we learned how lipids are classified as well as the role that the derived lipid, cholesterol, plays in the animal cell.