Lesson Explainer: Lipids Biology

In this explainer, we will learn how to describe the structure, synthesis, and properties of lipids.

Did you know that your brain is made of over fat? Lipids, commonly called “fats” or “oils,” are an important part of our diet and also play several essential roles in the cells of living organisms. Lipids are especially important as the main components of the cell membrane. Special membrane layers wrapped around our brain cells are what allow them to communicate so rapidly. Lipids also function in energy storage, as waterproof coverings, and they help to insulate and protect our organs.

Lipids are similar to carbohydrates because they are primarily made of carbon, hydrogen, and oxygen atoms bonded together, but lipids have very different properties. Lipids are insoluble in water but soluble in organic solvents like alcohols. Lipids also possess relatively more carbon and hydrogen and less oxygen than carbohydrates. Oil, a common lipid, is shown being poured into water in the photo below. The two substances remain distinctly separate.

We will start by taking a look at one of the simplest types of lipids, a fatty acid. A fatty acid is a molecule composed of a carboxyl group attached to a long chain of carbon atoms bonded with hydrogen atoms. This is called a “hydrocarbon chain,” and it is a characteristic all lipids share. The hydrocarbon chain is also what gives lipids their nonpolar property and makes them insoluble in water. The basic structure of a fatty acid is shown in Figure 2.

Every carbon atom is able to make 4 chemical bonds. In the hydrocarbon chain, each carbon atom bonds with two other carbon atoms and two hydrogen atoms, for a total of 4 chemical bonds. The exception being the carbon atom at the end of the chain, which bonds with three hydrogen atoms and one other carbon. When every carbon atom in the hydrocarbon chain is bonded with the maximum possible number of hydrogen atoms, the molecule is referred to as “saturated” because every free place possible in the hydrocarbon chain is taken up by a hydrogen atom. A saturated fatty acid is shown in Figure 3.

Sometimes two carbon atoms in the hydrocarbon chain form a double bond. This means that they can each only bond with one fewer hydrogen atom, in most cases one instead of two. When a hydrocarbon chain possesses less than the maximum number of hydrogen atoms, the molecule is called “unsaturated.” A diagram of an unsaturated fatty acid is shown in Figure 3.

The bond between carbon atoms and hydrogen atoms is considered a “high energy” bond. Molecules, such as the carbohydrate glucose, that possess several of these carbon–hydrogen bonds are useful as substrates for cellular respiration, where that energy is transferred to molecules of ATP to be used in cellular processes. For this reason, fatty acids are particularly adapted to be used as a source of energy in cells.

You can see in Figure 3 that saturated fatty acids have a straight shape and unsaturated fatty acids are bent. The straight, saturated fatty acid molecules naturally pack closer together. For this reason, saturated fats are typically solids at room temperature (e.g., butter). In contrast, the bent, unsaturated fatty acid chains create more space between molecules, so they are typically liquids at room temperature (e.g., olive oil). Fatty acids are building blocks of more complex lipids such as phospholipids and triglycerides. A simplified diagram contrasting the chemical structure of common saturated and unsaturated fats is shown in Figure 4.

Triglycerides are composed of three fatty acid chains (“tri-” means “three”) and a glycerol molecule. Condensation reactions, also called dehydration synthesis, join the carboxyl group in each fatty acid with the three hydroxyl groups on the glycerol molecule and three molecules of water are released in the process. The bond that forms between the glycerol and the fatty acids is called an ester bond. A diagram illustrating the formation of a triglyceride is shown in Figure 5.

Triglycerides can later be broken apart in the opposite reaction, called hydrolysis. The hydrolysis reaction consumes one molecule of water for every ester bond that is broken. After hydrolysis, the separate glycerol and fatty acids can be used for cellular respiration, just like glucose. For this reason, triglycerides are a very efficient way for our cells to store energy. A diagram illustrating hydrolysis of a triglyceride is shown in Figure 6.

Phospholipids are similar to triglycerides, with one major difference: one of the fatty acid chains in a triglyceride is replaced with a phosphate group. This structural difference gives phospholipids unique properties. A diagram showing the general structure of a phospholipid is shown in Figure 7.

Fatty acid chains are extremely hydrophobic, meaning they repel water. The phosphate group is hydrophilic, meaning it is attracted to water. So, when you place phospholipids in an aqueous solution, the hydrophilic heads point toward the water and the hydrophobic tails turn inward, avoiding the water. This causes phospholipids to spontaneously form a bilayer membrane, completely on their own. This is the property that makes the phospholipids perfectly adapted to make up the membranes of cells and organelles. An illustration of a phospholipid bilayer is shown in Figure 8.

Example 2: Describing the Properties of Regions of a Phospholipid

The following is a basic outline of the structure of a phospholipid.

1. Which region is hydrophobic?
2. Which region is hydrophilic?

Answer

Part 1

A phospholipid is a type of compound lipid made of a phosphate group, a glycerol molecule, and two fatty acid chains. Fatty acids are molecules made of a carboxyl group attached to a long, hydrocarbon chain. Hydrocarbon chains are nonpolar, their electrons and protons are evenly distributed, and the molecule does not carry a charge. This makes hydrocarbon chains hydrophobic. They are repelled by water and attracted to each other.

Using this information, we can deduce that the region that is hydrophobic is X.

Part 2

On the other hand, a phosphate group is a charged, polar molecule. It is attracted to water, so it is considered to be hydrophilic. For this reason, the phospholipid molecule is said to have a hydrophilic “head” region and hydrophobic “tails.”

This means that the region that is hydrophilic is Y.

One last lipid that is important to mention is cholesterol. Cholesterol is a type of steroid. Steroids are molecules with a particular arrangement of carbon atoms in rings. Cholesterol is what we call a “derived lipid,” because cholesterol is not made of combinations of fatty acids and other molecules but is rather made from fatty acids that have been transformed by certain reactions. Cholesterol is important to animal cells because it provides the cell membrane with strength and structure. A diagram of the chemical structure of a cholesterol molecule is shown in Figure 9.

Lipids can be classified based on their chemical composition or structure. The three common categories of lipids are simple lipids, compound lipids, and derived lipids. We are already familiar with derived lipids as lipids that are not made of combinations of fatty acids and other molecules, an example of which is cholesterol.

Compound lipids are lipids that break apart into lipids and nonlipid molecules when hydrolyzed. An example of a compound lipid is a phospholipid.

Simple lipids are lipids that are formed when fatty acids react with alcohols. Examples of simple lipids include solid fats, liquid fats (or oils), and waxes. An example of oils—a liquid fat—is those that cover the feathers of water-dwelling birds to help prevent water from reaching their skin. This also prevents the feathers from becoming saturated with water and being too heavy to allow the bird to fly! A wax is a molecule that is made of a long-chain alcohol bonded to a fatty acid. Waxes are found in nature as a coating on the surface of leaves, also called a cuticle. The cuticle prevents the leaf from losing excessive amounts of water. Table 1 summarizes these classifications, with some examples.

Table 2 summarizes the structures and functions of the 4 types of lipids. Lipids are most easily identified by their hydrocarbon chains and nonpolar nature. Different types of lipids have different properties that make them adapted to different functions, from energy transfer to cell structure.

Let’s review what we have learned in this explainer.