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 60% 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.

Key Term: Lipid

Lipids are macromolecules that are characterized by hydrocarbon chains. Lipids are insoluble in water and soluble in organic solvents.

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.

Oil being poured into water

Figure1

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.

Key Term: Fatty Acid

A fatty acid is a type of lipid that consists of a carboxyl group attached to a hydrocarbon chain. Fatty acids can be saturated or unsaturated.

Key Term: Hydrocarbon Chain

A chain of carbon atoms connected by single or double bonds. The free spaces are filled by hydrogen atoms.

Key Term: Nonpolar Molecule

A nonpolar molecule has an equal arrangement of electrons and protons so no part of the molecule possesses a charge. Nonpolar molecules are hydrophobic.

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.

Key Term: Saturated

We refer to a hydrocarbon chain as saturated when all the carbon atoms are linked by single bonds and the molecule possesses the maximum number of hydrogen atoms.

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.

Key Term: Unsaturated

We refer to a hydrocarbon chain as unsaturated when one or more carbon atoms share a double bond.

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.

Key Term: Triglyceride

A triglyceride is a type of lipid that is composed of a glycerol molecule bonded to three fatty acid chains. Triglycerides are important energy storage molecules.

Definition: Condensation Reaction (Dehydration Synthesis)

A condensation reaction is a chemical reaction in which a chemical bond is formed, causing a molecule of water to be released.

Definition: Ester Bond

An ester bond forms between a carboxyl group of an acid (e.g., fatty acid) and a hydroxyl group of an alcohol (e.g., glycerol) as a result of a condensation reaction.

Example 1: Recalling the Type of Reaction That Forms a Triglyceride

What type of reaction forms a triglyceride from a glycerol molecule and three fatty acids?

Answer

A triglyceride is a type of lipid that is used for energy storage. As stated in the question, triglycerides are composed of a glycerol molecule that is bonded to three fatty acid chains. Triglycerides are used for energy storage because the glycerol and fatty acids can be separated again and the separate molecules can be used as substrates in cellular respiration. In order for a fatty acid to become attached to a molecule of glycerol, the hydroxyl group (OH) on the carboxyl end of the fatty acid and one of the three hydroxyl groups on the glycerol molecule rearrange so that the two molecules share a bond with one atom of oxygen. This frees two hydrogen atoms and one oxygen atom that bond to form a molecule of water. This chemical reaction is called a “condensation” reaction because water is released in the process.

Therefore, with this information, we know that the type of reaction that forms a triglyceride is a condensation reaction.

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.

Definition: Hydrolysis

Hydrolysis is a reaction that breaks a chemical bond between molecules through the addition of a water molecule.

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.

Key Term: Phospholipid

A phospholipid is a type of lipid that consists of a glycerol molecule attached to two fatty acids and a phosphate group. Phospholipids are the main components of cell membranes.

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.

Key Term: Hydrophobic

Hydrophobic is a word that means “water fearing.” It is used to describe nonpolar molecules that are repelled by water but attracted to each other.

Key Term: Hydrophilic

Hydrophilic is a word that means “water loving.” It is used to describe charged or polar molecules that are attracted to or soluble in water.

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.

Key Term: Cholesterol

Cholesterol is an example of a type of lipid called a steroid. Cholesterol is especially important for the role it plays in strengthening the cell membrane of animal cells.

Example 3: Evaluating Statements about the Functions of Lipids in Plant Cells and Animal Cells

State whether the following statements about lipids are true or false.

  1. Functions of lipids in animal cells include being a major component of the cell membrane and being used as a respiratory substrate.
  2. Functions of lipids in plant cells include being broken down to provide energy in photosynthesis and being a major component of the cell wall.
  3. Lipids are insoluble in water.

Answer

Part 1

Fatty acids, triglycerides, phospholipids, and cholesterol can all be found in animal cells. Fatty acids are simple hydrocarbon chains with a carboxyl group at one end. They function as a source of energy since they can be used as a substrate for cellular respiration. Triglycerides are made of three fatty acids (“Tri-” means “three”) bonded to a glycerol molecule. One of triglycerides’ functions is energy storage since they can be broken down using hydrolysis and the resulting, smaller molecules can be used as substrates for cellular respiration. Phospholipids are composed of a phosphate group, a glycerol molecule, and two fatty acid chains. Phospholipids have a hydrophilic “head” region where the phosphate group is located, and the hydrocarbon “tails” are hydrophobic. These properties make the phospholipids spontaneously form what we call a “phospholipid bilayer” when placed in an aqueous solution. This phospholipid bilayer is a major component of cell membranes.

Since phospholipids are major components of the cell membrane and fatty acids are used as a respiratory substrate, we can conclude that this statement is true.

Part 2

The lipids found in plant cells include triglycerides, phospholipids, and fatty acids, but not cholesterol. In plant cells, lipids function as respiratory substrates and also store energy and are major components of the cell membrane. Cholesterol, which provides structure to animal cells, is not found in plant cells because plant cells have a cell wall composed of the rigid carbohydrate, cellulose, which provides support and structure instead.

Therefore, this statement is false.

Part 3

Lipids are a group of biological macromolecules that have some characteristics in common. Lipids are mostly made of atoms of carbon, hydrogen, and oxygen. Lipids all possess chains of hydrogen and carbon atoms bonded together, called a “hydrocarbon chain.” Lipids are nonpolar molecules, meaning that they do not possess an electrical charge and they are not soluble in water, but they are soluble in organic solvents like alcohol.

So, we can conclude that this statement is true.

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.

Example 4: Identifying Lipids from a List of Types of Biological Molecules

Examples of biological molecules are provided:

  1. Triglycerides
  2. Nucleic acids
  3. Phospholipids
  4. Cholesterol
  5. Cellulose

Which molecules from the examples given are classified as types of lipids?

Answer

Lipids are biological macromolecules that possess hydrocarbon chains and are not soluble in water. They function in cells as substrates for cellular respiration, energy storage molecules, and components of the cell membrane. A type of lipid that is a substrate for cellular respiration is a fatty acid. It is made of a hydrocarbon chain attached to a carboxyl group. A type of lipid that functions in energy storage is a triglyceride. Triglycerides can be broken down to release fatty acids that are used to generate cellular energy. Lipids that you can find in the cell membrane are phospholipids and cholesterol. Phospholipids form the main part of the cell membrane called the “phospholipid bilayer.” Cholesterol is also found in the cell membrane of animal cells, where it helps to provide strength and structure. On the other hand, one of the functions of nucleic acids is information storage, and they are made of nucleotides, not hydrocarbon chains. Moreover, cellulose is a type of carbohydrate that strengthens the cell walls of plant cells. It is not a lipid.

So, the molecules from the list that are classified as lipids are I, III, and IV.

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

Key Points

  • Lipids are not soluble in water and possess hydrocarbon chains.
  • Hydrocarbon chains can be saturated or unsaturated, depending on the presence of double bonds between carbon atoms.
  • Lipids functions include energy transfer, energy storage, and being components of the cell membrane.
  • Some common types of lipids are fatty acids, triglycerides, phospholipids, and cholesterol.
  • Lipids can be classified as simple, compound, or derived based on their chemical structure.

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