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26.3: Lipids

Created by: CK-12

Lesson Objectives

  • Describe the structure of a triglyceride.
  • Distinguish between saturated and unsaturated fats.
  • Describe the structure of a phospholipid and a lipid bilayer.
  • Describe the structure and properties of waxes.

Vocabulary

  • lipid
  • triglyceride
  • saturated fat
  • unsaturated fat
  • phospholipid
  • wax

Check Your Understanding

Recalling Prior Knowledge

  • What is an esterification reaction?
  • Are polar molecules more soluble in polar solvents or nonpolar solvents?

Certain large molecules known as fats are an essential component of a healthy body, though excessive consumption of fats is unhealthy. Fats are one member of a class of biological compounds known as lipids. In this lesson, you will learn about the structure and function of various lipids.

Triglycerides

Lipids are a class of water-insoluble compounds that includes oils, fats, and waxes. Oils and fats are based on the same general structure, but fats are solids at room temperature, while oils are liquids. Butter is an example of a fat that is derived from animals. Some oils include olive oil and canola oil, which are obtained from plants. Lipids are an essential part of a healthy diet, though excess dietary fat can be harmful. Lipids store energy in the body and are also needed to keep cell membranes healthy.

One type of lipid is called a triglyceride, which is an ester derived from the condensation reactions of three fatty acid molecules with one molecule of glycerol.

A triglyceride is a lipid that is formed by the esterification of glycerol with three fatty acid molecules.

Glycerol is a triol, an alcohol that contains three hydroxyl functional groups. A fatty acid is a long hydrocarbon chain, generally from 12 to 24 carbons in length, with a carboxyl group on one end. Each of the three fatty acid molecules undergoes an esterification reaction with one of the hydroxyl groups of the glycerol molecule. The result is a large triester molecule referred to as a triglyceride (Figure above).

Triglycerides are used for long-term storage of energy in the human body. Because of the long carbon chains, triglycerides are nearly nonpolar molecules, so they do not dissolve readily in polar solvents such as water. However, oils and fats are soluble in nonpolar organic solvents, such as hexane or ethers.

Fats may be either saturated or unsaturated. A saturated fat is a fat that consists of triglycerides whose carbon chains consist entirely of carbon-carbon single bonds. Therefore, the carbon chains are saturated with the maximum number of hydrogen atoms possible. An unsaturated fat is a fat that consists of triglycerides whose carbon chains contain one or more carbon-carbon double bonds. A fat with one double bond is called monounsaturated, while a fat with multiple double bonds is called polyunsaturated (Figure below).

Saturated fats are derived from fatty acids whose carbon chains contain all single carbon-carbon bonds. A monounsaturated fat contains one carbon-carbon double bond, while a polyunsaturated fat contains at least two double bonds.

Unsaturated fats are generally considered to be healthier than saturated fats. One reason is that they contain fewer calories than an equivalent amount of saturated fats. High consumption of saturated fats is linked to an increased risk of cardiovascular disease. Some examples of foods with high concentrations of saturated fats include butter, cheese, lard, and some fatty meats. Foods with higher concentrations of unsaturated fats include nuts, avocado, and foods that contain various vegetable oils, such as canola oil and olive oil. Figure below shows the percentages of different types of fat in some common foods.

Some common foods and oils, along with their percentages of saturated, monounsaturated, and polyunsaturated fats.

Phospholipids

A phospholipid is a lipid that contains a phosphate group and one or more long hydrocarbon chains. Phospholipids are the primary component of cell membranes. Each phospholipid molecule consists of a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails (Figure below).

A phospholipid consists of a head and one or more tails. The head of the molecule contains the phosphate group and is hydrophilic, meaning that it will dissolve in water. The tail of the molecule is a long carbon chain, which is hydrophobic and does not dissolve well in water.

Following the rule of “like dissolves like,” the hydrophilic head of the phospholipid molecule dissolves readily in water. The long carbon chains of a phospholipid are nonpolar, so they avoid water because of their insolubility. In water, phospholipids spontaneously form a double layer called a lipid bilayer, in which the hydrophobic tails of phospholipid molecules are sandwiched between two layers of hydrophilic heads (Figure below). In this way, only the heads of the molecules are exposed to the water, while the hydrophobic tails interact only with each other.

In an aqueous solution, phospholipids form a bilayer where the hydrophobic tails point towards each other, and only the hydrophilic heads are exposed to the water.

Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of molecules and ions in and out of the cell. However, an important function of the cell membrane is to allow selective passage of certain substances. This is accomplished by embedding various protein molecules that span the lipid bilayer (Figure below). These proteins form channels through which certain specific ions and molecules are able to move. Many membrane proteins are also attached to carbohydrates on the outside of the lipid bilayer. These carbohydrates serve a number of purposes that we are only beginning to understand, including the ability to provide identifying information about the type of cell to which it is attached.

The phospholipid bilayer of a cell membrane contains embedded protein molecules which allow for selective passage of ions and molecules through the membrane.

Waxes

Another type of lipid molecule is waxes. Waxes are esters of long-chain fatty acids and long-chain alcohols. Waxes are soft solids with low melting points that are insoluble in water. Figure below shows the structure of cetyl palmitate, a natural wax present in sperm whales.

Cetyl palmitate belongs to the category of compounds called waxes. It is derived from a fatty acid that is 15 carbons in length and an alcohol that contains 16 carbon atoms.

One of the best known natural waxes is beeswax, though many other animals and plants also synthesize waxes. Waxes can be found on the leaves of plants and on the skin, hair, or feathers of animals, where they function to keep these structures pliable and waterproof. Humans take advantage of the protective properties of natural and synthetic waxes in such applications as floor polish and car wax.

Lesson Summary

  • Lipids are a class of biomolecules that includes, oils, fats, and waxes. Lipids do not dissolve well in water because of their long carbon chains.
  • A triglyceride is derived from the esterification of glycerol with three fatty acids.
  • Phospholipids form a bilayer in which only the hydrophilic heads of the molecules are exposed to water. These bilayers form the structure of cell membranes.
  • Waxes are soft, low-melting solids formed from long-chain fatty acids and alcohols.

Lesson Review Questions

Reviewing Concepts

  1. What physical property of lipids is distinctly different from those of other biomolecules, such as carbohydrates and amino acids?
  2. Describe the difference between fats and oils.
  3. Explain how glycerol is capable of undergoing a reaction with three fatty acid molecules in the production of a triglyceride.
  4. Why are fats with one or more double bonds called unsaturated?
  5. How do phospholipids behave in water?
  6. What are the roles of phospholipids and proteins in a cell membrane?
  7. What organic compounds combine to form a wax?

Problems

  1. What type of organic reaction is capable of turning an unsaturated fat into a saturated fat? Explain.
  2. What are the molecular and condensed structural formulas of the wax formed from the esterification of 1-decanol with octanoic acid? Name the molecule.

Further Reading / Supplemental Links

Points to Consider

Nucleic acids form the genetic material that is passed from generation to generation in the form of deoxyribonucleic acid (DNA).

  • What are the structures of nucleic acids?
  • How does the nucleic acid sequence of DNA lead to the production of different proteins?

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