2.19.1 Acylglycerol

Introduction

The most abundant lipids in living things are acylglycerols. They are the main storage form of energy in both plants and animals. They are made of glycerol and fatty acids joined together by ester bonds.

Definition

Acylglycerols: Chemically, acylglycerols can be defined as esters of glycerol and fatty acids.

In simple words: Acylglycerols are lipids made of glycerol and fatty acids joined together.

Core Concept

• Acylglycerols are the most abundant lipids in living things.
• They are esters of glycerol and fatty acids.
• An ester is the compound produced as the result of a chemical reaction of an alcohol with an acid, and a water molecule is released. Such a reaction is called esterification.
• Acylglycerols are the main storage form of energy in plants and animals.

Detailed Explanation

Components of Acylglycerols

Acylglycerols are made of two components:

• Glycerol
• Fatty Acids

Glycerol

• Glycerol is a trihydroxy alcohol.
• It contains three carbons, each bearing an -OH group.
• Its molecular formula is:

Structure: CH₂OH - CHOH - CH₂OH

Fatty Acids

• A fatty acid is a type of organic acid containing one carboxylic acid group attached to a hydrocarbon.
• Fatty acids contain an even number of carbons from 2 to 30.
• Each fatty acid is represented as:

Structure: R - COOH

• The R group is a hydrocarbon chain.
• The -COOH group is the carboxylic acid group.

Esterification

Definition: An ester is the compound produced as the result of a chemical reaction of an alcohol with an acid, and a water molecule is released. Such a reaction is called esterification.

In simple words: Esterification is the reaction between an alcohol and an acid to form an ester and water.

General Equation:

Alcohol + Acid → Ester + Water

In Acylglycerol Formation:

Glycerol + Fatty Acid → Acylglycerol + Water

Types of Acylglycerols

Based on the number of fatty acids attached to glycerol, acylglycerols are classified into three types:

Formation:

• When a glycerol molecule combines chemically with one fatty acid, a monoacylglycerol (monoglyceride) is formed.
• When two fatty acids combine with a glycerol, a diacylglycerol (diglyceride) is formed.
• When three fatty acids combine with one glycerol molecule, a triacylglycerol (triglyceride) is formed.

• Triacylglycerols are also called neutral lipids.
• This is because all three -OH groups of glycerol are occupied by fatty acids, and no charge-bearing -OH group is left.

Structure of a Triacylglycerol

Glycerol + 3 Fatty Acids → Triacylglycerol + 3 Water

Diagrammatic Representation:

\beginaligned &CH₂OH \\ &CHOH \\ &CH₂OH \endaligned + 3 R-COOH → \beginaligned &CH₂O-CO-R \\ &CHO-CO-R \\ &CH₂O-CO-R \endaligned + 3 H₂O

Examples of Acylglycerols

Intermediate Level

Structure of Glycerol

• Trihydroxy alcohol
• 3 carbons
• Each carbon has an -OH group
• Formula: C₃H₈O₃

Structure: CH₂OH - CHOH - CH₂OH

Structure of Fatty Acids

• Organic acid
• Contains one carboxylic acid group (-COOH)
• Attached to a hydrocarbon chain (R group)
• Even number of carbons (2 to 30)
• Formula: R - COOH

Types of Fatty Acids:

Esterification Reaction

Definition: A chemical reaction between an alcohol and an acid to form an ester and water.

General Equation: Alcohol + Acid → Ester + Water

In Acylglycerol Formation: Glycerol + Fatty Acid → Acylglycerol + Water

• The -OH group of glycerol reacts with the -COOH group of a fatty acid.
• A water molecule is released.
• An ester bond is formed between the glycerol and the fatty acid.

Types of Acylglycerols in Detail

Triacylglycerols as Neutral Lipids

• Triacylglycerols are also called neutral lipids.
• This is because all three -OH groups of glycerol are occupied by fatty acids.
• No charge-bearing -OH group is left.
• Therefore, they are nonpolar and hydrophobic.

Biological Importance of Acylglycerols

Examples of Acylglycerols

Important Points

• 1. Acylglycerols can be defined as esters of glycerol and fatty acids.
• 2. They are the most abundant lipids in living things.
• 3. An ester is the compound produced as the result of a chemical reaction of an alcohol with acid, and a water molecule is released.
• 4. Such a reaction is called esterification.
• 5. Glycerol is a trihydroxy alcohol which contains three carbons, each bearing an -OH group.
• 6. A fatty acid is a type of organic acid containing one carboxylic acid group attached to a hydrocarbon.
• 7. Fatty acids contain an even number of carbons from 2 to 30.
• 8. Each fatty acid is represented as R-COOH, where R is a hydrocarbon tail.
• 9. When a glycerol molecule combines chemically with one fatty acid, a monoacylglycerol (monoglyceride) is formed.
• 10. When two fatty acids combine with a glycerol, a diacylglycerol (diglyceride) is formed.
• 11. When three fatty acids combine with one glycerol molecule, a triacylglycerol (triglyceride) is formed.
• 12. Triacylglycerols are also called neutral lipids as all three -OH groups of glycerol are occupied by fatty acids and no charge-bearing -OH group is left.
• 13. Examples of acylglycerols: Tristearin, Triolein, Tripalmitin.

Remember

• - Acylglycerols = esters of glycerol and fatty acids
• - Most abundant lipids in living things
• - Ester = compound formed from alcohol + acid; water released
• - Esterification = reaction that forms esters
• - Glycerol = trihydroxy alcohol; 3 carbons; 3 -OH groups
• - Fatty acid = R-COOH; even number of carbons (2–30)
• - Monoacylglycerol = glycerol + 1 fatty acid
• - Diacylglycerol = glycerol + 2 fatty acids
• - Triacylglycerol = glycerol + 3 fatty acids
• - Triacylglycerols = neutral lipids
• - Examples: Tristearin, Triolein, Tripalmitin

Common Mistakes

• 1. Confusing glycerol and fatty acids – Glycerol is a trihydroxy alcohol; fatty acids are organic acids with a hydrocarbon chain.
• 2. Thinking all acylglycerols are triglycerides – Acylglycerols can be mono-, di-, or tri-.
• 3. Confusing esterification with hydrolysis – Esterification forms an ester and water; hydrolysis breaks an ester using water.
• 4. Thinking triglycerides are polar – Triglycerides are nonpolar and hydrophobic.
• 5. Forgetting that triacylglycerols are neutral lipids – They have no free -OH groups, so they are neutral.
• 6. Thinking fatty acids have an odd number of carbons – Fatty acids have an even number of carbons (2 to 30).
• 7. Confusing saturated and unsaturated fatty acids – Saturated = no double bonds; Unsaturated = one or more double bonds.
• 8. Thinking all acylglycerols are solid – Acylglycerols with unsaturated fatty acids are liquid at room temperature (oils).
• 9. Forgetting the importance of acylglycerols – They are the main storage form of energy, provide insulation, and protect organs.
• 10. Thinking glycerol is a fatty acid – Glycerol is an alcohol, not a fatty acid.
• 11. Confusing mono-, di-, and triglycerides – Mono = 1 fatty acid; Di = 2 fatty acids; Tri = 3 fatty acids.
• 12. Forgetting the formula of glycerol – C₃H₈O₃.

Quick Revision

• The most abundant lipids in living things are acylglycerol.
• Chemically, acylglycerols can be defined as esters of glycerol and fatty acids.
• An ester is the compound produced as the result of a chemical reaction of an alcohol with acid and a water molecule is released such a reaction is called esterification.
• Glycerol is a trihydroxy alcohol which contains three carbons, each bears an OH group.
• A fatty acid is a type of organic acid containing one carboxylic acid group attached to a hydrocarbon.
• Fatty acids contain even number of carbons from 2 to 30.
• Each fatty acid is represented as R-COOH, where R is a hydrocarbon tail.
• When a glycerol molecule combines chemically with one fatty acid, a monoacylglycerol (monoglyceride) is formed.
• When two fatty acids combine with a glycerol a diacylglycerol (diglyceride) is formed and when three fatty acids combine with one glycerol molecule a triacylglycerol (triglyceride) is formed.
• Triacylglycerols are also called neutral lipid as all three OH groups of glycerol are occupied by fatty acids and no charge bearing OH group is left.

60-Second Revision

• Acylglycerols = esters of glycerol and fatty acids
• Most abundant lipids in living things
• Ester = compound formed from alcohol + acid; water released
• Esterification = reaction that forms esters
• Glycerol = trihydroxy alcohol; 3 carbons; 3 -OH groups
• Fatty acid = R-COOH; even number of carbons (2–30)
• Monoacylglycerol = glycerol + 1 fatty acid
• Diacylglycerol = glycerol + 2 fatty acids
• Triacylglycerol = glycerol + 3 fatty acids
• Triacylglycerols = neutral lipids
• Examples: Tristearin, Triolein, Tripalmitin
• Warning: Acylglycerols are not all triglycerides. Glycerol is not a fatty acid. Triglycerides are neutral lipids because they have no free -OH groups. Esterification is not the same as hydrolysis. Fatty acids have an even number of carbons.