Triacylglycerols: Structure, Function, and Biological Importance

Triacylglycerols

Structure and Classification

Triacylglycerols (also known as triglycerides) are a class of lipids composed of three fatty acid chains esterified to a single glycerol molecule. They are the main constituents of body fat in humans and other animals, as well as vegetable fat.

• Glycerol: A three-carbon alcohol that serves as the backbone for triacylglycerols.

• Fatty acids: Long hydrocarbon chains with a carboxyl group at one end; can be saturated or unsaturated.

• Ester bonds: The linkage formed between the hydroxyl groups of glycerol and the carboxyl groups of fatty acids via dehydration synthesis.

Triacylglycerols can be classified as:

• Simple triacylglycerols: All three fatty acids are identical.

• Mixed triacylglycerols: Two or three different fatty acids are attached to the glycerol backbone.

Example: A triacylglycerol containing palmitic acid (16:0), oleic acid (18:1), and linoleic acid (18:2) is a mixed triacylglycerol.

Chemical Structure

The general structure of a triacylglycerol can be represented as:

• Glycerol backbone with three esterified fatty acids.

• Fatty acids may vary in length and degree of saturation.

Key Bond: The bond connecting the fatty acid chains to the glycerol backbone is an ester bond.

$\text{Glycerol} + 3\ \text{Fatty Acids} \xrightarrow{\text{Dehydration}} \text{Triacylglycerol} + 3\ \text{H}_2\text{O}$

Physical Properties

• Triacylglycerols are nonpolar and hydrophobic.

• They are stored in adipocytes (fat cells) in animals.

• Can exist as oils (liquid at room temperature, usually from plants) or fats (solid at room temperature, usually from animals).

Biological Functions

Triacylglycerols serve several important functions in living organisms:

• Energy Storage: Triacylglycerols are the primary long-term energy storage molecules in animals. They store more energy per gram than carbohydrates due to their highly reduced state.

• Insulation: Fat deposits help maintain body temperature by providing thermal insulation.

• Protection: Fat cushions and protects vital organs.

Example: Adipocytes are specialized cells that synthesize and store triacylglycerols.

Comparison Table: Simple vs. Mixed Triacylglycerols

Type	Fatty Acid Composition	Example
Simple Triacylglycerol	All three fatty acids are the same	Tripalmitin (three palmitic acids)
Mixed Triacylglycerol	Two or three different fatty acids	Palmitic, oleic, and linoleic acids

Practice Questions (from notes)

• What type of bond connects the fatty acid chains to the glycerol backbone in triacylglycerols? Answer: Ester bonds.

• Draw the structure of a triacylglycerol with three 16:0 fatty acids. Is it simple or mixed? Answer: Simple triacylglycerol.

• The functions of triacylglycerols in animals include: Storage for long-term energy, insulation, and protection.

Additional info:

• Triacylglycerols are synthesized via dehydration reactions catalyzed by enzymes called acyltransferases.

• Fatty acids can be mobilized from triacylglycerols by hydrolysis, a process called lipolysis.