Problem 129 - Chemistry The Molecular Nature of Matter

Question

Write the structure of a triacylglycerol that could be made from palmitic acid, oleic acid, and linoleic acid.

Step-by-Step Solution

Answer

A triacylglycerol composed of palmitic acid, oleic acid, and linoleic acid would have a glycerol backbone attached to these three fatty acids, esterified at each of the three hydroxyl groups. Palmitic acid would have a straight chain; oleic and linoleic acids would include kinks representing the double bonds in their structures.

Step 1: Understand the Components

Triacylglycerols, also known as triglycerides, are composed of a glycerol molecule that is esterified with three fatty acid chains. In this exercise, the three fatty acids given are palmitic acid, oleic acid, and linoleic acid. Palmitic acid is a saturated fatty acid with a 16-carbon chain. Oleic acid is an unsaturated fatty acid with one double bond (cis) and an 18-carbon chain. Linoleic acid is a polyunsaturated fatty acid with two double bonds (both cis) and an 18-carbon chain.

Step 2: Structure of Glycerol

Draw the glycerol backbone, which consists of a three-carbon chain, with each carbon atom bearing a hydroxyl group (OH). The structure is horizontal, with the carbons connected sequentially.

Step 3: Attach the Fatty Acids

Attach each fatty acid to the glycerol backbone through an ester bond, which forms by the reaction between the carboxyl group (COOH) of the fatty acid and the hydroxyl group (OH) of the glycerol. The position of the fatty acids on the glycerol molecule can vary, but typically, palmitic acid might be at the sn-1 position, oleic acid at the sn-2 position, and linoleic acid at the sn-3 position.

Step 4: Draw the Complete Structure

Combine the structures from the previous steps to show the complete triacylglycerol molecule. The palmitic acid chain will be straight, while the oleic and linoleic acid chains will have kinks at the sites of the double bonds. Remember to depict the double bonds correctly: a single line for single bonds and a double line for double bonds.

Key Concepts

Fatty Acids

Fatty acids are essential building blocks of lipids, particularly in the structure of triacylglycerols, which serve as a major form of energy storage in animals. There are various types of fatty acids, characterized by the length of their carbon chain and the presence or absence of double bonds.

Saturated fatty acids, like palmitic acid found in the given exercise, have no double bonds between carbon atoms and a straight chain. This allows for tight packing and solid form at room temperature. Unsaturated fatty acids like oleic and linoleic acid contain one or more double bonds, introducing kinks in the chain that prevent close packing, thus they're usually liquid at room temperature.

The presence of double bonds not only influences the physical properties of the fatty acid but also its biological roles and nutritional aspects. When drawing the structure of a triacylglycerol, it's crucial to correctly depict the carbon chain length and the position of double bonds for each fatty acid.

Ester Bond

An ester bond is a critical chemical linkage in the structure of lipids, specifically triacylglycerols. It is formed through a dehydration reaction between the carboxyl group of a fatty acid and the hydroxyl group of a glycerol molecule.

During this reaction, a molecule of water is released, and an ester bond is created, resulting in the attachment of the fatty acid to the glycerol backbone. This bond is depicted in molecular drawings as a single line connecting the glycerol and the fatty acid, generally positioned between the carbon atom of the glycerol's hydroxyl group and the carbon atom of the fatty acid's carboxyl group.

Given the relevance of ester bonds in triacylglycerol structure and function, it is essential for students to identify and accurately represent these bonds when drawing lipid molecules. Ester bonds are responsible for the energy-rich nature of fats, as they can be broken down during metabolism to release energy.

Glycerol Backbone

The glycerol backbone is the structural foundation to which fatty acids are attached in triacylglycerol molecules. Glycerol is a three-carbon alcohol, each bearing a hydroxyl group (OH), that serves as the 'anchor' for fatty acids.

In the exercise, students are asked to draw the glycerol structure as a horizontal chain with the three carbon atoms linked sequentially. It's important to correctly place each hydroxyl group on the respective carbon to ensure proper formation of ester bonds during the creation of the triacylglycerol structure.

When visualizing the glycerol molecule, imagine it as the central 'spine' of the triacylglycerol with the three fatty acids extending from it like branches. Accurate representation of the glycerol is vital as it helps students understand how fats are formed and stored in living organisms, as well as how they are broken down into their individual components during digestion.