Ch.23 Lipids

McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition

McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook

McMurry 8th Edition

Ch.23 Lipids

Problem 23

Chapter 23, Problem 23

A membrane lipid was isolated and completely hydrolyzed. The following products were detected: ethanolamine, phosphate, glycerol, palmitic acid, and oleic acid. Propose a structure for this membrane lipid, and name the family to which it belongs.

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Identify the components of the membrane lipid based on the hydrolysis products: ethanolamine, phosphate, glycerol, palmitic acid, and oleic acid. These components suggest the lipid is a phospholipid, as it contains a phosphate group and glycerol backbone.

Recall the general structure of a phospholipid: it consists of a glycerol backbone, two fatty acid chains (hydrophobic tails), and a polar head group that includes a phosphate group and an additional molecule (in this case, ethanolamine).

Assign the fatty acids to the glycerol backbone. Palmitic acid (a saturated fatty acid) and oleic acid (an unsaturated fatty acid) will be esterified to the first and second hydroxyl groups of glycerol, respectively.

Attach the phosphate group to the third hydroxyl group of glycerol. Then, link the ethanolamine molecule to the phosphate group to form the polar head group. This structure is characteristic of a phosphatidylethanolamine.

Conclude that the membrane lipid belongs to the phospholipid family, specifically the phosphatidylethanolamine subclass, as it contains ethanolamine as the polar head group.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Phospholipids

Phospholipids are a class of lipids that are a major component of all cell membranes. They consist of a glycerol backbone, two fatty acid tails, and a phosphate group. The hydrophilic (water-attracting) phosphate head and hydrophobic (water-repelling) fatty acid tails create a bilayer structure that is fundamental to membrane integrity and function.

Fatty Acids

Fatty acids are long hydrocarbon chains that can be saturated (no double bonds) or unsaturated (one or more double bonds). In the context of membrane lipids, the presence of palmitic acid (a saturated fatty acid) and oleic acid (an unsaturated fatty acid) influences the fluidity and flexibility of the membrane, affecting its overall properties and functionality.

Glycerol Backbone

The glycerol backbone is a three-carbon molecule that serves as the structural foundation for various lipids, including triglycerides and phospholipids. In phospholipids, two of the hydroxyl groups of glycerol are esterified with fatty acids, while the third is linked to a phosphate group, which can further bond to other molecules like ethanolamine, contributing to the lipid's identity and function.

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Related Practice

Textbook Question

Complete hydrogenation of triacylglycerol C in Problem 23.20 yields a triacylglycerol of what fatty acid composition? Would the hydrogenation product of triacylglycerol C be more like the hydrogenation product of triacylglycerol A or B? Explain.

Textbook Question

Draw an 18-carbon unsaturated fatty acid that contains two carbon–carbon double bonds, one on carbon 6 and one on carbon 9 (count starting with the carboxyl carbon). Is this a “straight-chain” molecule or a “bent” molecule?

Textbook Question

As noted earlier (Section 22.3), he first step in glycolysis, which occurs within cells, is phosphorylation of glucose to glucose 6-phosphate. Why does this step prevent passive diffusion of glucose back out of the cell?

Textbook Question

Are the carbon–carbon double bonds in naturally occurring fatty acids primarily cis or trans?

Textbook Question

Which of the following terms apply to the compound shown below? (Hint: Look at the functional groups and the bonds involved to begin analyzing the compound part by part in comparison to the lipids discussed in this chapter.)

e. A lipid

Textbook Question

Draw an 18-carbon saturated fatty acid. Is this a “straight-chain” molecule or a “bent” molecule?