Fatty Acid and Lipid Metabolism

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CHAPTER 27 Fatty Acid Degradation
CHAPTER 28 Fatty Acid Synthesis
CHAPTER 29 Lipid Synthesis: Storage Lipids, Phospholipids, and Cholesterol

Some birds can fly thousands of miles over water without stopping to eat. Bears can hibernate for months without the need to wake and forage. Boundaries allow for the existence of cells and, by extension, organisms. All of these circumstances are possible because of a crucial class of biomolecules—lipids. We have already encountered lipids as a storage form of energy (Chapter 11) and as components of membranes (Chapter 12). Energy is stored as triacylglycerols, a glycerol molecule esterified to three fatty acids. When energy is required, the fatty acids are liberated and oxidized to provide ATP. When fuel is abundant, fatty acids are synthesized and incorporated into triacylglycerols and stored in adipose tissue.

Triacylglycerols are the most efficient fuels because they are more reduced than carbohydrates. However, this increase in energy efficiency comes at the cost of biochemical versatility. Unlike carbohydrates, lipid metabolism requires the presence of molecular oxygen to form ATP.

Not only are lipids important fuel molecules, but they also serve a structural purpose. The common forms of membrane lipids are phospholipids, glycolipids, and cholesterol. A phospholipid is built on a backbone of either glycerol or sphingosine, an amino alcohol. These lipids contain a phosphoryl group and an alcohol, in addition to fatty acids. Sphingosine-based lipids are further decorated with carbohydrates to form glycolipids. Cholesterol is another key membrane lipid. Cholesterol does not contain fatty acids; rather, it is built on a steroid nucleus. Cholesterol is crucial for membrane structure and function as well as being a precursor to the steroid hormones. Triacylglycerols and cholesterol are transported in the blood throughout the body as lipoprotein particles.

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In Chapter 27, we will examine how triacylglycerols are processed to yield fatty acids and how the fatty acids are degraded in a process called β oxidation that ultimately results in the synthesis of much ATP. In Chapter 28, we will see how fatty acids are synthesized and how fatty acid degradation and synthesis are coordinated. In Chapter 29, we will study lipid synthesis and transport, with a particular emphasis on cholesterol synthesis and its regulation.

✓ By the end of this section, you should be able to: