MCAT Biochemistry Review
Chapter 11: Lipid and Amino Acid Metabolism
For weeks before the winter season begins, bears and certain mammals increase their food intake to prepare for hibernation. During this time, they increase their weight by storing energy. Different organisms store fuel and supplies in different ways. Hamsters store extra food in pouches in their cheeks. Cacti absorb and conserve water in preparation for dry seasons. But hibernating animals store extra calories as fat. Over the course of the winter, fat stores are mobilized and metabolized for basic bodily functions, which are minimal during hibernation. Come spring and summer, these reserves will be replenished in preparation for the next winter season. Humans also store extra energy as fat. While we may not hibernate through the winter, fat stores allow us to store energy to use during prolonged periods without food.
As discussed in Chapter 8 of MCAT Biochemistry Review, lipids play a major role in maintaining the structure and function of cells; however, they also have important roles as storage molecules for energy and in biological signaling. In this chapter, we'll examine the metabolism of lipids, starting with ingestion of food particles and continuing through absorption, transport, and energy catabolism. We will also cover energy storage via lipid synthesis, as well as the metabolism of cholesterol and ketone bodies. In addition, we will learn about how protein degradation feeds into lipid and carbohydrate pathways, and the urea cycle.
11.1 Lipid Digestion and Absorption
In addition to being a major source of energy in the body, lipids serve a variety of other functions in the body. For instance, some fat-soluble vitamins play roles as coenzymes; prostaglandins and steroid hormones are necessary in the control and maintenance of homeostasis. Aberrant lipid metabolism may also be associated with clinical manifestations such as atherosclerosis and obesity.
Dietary fat consists mainly of triacylglycerols, with the remainder comprised of cholesterol, cholesteryl esters, phospholipids, and free fatty acids. Lipid digestion is minimal in the mouth and stomach; lipids are transported to the small intestine essentially intact. Upon entry into the duodenum, emulsification occurs, which is the mixing of two normally immiscible liquids (in this case, fat and water). Formation of an emulsion increases the surface area of the lipid, which permits greater enzymatic interaction and processing. Emulsification is aided by bile, which contains bile salts, pigments, and cholesterol; bile is secreted by the liver and stored in the gallbladder. Finally, the pancreas secretes pancreatic lipase, colipase, and cholesterol esterase into the small intestine; together, these enzymes hydrolyze the lipid components to 2-monoacylglycerol, free fatty acids, and cholesterol. Figure 11.1 summarizes the digestion and absorption of dietary lipid components.
Figure 11.1. Absorption of Lipids
Emulsification is followed by absorption of fats by intestinal cells. Free fatty acids, cholesterol, 2-monoacylglycerol, and bile salts contribute to the formation of micelles, which are clusters of amphipathic lipids that are soluble in the aqueous environment of the intestinal lumen. Essentially, micelles are water-soluble spheres with a lipid-soluble interior. Micelles are vital in digestion, transport, and absorption of lipid-soluble substances starting from the duodenum all the way to the end of the ileum. At the end of the ileum, bile salts are actively reabsorbed and recycled; any fat that remains in the intestine will pass into the colon, and ultimately ends up in the stool.
Micelles diffuse to the brush border of the intestinal mucosal cells where they are absorbed. The digested lipids pass through the brush border, where they are absorbed into the mucosa and re-esterified to form triacylglycerols and cholesteryl esters and packaged, along with certain apoproteins, fat-soluble vitamins, and other lipids, into chylomicrons. Chylomicrons leave the intestine via lacteals, the vessels of the lymphatic system, and re-enter the bloodstream via the thoracic duct, a long lymphatic vessel that empties into the left subclavian vein at the base of the neck. The more water-soluble short-chain fatty acids can be absorbed by simple diffusion directly into the bloodstream.
Absorption in the small intestine and colon follows a characteristic pattern. This is a good time to review digestion, discussed in Chapter 9 of MCAT Biology Review, to create a complete schema for the absorption and metabolism all of the macronutrients.
MCAT Concept Check 11.1:
Before you move on, assess your understanding of the material with these questions.
1. When lipids leave the stomach, what stages of digestion have been accomplished? What enzymes are added to accomplish the next phase?
2. True or False: All lipids enter the circulation through the lymphatic system.
3. Describe the structure of a micelle.