The emulsification of fat aids digestion because the smaller and more numerous emulsification droplets present a greater surface area than the unemulsified fat droplets that originally entered the duodenum. Fat digestion occurs at the surface of the droplets through the enzymatic action of pancreatic lipase, which is aided in its action by a protein called colipase (also secreted by the pancreas) that coats the emulsification droplets and "anchors" the lipase enzyme to them. Through hydrolysis, lipase removes two of the three fatty acids from each triglyceride molecule and thus liberates free fatty acids and monoglycerides (fig. 18.34). Phospholipase A likewise digests phospholipids such as lecithin into fatty acids and lysolecithin (the remainder of the lecithin molecule after the removal of two fatty acids).
Free fatty acids, monoglycerides, and lysolecithin, which are more polar than the undigested lipids, quickly become associated with micelles of bile salts, lecithin, and cholesterol to form "mixed micelles" (fig. 18.35). These micelles then move to the brush border of the intestinal epithelium where absorption occurs.
■ Figure 18.34 The digestion of triglycerides. Pancreatic lipase digests fat (triglycerides) by cleaving off the first and third fatty acids. This produces free fatty acids and monoglycerides. Sawtooth lines indicate hydrocarbon chains in the fatty acids.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.