13 Digestion

13 Digestion - Acquiring energy substrates: diges2on...

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Unformatted text preview: Acquiring energy substrates: diges2on Animals cannot make their own nutrients from inorganic precursors, so all animals rely on inges2ng other organisms to provide energy substrates and raw materials for synthesizing their own macromolecules. We call the material that is ingested “food.” Food typically includes macromolecules that can’t be used by the animal “as is.” Instead the macromolecules must be broken into smaller components by the processes of diges2on. Diges2on can be broken into four stages. Diges2on takes place along the diges2ve tract, which is divided  ­ ­both anatomically and func2onally ­ ­ into dis2nct regions, and each region carries out specific phases of diges2on. As long as digested food is inside the diges2ve tract it is useless to the animal. The products of diges2on must be absorbed across the wall of the diges2ve tract and must enter the bloodstream to be used. Here you see an illustra2on of the diges2ve tract structure that is typical of three different kinds of animals. Although the structure varies among species, one feature that diges2ve tracts have in common is a large surface area, which facilitates absorp2on. This schema2c diagram of the human diges2ve tract shows: (1)  The major components of a mammalian diges2ve tract, and (2)  The major exocrine glands that play important roles in diges2on (3)  Each component and each gland contributes uniquely to the process of diges2on. This figure shows the real anatomy of the components illustrated in the previous slide. The chemical components of food are called “nutrients.” Nutrients can be categorized as “macronutrients” or “micronutrients,” depending on how much of them we need to ingest every day. The major macronutrients are: carbohydrates—sugars and polymers made up of sugars proteins—polymers made up of amino acids lipids (or fats)—made up of hydrophobic carbon ­based molecules such as faUy acids (and maybe nucleic acids) The diges2on of each macromolecule takes place in par2cular loca2ons along the diges2ve tract. Structure and func2on of some sub ­regions of the diges2ve tract (also called the gastrointes2nal tract or the gut) The wall of the stomach contains a thick layer of smooth muscle fibers, and the epithelium that lines the stomach includes many (exocrine) glands that secrete proteases or HCl. A sphincter is a ring of smooth muscle that surrounds a passage from one structure to another Smooth muscles have some features in common with skeletal muscles, but they also differ in major ways: Similari2es: (1)  force is generated when myosin in thick filaments binds cyclically to ac2n in thin filaments, (2)  ATP has the same role as in skeletal muscle fibers, and (3)  [Ca2+]cytoplasm plays a cri2cal role in regula2ng the binding of myosin to ac2n. Differences: (1)  Each fiber is very small and includes only one nucleus (2)  Fibers are innervated by autonomic neurons, not by spinal motor neurons, and in most smooth muscle 2ssues, both branches of the autonomic nervous system innervate each fiber. (3)  In some smooth muscle 2ssues, fibers are coupled to one another via gap junc2ons, so excita2on spreads from one muscle fiber to another, producing a wave of contrac2on that travels along the 2ssue. (4)  No sarcomeres, so they aren’t striped (that’s why they’re called smooth.) The wall of the small intes2ne includes layers of smooth muscle in which the fibers are coupled together via gap junc2ons. Peristal2c contrac2on of the muscles moves food along the diges2ve tract from the esophagus to the anus. The intes2nes are lined with epithelial 2ssue that has two levels of folding: villi and microvilli. The villi are folds of the en2re epithelial layer lining the tract. The microvilli are folds of the plasma membrane at the apical surface of each epithelial cell. The microvilli form a brush border. Both the villi and the microvilli greatly increase the surface area across which the products of diges2on can be absorbed. Overall view of diges2on in a mammal Ac2vity in neurons from both branches of the autonomic nervous system affects func2on of the diges2ve organs. In addi2on, a complex set of hormones modulates the func2on of these organs. We return to where we began, with ea2ng. In the last decade or two, we have learned that appe2te and ea2ng behavior are controlled by the brain, but ac2vity in the brain responds to several hormones that are produced in the diges2ve organs. The four hormones shown on this diagram (no2ce the source of each) are all only par2ally understood. This is a hot topic of research. No2ce that all of these hormones act through the hypothalamus. ...
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