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Overview of the Digestive System. Digestive System. Consider for a moment a Big Mac
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Digestive System • Consider for a moment a Big Mac • The purpose in your eating a Big Mac, other than simple hedonism, is to assimilate the nutrients it represents and make them available to build, repair and maintain your own tissues, as well as provide energy for studying and occasional other pursuits.
Digestive System • "Exactly what nutrients are present in a Big Mac that I can assimilate?" • A Big Mac is composed of: • "Two all beef patties (largely proteins and triglyceride, or neutral fat), special sauce (??), lettuce (cellulose, which you cannot assimilate to any great extent), cheese (again, protein and triglyceride), pickles (cellulose, starch), onions (cellulose, starch) on a sesame seed bun (starch)."
Digestive System • In terms of raw materials, a 200 gram Big Mac consists of roughly: • 25 grams of protein • 28 grams of fat • 47 grams of carbohydrate • Add these all up and you get a total of 530 Calories.
Digestive System • MacDonald's comes close to full disclosure in this regard, • but what they don't tell you is that in order to take advantage of these nutrients, you have to provide the means to carefully break them down into much smaller molecules that can be imported into blood. • Luckily, your digestive system takes care of this very complex process so efficiently that most of the time you don't even need to think about it.
Digestive System • At its simplest, the digestive system is a tube running from mouth to anus. This tube is like an assembly line, or more properly, a disassembly line.
Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination around your body.
The breakdown of foodstuffs like a Big Mac is accomplished through a combination of mechanical and enzymatic processes. • To accomplish this breakdown, the digestive tube requires considerable assistance from accessory digestive organs such as the salivary glands, liver and pancreas, which dump their secretions into the tube. • The name "accessory" should not be taken to mean dispensable; indeed, without pancreatic enzymes you would starve to death in short order.
Digestive System • In many ways, the digestive system can be thought of as a well-run factory in which a large number of complex tasks are performed. • The three fundamental processes that take place are:
Digestive System • Secretion: Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood • Absorption: Transport of water, ions and nutrients from the lumen, across the epithelium and into blood • Motility: Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents
Digestive System • Control of digestive function is achieved through a combination of electrical and hormonal messages which originate either within the digestive system's own nervous and endocrine systems, as well as from the central nervous system and from endocrine organs such as the adrenal gland.
Digestive System • Different parts of these systems are constantly talking to one another. • The basic messages are along the lines of "I just received an extraordinary load of food, so I suggest you get prepared" (stomach to large intestine) or • "For goodness sake, please slow down until I can catch up with what you've already given me" (small intestine to stomach).
Basic Functional Anatomy of the Digestive System • Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. In some species, saliva contains amylase, an enzyme that digests starch. • Esophagus: A simple conduit between the mouth and stomach - clearly important but only marginally interesting compared to other regions of the tube. • Stomach: Where the real action begins - enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form. • Liver: The center of metabolic activity in the body - its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats.
Basic Functional Anatomy of the Digestive System • Pancreas: Important roles as both an endocrine and exocrine organ - provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein. • Small Intestine: The most exciting place to be in the entire digestive system - this is where the final stages of chemical enzymatic digestion occur and where almost almost all nutrients are absorbed. • Large Intestine: Major differences among species in extent and importance - in all animals water is absorbed, bacterial fermentation takes place and feces are formed.
Review of Food Chemistry • The diet of any animal contains hundreds if not thousands of different molecules, • The bulk of the ingested nutrients are in the form of huge macromolecules that cannot be absorbed into blood without first being reduced to much simpler and smaller forms – • Even table sugar (sucrose) cannot be absorbed without first being enzymatically ripped apart into glucose and fructose. • The most important enzymatic reaction in digestion of foodstuffs is hydrolysis - the breaking of a chemical bond by the addition of a water molecule.
Proteins • Proteins are polymers of amino acids linked together by peptide bonds. • Very short proteins, typically 3 to 10 amino acids in length, are called peptides. • Although very small peptides can be absorbed to a limited degree, for all intents and purposes, proteins must be reduced to single amino acids before they can be absorbed. • Enzymes that hydrolyze peptide bonds and reduce proteins or peptides to amino acids are called proteasesor peptidases.
Lipids • Fatty acids are present in only small amounts in animal and plant tissues, but are the building blocks of many important complex lipids. • True fatty acids possess a long hydrocarbon chain terminating in a carboxyl group.
Lipids • The most abundant storage form of fat in animals and plants, and hence the most important dietary lipid, is neutral fat or triglyceride. • Triglycerides cannot be efficiently absorbed, and are enzymatically digested by pancreatic lipase
Carbohydrates • Monosaccharides or simple sugars are either hexoses (6-carbon) like glucose, galactose and fructose, or pentoses (5-carbon) like ribose. • These are the breakdown products of more complex carbohydrates and can be efficiently absorbed across the wall of the digestive tube and transported into blood. • Disaccharides are simply two monosaccharides linked together by a glycosidic bond. The disaccharides most important in nutrition and digestion are: • lactose or "milk sugar": glucose + galactose • sucrose or "table sugar": glucose + fructose • maltose: glucose + glucose
Carbohydrates • Polysaccharides are the most abundant dietary carbohydrate • Starch is a major plant storage form of glucose • Cellulose is the other major plant carbohydrate • Glycogen is the third large polymer of glucose and is the major animal storage carbohydrate.
