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GHC BIOL 2190 - Carbohydrates

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CHAPTER 5CarbohydratesI. Structures and FunctionA. Background1. Composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio often written as (CH2O)n where n=the number of times the ratio is repeated. For example, if n=6, then (CH2O)6 = C6H12O6.2. Provides main source of fuel for cells (at 4 kcal/gram)3. Divided into two main classesa. Simple sugars such as monosaccharides, disaccharides, and oligosaccharides.b. Complex sugars such as polysaccharides.B. MONOSACCHARIDES (glucose, fructose, and galactose are isomers of each other)1. Glucose is also called dextrose or blood sugar.a. Acts as the principle monosaccharide that acts as a buildingblock form any other carbohydrates.b. In the body it forms a six carbon (hexose) ring structure.2. Fructose is also called levulose.a. Composed of a six carbon ring structure.b. Commonly found in fruit, honey, and corn syrup and is used in soft drinks and frozen desserts.c. Accounts for 8 to 10% of our energy intake.d. After absorption by the small intestine, fructose is converted into glucose by the liver.e. If fructose consumption is high, it is also converted into glycogen, lactic acid, or fat.3. Galactosea. Has a six carbon ring structure.b. This is not usually found in nature but exists mostly as a unit of the disaccharide lactose which is found easily in nature.c. Easily converted into glucose in the liver or can be stored as glycogen.4. Ribose sugar a. Has a five carbon ring and is not an isomer of glucose.b. Used as part of the RNA strand, which is an important part of a cell’s genetic material?C. DISACCHARIDES1. Maltosea. Formed by condensation synthesis reaction between two glucose molecules.b. The glucose molecules form an alpha bond between them which is easily broken during digestion.c. Maltose is commonly used in the production of many alcohols by fermentation (recall anaerobic cellular respiration).2. Sucrosea. Formed by a glucose and a fructose attached by an alpha bond during condensation synthesis.b. Major sources are table sugar and in plants such as sugar cane and maple syrup.3. Lactosea. Formed by the addition of glucose and a galactose by beta bonds during condensation synthesis reaction.b. Beta bonds are more difficult to digest hence the large portion of the population that is lactose intolerant.c. Primary sugar found in milk and milk products.D. OLIGOSACCHARIDES1. Raffinose and stachyose are common examples2. These are found in beans3. Because they contain large numbers of beta bonds they are difficultto digest by digestive enzymes.4. Instead bacteria in the large intestine break apart these oligosaccharides, producing gas and other byproducts.E. POLYSACCHARIDES1. Starcha. Two types of plant starch=Amylose and amylopectin.b. Long chains of a 1000 or more glucose molecule.c. Amylose is a straight chain polymer while amylopectin is a branched polymer.d. Food sources include potatoes, breads, pasta, and rice.e. Amylopectin raises blood sugar levels quicker because of the branched configuration which enables amylase to have more points of attack.2. Glycogena. Storage form of glucose in the human body.b. Long branched chains of glucose that are more complex than amylopectin.c. Even more digestible because of its branched structure.3. Fibera. Dietary fibers are chemically composed of non-starch polysaccharides:i. Cellulose and hemicellulose which is found in wheat, rye, rice, and vegetables (insoluble).ii. Pectin, gums, and mucilage which is found in citrus fruits, oat products and beans (soluble).b. Dietary fibers are also composed of a non-carbohydrate material called lignin.c. All dietary fibers come from plants and are not digested in the stomach.d. The water soluble fibers however are metabolized by bacteria in the intestines.II. Carbohydrate Digestion and AbsorptionA. Begins in the mouth1. Salivary amylase breaks some starches into maltose during mastication.2. Continues digestion through the esophagus but stops in the stomach because the high acid environment inactivates salivary enzymes.B. Pancreatic amylases located in the small intestine further break down starches into maltose.C. Enzymes in the wall of the small intestine break down disaccharides intomonosaccharides:1. Maltose is acted on by maltase.2. Sucrose is acted on by sucrase.3. Lactose is acted on by lactase.D. Glucose and galactose are actively absorbed in the small intestine (see figure 5-6).E. Fructose is absorbed through facilitated diffusion which although requires a carrier does not require energy.F. Single sugars are then transported via the portal vein that goes directly tothe liver where the liver utilizes several options:1. Transforms the monosaccharides into glucose and releases it directly into the bloodstream for use.2. Produces glycogen for short term storage.3. Produces fat for long term storage.III. Functions of Glucose and other sugarsA. Yields Energy (recall 4 kcal/g)B. Spares protein from use as an energy source (i.e. no gluconeogenesis)C. Preventing Ketosis1. Adequate intake of carbohydrates is necessary for the complete metabolism of fats to carbon dioxide and water.2. If low carbohydrate intake leads to incomplete breakdown of fatty acids, ketone bodies form resulting in ketosis.3. The excess ketone bodies spill over into the urine and begin to draw sodium and potassium ions as well.4. This results in an ion imbalance and acidic blood that can lead to coma and death if not treated.D. Imparting flavor and sweetness to foods (in order of sweetness from high to low= fructose, sucrose, glucose, maltose, lactose).IV. Functions of Dietary FiberA. Dietary fiber1. Adds mass and water to fecesa. Feces then becomes larger and softer and easier to eliminateb. Less pressure is therefore needed to expel stool.2. Little or no fiber in the diet can cause constipation.a. More pressure is needed to defecate.b. May cause diverticula and hemorrhoids.i. Diverticulosis=no symptomsii. Diverticulitis=pouches fill with food stuffs and eventually become inflamed.3. Aids in weight control because it fills you up but yields little energy.4. Appear to be linked to reduction of colon cancer.5. Higher intakes of fiber are correlated with lowering cholesterol andbile acid absorption.V. Recommended Carbohydrate IntakesA. RDA is 130 grams of carbohydrate for adults with 20-35 grams of this being dietary fiber and no more than 10% of total kcal/day from sugars.B. Problems associated with high sugar intakes:1. May not contribute the right proportion of energy to the


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