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B3 CARBOHYDRATES B.3.4 List the major functions of carbohydrates in the human body. Major functions of carbohydrates in the human body include: Immediate energy source (glucose) Short term energy reserves (glycogen) Precursors for other biologically important molecules B.3.1 Describe the structural features of monosaccharides. *Structural features of Monosaccharides: - Emperical Formula = CH2O Contain a carbonyl group (C=O) Contain at least two hydroxyl groups (–OH) Between 3 and 6 carbons [e.g. C5H10O5 pentoses ; C6H12O6 hexoses] Many structural/optical isomers possible Ring and open chain structures possible B.3.2 Draw the straight-chain and ring structural formulas of glucose and fructose. [Students should be made aware of the structural difference between alpha and beta isomers.] Alpha-fructose Beta-fructose B.3.3 Describe the condensation of monosaccharides to form disaccharides and polysaccharides. Examples include: - disaccharides lactose, maltose and sucrose - polysaccharides starch (alpha-glucose), glycogen (alpha-glucose) and cellulose (beta-glucose). B.3.5 Compare the structural properties of starch and cellulose, and explain why humans can digest starch but not cellulose.] [Both are polymers of glucose units. Starch has two forms: amylase, which is a straight-chain polymer (alpha-1,4 linkage), and amylopectin, which is a branched structure with both alpha 1-4, and alpha 1,6 linkages. Cellulose has a beta 1,4 linkage; this can be hydrolysed by the enzyme cellulose, which is absent in most animals, including mammals.] Polysaccharide Role amylose (in starch) Energy reserve in plants α-glucose amylopectin (in starch) glycogen cellulose Monomer Energy reserve in animals Structural material in plants β-glucose Structural Features Properties Straight chains with α1-4 linkages Branched chains with α1-4 linkages and a few α1-6 linkages Straight chains with β1-4 linkages Chains coil to give a globular structure Slightly soluble and readily hydrolyzed by enzymes in humans Chains pack side by side to give fibres Insoluble and cannot be hydrolyzed by enzymes in humans Role in Human Nutrition Source of glucose for energy Dietary fibre B.3.6 State what is meant by the term dietary fibre. [Dietary fibre is mainly plant material that is not hydrolysed by enzymes secreted by the human digestive tract but may be digested by microflora in the gut. Examples include cellulose, hemicellulose, lignin and pectin.] B.3.7 Describe the importance of a diet high in dietary fibre. [Aim 8: Dietary fibre may be helpful in the prevention of conditions such as diverticulosis, obesity, Crohn’s disease, hemorrhoids and diabetes mellitus.] Dietary Fibre mainly plant material that is part of fruits, grains and vegetables that the human body cannot digest due to the absence of the needed enzymes; however, may be digested by bacteria in the gut Examples: - Cellulose plant cell walls are composed of multiple layers of cellulose; about 33% of all plant material; the most common organic compound on earth; a polysaccharide - Hemicelluloses another component of cell walls; a polysaccharide - Lignin commonly derived from wood; provides mechanical support for stems and leaves; supplies the strength and rigidity of cell walls; a biopolymer - Pectin major binding component of the cell walls of plants and fruits; a polysaccharide Lignin Importance of diet high in dietary fibre: Type of Dietary Fibre A. Water Insoluble B. Water Soluble Examples - Cellulose - Lignin - Pectins Sources - whole grain foods - fruits & vegetable - whole grain foods - fruits & vegetable Function - absorb water to provide bulk; move food through the digestive system - undergo fermentation in large intestine by bacteria to produce short-chain fatty acids (e.g. propanoic/butanoic acid) - these fatty acids stabilize lipid and blood glucose levels; may therefore help prevent diabetes - they also stimulate the production of antibodies and therefore provide immune protection Dietary Fibre may also help prevent: 1. Irritable Bowel (Intestine)Syndrome(IBS) includes constipation, bloating, abdominal pain etc...all arise from malfunctioning bowels (↑ dietary fibre ↓ these symptoms) 2. Obesity excess body mass leading to many, many health issues (↑ fibre = feeling full on a diet with ↓ carbs and fats = ↓ weight) 3. Crohn’s Disease inflammatory bowel disease; cause unknown (↑ dietary fibre may help in its prevention) 4. Hemorroids enlarged, weak, swollen blood vessels in and around the anus and rectum that can burst; can be caused by pressure in the abs as a result of constipation [↑ dietary fibre makes the bulk move through the large intestine more easily (i.e. prevents constipation)] 5. Constipation & Diverticulosis ↓ fibre = hard stool that does not pass easily = constipation ↑ pressure required to move stool causes diverticulosis; that is, bulges in the colon leading to pain