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5. Carbohydrates.doc

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Animal Science
ANSC 3120

Carbohydrates Classification and Structure  CH O 2 6 CO + 62H O  C 2 O + 6 O 6 12 6 2 o Photosynthesis o C H6O12s6the start of most life on earth  Average North American consumes ~ 50% of energy from CHO  Classification o All glucose most common  Starch  Cellulose  Glycogen o ...Sugars  Monosaccharides • Can’t be further ‘hydrolyzed’  Disaccharides • 2 monosaccharides bound covalently o ...Starches & Fibres  Polysaccharides • Up to hundreds or thousands of sugar units  Monosaccharides o Triose: 3 C – cells, metabolites of glucose o Tetrose: 4 C – cells, metabolites of glucose o Pentose: 5 C – Cells, component of DNA and RNA o Hexose: 6 C – nutritionally most important Reducing sugar Carbohydrates 1  Glucose Enantiomers Start numbering at aldehyde/ketone end right left Not in nature o Enantiomers  One class of stereoisomers • Same molecular formula and sequence • Differ in 3D orientation • Only occurs at ‘chiral carbons’ which have 4 different side chains  “Mirror images” o Optical activity  Bend light (polarized) positive or negative Carbohydrates 2  Open chain o Fisher Projection o Not ‘normal’ in solution Carbohydrates 3 Carbohydrates 4  Disaccharides o Maltose  Glucose + glucose  Digestion product of starch o Lactose  Glucose + galactose  Milk sugar  Lactose in tolerance o Sucrose  Glucose + fructose  Major disaccharide in diet  Sugar cane  Sugar beets  Fruits  Expensive to extract; ~ 50% fructose and ~ 50% glucose will have a similar sweetness and it is cheaper to produce than sucrose o High Fructose Corn Syrup Corn starch amylase free glucose isomerise ~50% fructose ~50 glucose Pop/processed foods Carbohydrates 5  Polysaccharides o 3 – 6 sugars are also known as oligosaccharides o Greater than 6 sugars are polysaccharides o Starch Combine to form  Amylose: α-1, 4 linkage (unbranched) optimally condensed  Amylopectin: α-1, 4 & α-1, 6 (branched) granules o Dextrins (eg α-1, 6 disaccharides)  Intermediate product of starch digestion o Glycogen  Α-1, 4 and α-1, 6 granule  Highly branched  more than amylopectin s Carbohydrates 6 Dietary Fibre  Non-digestible complex CHO  Structural part of plants Insoluble Soluble Lignin Gums Cellulose Some Hemicellulose Pectins Mucilages Remains intact through the intestinal Forms gel (do dissolve in water) Tract. Increases bulk in intestine slows down absorption of food  Fibre o Cellulose  Linear polymer: β-1, 4 linked glucose *  Insoluble, low fermentation  better in ruminants  Wheat bran  Hay o Pectin  Backbone of α-1, 4 linked galacturonic acid *  Soluble, highly fermentable • Gel forming  jams, jellies  Fruits  Beets,  Potatoes with extensive grinding o Hemicellulose  Mixed polymers of β-linked pentoses & hexoses *  Variable solubility and fermentability o Lignin (non-carbohydrate) Carbohydrates 7  Polymer of phenylpropane derivatives *  Insoluble, unfermentable  Plant structured strength  Not chemically a CHO  Used as internal (endogenous) digestibility marker  Hemicellulose o Highly variable backbone and side chains: β-linked o Solubility varies o Fermentability varies  Lignin o Polymer of phenylpropane derivatives * o ∴ not carbohydrate o Insoluble Carbohydrates 8 o Unfermentable  Little bit in ruminants  small error in lignin marker studies o Binds hydrophobic molecules  Health Benefits of Dietary Fibre o Maintains function and health of the gut o Decrease constipation and diarrhoea Insoluble fibre  increase fecal bulk o Decrease in diverticulitis o Increase satiety o Weight control?  Health Benefits of Soluble Fibre o Decrease cholesterol and cardiovascular risk o Also increases fecal water holding o Better consistency o Mechanism?  Binds bile acids, excreted • Also PCBs dioxins • Formed from cholesterol in liver (stored and reused)  High fibre = low saturated fat?  Soluble fibre interrupts “enterohepatic circulation” o Effects on blood glucose?  May decrease blood glucose response  Decrease the rate of starch/ sugar digestion Carbohydrate Digestion  Mouth o α-amylase starts starch breakdown  sensing of available of CHO in mouth o Stomach  α-amylase inactivated o Small Intestine  Intestinal Lumen  Pancreatic amylase Carbohydrates 9 Facilitated diffusion – no direct energy cost More active transport - Sodium gradient Active transport – ATP used to maintain Na gradient  Carbohydrates – Function o Glucose – primary energy source for most cells o Essential for erythrocytes (no mitochondria!), central nervous system, brain (11%) o Skeletal muscle – exercise o Protein-sparing nutrient  Prevents breakdown of protein to make glucose (gluconeogenesis)  What about fat? • Can’t be used to make glucose o Except for glycerol backbone o Prevents Ketosis  Prevents ketone buildup from fat breakdown Carbohydrates 10 • Eg Atkins Diet – low CHO  Ketosis, acidosis Carbohydrate Metabolism  Key Points o Know the ‘purpose’ of each pathway, “key” steps and enzymes highlighted in class o Details about each step, enzymes and structure are important but you have been tesed on
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