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The carbohydrates Nature of carbohydrates: • Energy production system: - 3 steps to produce energy from a basic fuel supply: (1) In human body, the body digests its basic fuel, carbohydrates, changing it to glucose. (2) The body then absorb & through blood circulation, carries this refined fuel to cells that need glucose. (3) Glucose is burned in these cells, & energy produced through the process of metabolism. • The carbohydrates are a group of naturally occurring carbonyl compounds (aldehydes or ketones) that also contain several hydroxyl groups. • A carbohydrates is composed of carbon (C), hydrogen (H), and oxygen (O). • (CH2O)n when n = 5 then C5H10O5 • Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose, and other sugars are converted into glucose in the liver. Classes of carbohydrates: • It is taken up by the cells and either broken down to obtain energy (glycolysis) or converted into other metabolites. • Glucose is the major metabolic fuel of mammals and a universal fuel of the fetus. It is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage; ribose and deoxyribose in nucleic acids; and galactose in lactose of milk, in glycolipids, and in combination with protein in glycoproteins and proteoglycans. 1 • Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance. • CARBOHYDRATES ARE ALDEHYDE OR KETONE DERIVATIVES OF POLYHYDRIC ALCOHOLS 1- Monosaccharide: Are those carbohydrates that cannot be hydrolyzed into simpler carbohydrates: They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms; and as aldoses or ketoses depending upon whether they have an aldehyde or ketone group. Examples are listed in Table 1. – They are quickly absorbed from the intestine into the blood stream and carried to the liver. – Glucose, fructose, lactose. 2- Disaccharide: Are condensation products of two monosaccharide units. - Sucrose, Lactose, Maltose. 3- Oligosaccharides: Are condensation products of more than two to ten monosaccharides; maltotriose is an example 4- Polysaccharide: Are condensation products of more than ten monosaccharide units; examples are the starches and dextrins, which may be linear or branched polymers. - Starch, Glycogen, Dietary fiber. Monosaccharide • Glucose: - Is the form of sugar circulating in the blood and is the primary fuel to the cells. - The blood supply comes mainly from the digestion of starch, & sometimes called dextrose. - Is a moderately sweet sugar. 2 • Fructose: is found mainly in fruits, consider the sweetest of the simple sugars. • Galactose: comes mainly from the digestion of milk sugar, or lactose. Disaccharides • Sucrose: - Is common table sugar, its two sugar units are glucose & fructose. • Lactose: - Is the milk sugar, formed in mammary glands - Its two single sugar units are glucose & Galactose. - Not found in plants. - Aids in the absorption of calcium & phosphorus. • Maltose: - The two single sugar units that compose maltose are double glucose molecule, occurs as a breakdown product of the starches contained in malt (“malt sugar” and as an intermediate in the digestion of starch. Polysaccharides • Polysaccharides are sometimes classified as hexosans or pentosans, depending upon the identity of the constituent monosaccharides. • Starch and glycogen are storage polymers of glucose in plants and animals, respectively. • Starch is the major source of energy in the diet. • 1) Starch: - They are found in grains, legumes, and other vegetables, and some fruits. - Breakdown more slowly and supply energy over a longer period of time. - Most important dietary carbohydrate 3 - The major food sources of starch include grains in the form of pasta, crackers, bread, and other baked goods, legumes in the form of beans, & peas, potatoes, rice, corn, & bulgur. - Whole grains: is used for food products such as flours, breads, or cereals that are produced from unrefined grain, which is grain that still retains its outer bran layers and inner germ endosperm and their nutrients (i.e. dietary fiber, minerals, & vitamins) - Enriched grains: are refined grain products to which key nutrients, usually minerals (i.e. iron), and vitamins (i.e. A, C, D, thiamin, riboflavin, & niacin), have been added. • 2) Glycogen: found in animal muscle tissue, and liver • 3) Dietary fiber Dietary fiber • Dietary fiber: in humans there is a lack in the necessary enzymes to digest dietary fiber, used to treat gastrointestinal problems. - Cellulose: helps move the food mass along, stimulates normal muscle action in the intestine, the main sources are the stems and leaves of vegetables and the coverings of seeds and grains. - Non-cellulose polysaccharides: they absorb water and swell to a larger bulk, thus slowing the emptying of the food mass from the stomach, and preventing spastic colon. - Lignin: is a large compound that forms the woody part of certain plants, it also combines with bile acids and cholesterol in the human intestine, preventing their absorption Complex carbohydrates • Contain other sugar derivatives such as amino sugars, uronic acids, and sialic acids. 4 • They include proteoglycans and glycosaminoglycans, associated with structural elements of the tissues; and glycoproteins, proteins containing attached oligosaccharide chains. • They are found in many situations including the cell membrane. Other sweeteners • Used as sugar replacers • 2 types: 1) Nutritive sweeteners: - They are sweeteners such sugar alcohols that contribute to total calorie intake. - Sugar alcohols (sorbitol, mannitol, & xylitol) - 2-3 kcalories/gram. - Sorbitol: sucrose substitute. - They are absorbed slowly in the small intestine. - Use for diabetic patients - Excessive amount= diarrhea. - lowered risk than sugar to cause dental caries. 2) Nonnutritive sweeteners: - Alternative sweeteners. - Are sugar substitutes that do not have any caloric value. - Sweet taste without energy. - Commonly used are aspartame & saccharin. - Sweeter than table sugar. Functions of carbohydrates • 1) Primary energy function: (a) Basic fuel supply: - Primary fuel for the body. 5 - Fuel factor for carbohydrates is 4 kcal/gram. (b) Reserve of fuel supply: - Glycogen reserves fuel supply - Individual must eat carbohydrate foods regularly to meet energy demand (to maintain blood glucose level & prevent a breakdown of fat and protein in tissue). • 2) Special tissue functions: - Liver: glycogen reserves in the liver & muscles, protect cells from depressed metabolic function. - Protein & fat: (1) Carbohydrates protects protein to be used in tissue growth & maintenance (2) Prevent rapid breakdown of fat & produce ketones (strong acid), this action called antiketogenic effect. - Heart: fatty acids are the regular fuel for energy, in emergency situation glycogen. - Central nervous system: the brain need continues supply of glucose, otherwise low sugar cause brain damage. Table 1. Classification of important sugars Aldoses Ketoses Trioses (C3H6O3) Glycerose Dihydroxyacetone Tetroses (C4H8O4) Erythrose Erythrulose Pentoses (C5H10O5) Ribose Ribulose Hexoses (C6H12O6) Glucose Fructose 6