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    Carbohydrates Carbohydrates Presentation Transcript

    • Chapter 20: Carbohydrates K.Dunlap Chem 104
    • Carbohydrates • Composed of carbon, oxygen, hydrogen • two-third of the human diet is composed of carbohydrates • most carbohydrates are produced by the photosynthesis in green plants • the three elements that make up all carbohydrates are arranged as alcohols, aldehydes, or ketones •The three main classes of carbohydrates are: monosaccharides, disaccharides, polysaccharides
    • Carbohydrates • Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. – Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8. – Aldose: a monosaccharide containing an aldehyde group. – Ketose: a monosaccharide containing a ketone group.
    • 1) Monosaccharides • also known as simple sugars can not be broken down into smaller CHO • glucose, galactose, fructose, ribose, and deoxyribose • hexoses and pentoses
    • aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal forming a ring with an oxygen bridge between 2 carbon atoms. Rings with five carbons are called furanoses and rings with 6 carbons are call pyranoses
    • Smallest monosaccharides
    • Monosaccharides have chiral carbons
    • The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this –OH is on the right of the Fischer projection, this is a D sugar D-glucose
    • Glucose • the most biologically important monosaccharide • used directly by the body for energy • oxidized to carbon dioxide and water • Excess stored as adipose
    • Monosaccharides Galactose is in many plant gums and pectins • component of the disaccharide lactose Fructose is the sweetest of all the naturally occurring sugars • honey, fruits • component of the disaccharide sucrose
    • Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to the carbon 5: they are either on the opposite sides (α), or the same side (β) The α and β anomers of glucose.
    • Other monosaccarides -components of DNA and RNA
    • Disaccharides consist of 2 monosaccharide Sucrose • glucose and fructose • table sugar • 1/4 of total calories Lactose • glucose and galactose • milk Maltose • glucose and glucose • germinating grains
    • Disaccharides • Sucrose (table sugar) – Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets. – Glucose and fructose linked together with a -1,2glycosidic bond
    • Disaccharides • Lactose – Lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk. – Contains glucose and galactose linked together with a -1,4-glycosidic bond
    • Disaccharides • Maltose – Present in malt, the juice from sprouted barley and other cereal grains. – Maltose consists of two units of glucose joined by an -1,4-glycosidic bond.
    • Physical Properties • Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol – Sweetness relative to sucrose:
    • Polysaccharides - a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. Starch -2/3 of the human diet -Potatoes, rice, wheat, cereal grains -mixture of amylose and amylopectin Glycogen -only storage for glucose in the body -liver and muscle -similar in structure to amylopectin but, more branched
    • Cellulose - forms the structural component of the cell walls of plants -cotton, paper, linen, rayon -long unbranched chains of glucose (10010,000 glucose molecules) - linkages instead of present in starch linkages that are -many herbivores have the enzyme to breakdown linkages
    • Polysaccharides • Starch: a polymer of D-glucose. – Starch can be separated into amylose and amylopectin. – Amylose is composed of unbranched chains of up to 4000 glucose units joined by -1,4-glycosidic bonds. – Amylopectin contains chains up to 10,000 D-glucose units also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6glycosidic bonds.
    • Polysaccharides • Figure 20.3 Amylopectin.
    • Polysaccharides • Glycogen is the energy-reserve carbohydrate for animals. – Glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4and -1,6-glycosidic bonds. – The total amount of glycogen in the body of a wellnourished adult human is about 350 g, divided almost equally between liver and muscle.
    • Polysaccharides • Cellulose is a linear polysaccharide of Dglucose units joined by -1,4-glycosidic bonds. – It has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule. – Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds. – This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength. – It is also the reason why cellulose is insoluble in water.
    • Polysaccharides • Cellulose (cont’d) – Humans and other animals cannot use cellulose as food because our digestive systems do not contain glucosidases, enzymes that catalyze hydrolysis of glucosidic bonds. – Instead, we have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen. – Many bacteria and microorganisms have -glucosidases and can digest cellulose. – Termites have such bacteria in their intestines and can use wood as their principal food. – Ruminants (cud-chewing animals) and horses can also digest grasses and hay.
    • • Figure 20.4 Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by -1,4-glycosidic bonds.
    • Starch And Cellulose
    • 1. What are the three main classes of carbohydrates? 2. Cellulose is made from glucose, but humans can not digest cellulose. Explain. 3. How does your body store carbohydrates?
    • 4. Indicate whether each of the following is a monosaccharide, disaccharide, or polysaccharide. a) sucrose b) cellulose c) glucose d) lactose 5. Starch is a mixture of two types of polysaccharides. Name them. 6. Name the 2 monosaccharides in each of the following. a) sucrose b) maltose c) lactose