Presentation on theme: "Carbohydrates Introduction"— Presentation transcript:
1 Carbohydrates Introduction Carbohydrates, called sugars and starches, arepolyhydroxy aldehydes or ketones, or compoundsthat can be hydrolyzed to them.Carbohydrates can be classified as mono-saccharides, disaccharides, and polysaccharides.They are synthesized in green plants throughphotosynthesis, a process that uses the energyfrom the sun to convert carbon dioxide and waterinto glucose and oxygen.In the body, they are used for bursts of energyneeded during exercise in the form of glucose.
3 Monosaccharides Monosaccharides, the simplest carbohydrates, generally have 3 to 6 C atoms in a chain with acarbonyl group and many –OH groups.Monosaccharides with a carbonyl group at C1 arecalled aldoses and those with carbonyl group at C2are called ketoses.
4 Monosaccharides The simplest aldose is glyceraldehyde. The simplest ketose is dihydroxyacetone.They are constitutional isomers of each other,sharing the formula C3H6O3.
5 Monosaccharides A monosaccharide is characterized by the number of C atoms in its chain:A triose has 3 Cs.A tetrose has 4 Cs.A pentose has 5 Cs.A hexose has 6 Cs.These terms are then combined with the words aldoseand ketose:Glyceraldehyde is an aldotriose.Dihydroxyacetone is a ketotriose.
6 Monosaccharides Monosaccharides are sweet tasting, but their relative sweetness varies a great deal.They are polar compounds with high meltingpoints.The presence of so many polar functional groupscapable of hydrogen bonding makes themonosaccharides very water-soluble.
7 Monosaccharides Fischer Projection Formula All carbohydrates except dihydoxyacetone haveone or more chirality centers.Glyceraldehyde, the simplest aldose, has onechirality center, and has two possible enantiomers.
8 Monosaccharides Fischer Projection Formula The prefix D (Dextro) is used when the –OH group isdrawn on the right side of the chiral carbon.The prefix L (Levo) is used when the –OH group isdrawn on the left side of the chiral carbon.The wedged and dashed lines can be re-drawn ina Fischer projection formula:
9 Monosaccharides With More than One Chirality Center Glucose (dextrose) has four chirality centers andis drawn as:
10 Monosaccharides With More than One Chirality Center The configuration of the chirality center farthestfrom the carbonyl group determines whethera monosaccharide is D or L.
11 Monosaccharides With More than One Chirality Center All naturally occurring sugars are D sugars.
12 Monosaccharides Common Monosaccharides Glucose (dextrose) is blood sugar and the mostabundant monosaccharide.Normal blood glucose levels are mg/dL.Insulin regulates blood glucose levels bystimulating the uptake of glucose into tissues orits conversion to glycogen.Patients with diabetes produce insufficient insulinto adequately regulate blood sugar levels, so theymust monitor their diet and/or inject insulin daily.
14 Monosaccharides Common Monosaccharides Galactose is one of the components of thedisaccharide lactose.Patients with galactosemia lack an enzyme neededto metabolize galactose, which accumulates andcauses cataracts and cirrhosis.Fructose is one of the components of thedisaccharide sucrose.It is a ketohexose found in honey and almosttwice as sweet as table sugar with the samenumber of calories per gram.
15 The Cyclic Forms of Monosaccharides Haworth projections The hydroxyl and carbonyl groups can reacttogether to form a ring.
16 The Cyclic Forms of Monosaccharides The first step in cyclization is to rotate glucose 90o.
17 The Cyclic Forms of Monosaccharides Next, the chain must be twisted around, forminga six-membered ring:
18 The Cyclic Forms of Monosaccharides Cyclization yields two isomers, since the OH canbe located above or below the ring, an a anomerand a b anomer.These rings are called Haworth projections.
19 Reactions of Monosaccharides Reduction of the Aldehyde Carbonyl Group The carbonyl group of an aldose is reduced toan alcohol using H2 with Pd.
20 Reactions of Monosaccharides Oxidation of the Aldehyde Carbonyl Group The aldehyde group is easily oxidized to acarboxylic acid using Benedict’s reagent.
21 Disaccharides Disaccharides are carbohydrates composed of two monosaccharides.A disaccharide is formed when a hydroxyl groupof one monosaccharide reacts with a hydroxylgroup of a second monosaccharide
22 Disaccharides When this reaction occurs between two monosaccharides, the bond that joins the tworings is called a glycosidic linkage.
23 Disaccharides The glycosidic linkage joining the two rings can be alpha (a) or beta (b).If the bond is alpha (a):
24 Disaccharides If the bond is beta (b): Hydrolysis cleaves the C—O glycosidic linkageand forms two monosaccharides.
25 DisaccharidesHydrolysis of maltose yields 2 glucose molecules.
26 Focus on Health & Medicine Lactose Intolerance Lactose is the disaccharide in milk; it consists of1 galactose ring and 1 glucose ring joined by a14-b-glycosidic bond.
27 Focus on Health & Medicine Lactose Intolerance The disaccharide bond is cleaved by the enzymelactase in the body.Individuals who are lactose intolerant no longerproduce this enzyme.Without the enzyme, lactose cannot be digested,causing abdominal cramps and diarrhea.
28 Focus on Health & Medicine Sucrose and Artificial Sweeteners Sucrose (table sugar) is a disaccharide consistingof 1 glucose ring and 1 fructose ring.Sucrose is very sweet, butcontains many calories.To reduce caloric intake,many artificial sweetenershave been developed.
29 Polysaccharides Cellulose Polysaccharides contain three or more mono-saccharides joined together.Cellulose is an unbranched polymer made up ofrepeating glucose units joined by14-b-glycosidic linkages.Cellulose is found in the cell walls of all plants,where it gives support and rigidity to wood,plant stems, and grass.Humans do not posses the enzyme to hydrolyzecellulose (b-glycosidase) and cannot digest it.
30 Polysaccharides Cellulose Cellulose makes up the insoluble fiber in ourdiets, which is important in adding bulk to wasteto help eliminate it more easily.
31 Polysaccharides Starch Starch is a polymer made up of repeating glucoseunits joined by alpha glycosodic linkages.Starch is present in corn, rice, wheat, and potatoes.The first main type of starch is amylose:
32 Polysaccharides Starch The second type of starch is amylopectin:
33 Polysaccharides Starch Amylose is an unbranched polymer linkedby 14-a-glycosidic linkages.Amylopectin is a branched polymer linked by14-a and 16-a-glycosidic linkages.Both starch molecules can be digested by humansusing the enzyme amylase.
34 Polysaccharides Glycogen Glycogen is the major form of polysaccharidestorage in animals, similar in structure toamylopectin.It is stored mainly in the liver and in muscle cells.When glucose is needed for energy, glucoseunits are hydrolyzed from the ends of the glycogenpolymer.Because glycogen is highly branched, thereare many ends available for hydrolysis.