1Structure and function of Carbohydrates.pptx

1Structure and function of Carbohydrates.pptx

1. Structure and function of Carbohydrates

2. Carbohydrates- introduction Carbohydrates have the general molecular formula CH 2 O Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers (hence " polysaccharides "); that is, each is built from repeating units, monomers , much as a chain is built from its links. Carbohydrates are four types : Monosaccharides, disaccharides, oligosaccharides and polysaccharides

3. Monosaccharides-types and sources(1/4): Glucose Glucose (derived from the Greek word for " sweet ") is the primary form of sugar stored in the human body for energy. Probably the most common source of glucose is table sugar (sucrose). Glucose is also obtained from starch , the major storage form of carbohydrate in plants.

4. Monosaccharide types and sources (2/4): fructose and galactose Sources of fructose (fruit sugar) include fruit, honey and high-fructose corn syrup. Galactose is got from lactose in milk. After being absorbed by the body, fructose and galactose are converted into glucose by the liver and then used as fuel for the body In humans glucose is stored as glycogen

5. Monosaccharides -types and sources (3/4): Pentoses Pentoses: Sugar molecules that contain five carbon atoms are termed pentoses. Pentoses occur in food in very small amounts, although they are an essential part of the genetic material of every cell. The monosaccharide ribose is a part of RNA, or ribonucleic acid. Deoxyribose , another five-carbon monosaccharide, is part of DNA, or deoxyribonucleic acid.

6. Monosaccharides –types: (4/4) sugar alcohols Sugar alcohols: When a hydroxyl group, or --OH, replaces a hydrogen atom in the sugar molecule, it becomes a sugar alcohol or polyol. Our body does not completely absorb sugar alcohols, and as such, they do not produce much of a rise in blood sugar levels . These monosaccharides also have fewer calories per gram than other sugars. Sugar alcohols include erythritol, isomalt, lactitol, mannitol, sorbitol and xylitol . Xylitol has a sweetness level equal to table sugar. Sugar alcohols do occur in very small quantities in some fruits. However, these monosaccharides are manufactured in large quantities synthetically . Sugar alcohols are useful as low-calorie sweeteners in foods such as chewing gum , sugar-free candies , hard candies , chocolate and toothpaste .

7. Monosaccharide-structure (1/4) With few exceptions ( e.g. , deoxyribose ), monosaccharides have the chemical formula (CH 2 O)n , where n can be 3,4,5,6,7, or 8 and have two or more hydroxyl groups . Monosaccharides can be classified by the number n of carbon atoms they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on. The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses mannoheptulose and sedoheptulose . Monosaccharides with eight or more carbons are rarely observed as they are quite unstable.

8. Monosaccharide-structure (2/4) We can have open linear shape or ring shaped monosaccharide Functional group unique to linear shaped monosaccharide are carbonyl and hydroxyl groups

9. Monosaccharides structure-aldoses (3/4)

10. Monosaccharides structure-ketoses (4/4) Ketoses of biological interest usually have the carbonyl at position 2.

11. In a simple open-chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and (in ketoses) the carbon with the keto group. Monosaccharides with four or more carbons may contain multiple chiral carbons , so they typically have more than two stereoisomers. The number of distinct stereoisomers with the same diagram is bounded by 2 n , where n is the number of chiral carbons . Stereoisomerism in monosaccharides

12. Sample problem Deduce the number of stereoisomers from the following compounds, if any: Dihydroxyacetone (n = 0) Ribulose (n = 2) Fructose (n= 3)

13. Ring or cyclic monosaccharides For many monosaccharides (including glucose), the cyclic forms predominate , in the solid state and in solutions A monosaccharide often switches from the acyclic (open-chain) form to a cyclic form , through a nucleophilic addition reaction between the carbonyl group and one of the hydroxyls of the same molecule . The reaction creates a ring of carbon atoms closed by one bridging oxygen atom .

14. Ring formation

15. Structural Isomerism in ring form of the monoaccharides

16. Stereoisomerism in cyclic monosaccharides alpha and beta links Anomeric carbon

17. Disaccharides-Introduction A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water , from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water. One of the more known examples is sucrose , the other lactose . ' Disaccharide ' is one of the four chemical groupings of carbohydrates ( monosaccharide , disaccharide , oligosaccharide , and polysaccharide ).

18. Disaccharides-formation/hydrolysis

19. Disaccharides-classification There are two different types of disaccharides: reducing disaccharides , in which one monosaccharide, the reducing sugar, still has a free hemiacetal unit; and non-reducing disaccharides , in which the components bond through an acetal linkage between their anomeric centers and neither monosaccharide has a free hemiacetal unit. Cellobiose (2 glucose molecules) and maltose (2 glucose molecules) are examples of reducing disaccharides. Sucrose (glucose + fructose) and trehalose (2 glucose molecules) are examples of non-reducing disaccharides.

20. Disaccharides-properties The glycosidic bond can be formed between the OH of an anomeric carbon any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose ), different bond combinations ( regiochemistry ) and stereochemistry ( alpha- or beta- ) result in disaccharides that are diastereoisomers with different chemical and physical properties. Depending on the monosaccharide constituents , disaccharides are sometimes crystalline , sometimes water-soluble , and sometimes sweet-tasting and sticky-feeling .

21. Types of glycosidic bonds

22. Sample problem Deduce the number of stereoisomers from the following compounds, if any: Dihydroxyacetone (n=0) Ribulose (n=2) Fructose (n=3)