Carbohydrates, introduction, types and importance
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Carbohydrates are the most abundant biomolecules on Earth and serve important functions in living organisms. They include monosaccharides like glucose and fructose, oligosaccharides like sucrose and lactose, and polysaccharides like starch, cellulose, and glycogen. Monosaccharides are either aldoses or ketoses and commonly exist as cyclic structures with α and β anomers. Glycosidic bonds link monosaccharides into oligosaccharides and polysaccharides, which serve structural roles like cellulose and chitin or storage roles like starch and glycogen. Carbohydrates play key roles through their diverse structures and functions.
Carbohydrates, introduction, types and importance
- 1. Importance of Carbohydrates Carbohydrate is 1. the most abundant biomolecules on Earth 2. produced by the conversion of more than 100 billion metric tons of CO2 and H2O in photosynthesis 3. a major dietary staple and its oxidation is central in energy- yielding pathway in living cell 4. structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals. 5. lubricate skeletal joints and participate in recognition and adhesion between cells as glycoconjugates.
- 2. Definition • Carbohydrates are polyhydroxy aldehydes or ketones,having the empirical formula (CH2O)n There are three major size classes of carbohydrates: 1. Monosaccharides, 2. Oligosaccharides, and 3. Polysaccharides Monosaccharides (simple sugars) consist of a single polyhydroxy aldehyde or ketone unit. • The most abundant monosaccharide in nature is the six- carbon sugar D-glucose. • Monosaccharides of more than four carbons tend to have cyclic structures.
- 3. • Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. • The most abundant are the disaccharides, with two monosaccharide units is sucrose (cane sugar), combination of the six-carbon sugars D-glucose and D-fructose. • All common monosaccharides and disaccharides have names ending with the suffix “-ose.” • Polysaccharides are sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units e.g., cellulose, glycogen, starch.
- 4. Monosaccharides Are Aldoses and Ketoses • Monosaccharides are colorless, crystalline solids, • Freely soluble in water but insoluble in nonpolar solvents, • Most have a sweet taste. • carbon atoms at backbone are linked by single bonds. • In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group.
- 5. • If the carbonyl group is at an end of the carbon chain (that is, in an aldehyde group) the monosaccharide is an aldose; • if the carbonyl group is at any other position (in a ketone group) the monosaccharide is a ketose.
- 6. • Monosaccharides with four, five, six, and seven carbon atoms in their backbones are called, respectively tetroses, pentoses, hexoses, and heptoses. • D-glucose is an aldohexose and D-fructose is an the ketohexose.
- 7. Monosaccharides Have Asymmetric Centers • Assymetric or chiral carbon atom provides optically active isomeric forms of sugar called enantiomers expect dihydroxyacetone. • The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers. • By convention, one of these two forms is designated the D-isomer, the other the L-isomer.
- 8. Stereoisomeric models • Three ways to represent the two stereoisomers of glyceraldehyde. • The stereoisomers are mirror images of each other. • Ball and-stick models show the actual configuration of molecules.
- 9. Stereoisomeric models • In Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader; vertical bonds project behind the plane of the paper, away from the reader. • In perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away.
- 10. • When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D- isomer; when on the left, it is the L-isomer. • Two sugars that differ only in the configuration around one carbon atom are called epimers; • D-glucose and D-mannose, which differ only in the stereochemistry at C-2, are epimers, as are D-glucose and D-galactose (which differ at C-4)
- 11. Common Monosaccharides Have Cyclic Structures • In aqueous solutions, sugars predominates as cyclic (ring) form. • Carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group producing hemiacetal or hemiketal, each with two stereoisomers designated α and β • 6-carbon sugars in its ring form are called pyranoses
- 12. • Rings of 5-carbon sugars are called furanoses. • Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. • The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. • The α and β anomers of D-glucose interconvert in aqueous solution by a process called mutarotation.
- 13. • Haworth perspective formulas are commonly used to show the stereochemistry of ring forms of monosaccharides • It suggests 6-carbon pyranoses tends to assume either of two “chair” conformations • these conformations of a molecule are interconvertible only by breakaging covalent bonds.
- 14. Organisms Contain a Variety of Hexose Derivatives
- 15. Monosaccharides Are Reducing Agents • Oxidation of the anomeric carbon of glucose and other sugars makes them reducing sugar. • This is the basis for Fehling’s reaction. • The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. • The carbonyl carbon is oxidized to a carboxyl group. • For many years, this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of diabetes.
- 16. Oligosaccharides • It consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. • The most abundant are the disaccharides, with two monosaccharide units is sucrose (cane sugar), combination of the six-carbon sugars D-glucose and D-fructose.
- 17. Glycosidic Bond • It is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other. • This reaction represents the formation of an acetal from a hemiacetal (such as glucopyranose) and an alcohol (a hydroxyl group of the second sugar molecule).
- 18. Reducing Vs. Non-reducing • The oxidation of a sugar’s anomeric carbon by cupric or ferric ion occurs only with the linear form, which exists in equilibrium with the cyclic form(s). • The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end. Maltose is a reducing suger. • When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar.
- 19. Common oligosaccharides • Lactose is a disaccharide of D-glucose and D-galactose, occur naturally in milk. Anomeric carbon of glucose is available for oxidation making lactose, a reducing sugar. • Sucrose (table sugar) is a disaccharide of glucose and fructose. It is formed by plants. sucrose contains no free anomeric carbon atom; the anomeric carbons of both monosaccharide units are involved in the glycosidic bond. Sucrose is therefore a nonreducing sugar. • It is the principal form in which sugar is transported from the leaves to other parts of the plant body. • Trehalose- a disaccharide of D-glucose, is a nonreducing sugar, and it is a major constituent of the circulating fluid (hemolymph) of insects, serving as an energy-storage compound.
- 20. Polysaccharides • These are sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. • They are also called glycan. • They may be Homopolysacchrides containing only one kind of monomers; • Heteropolysaccharides contains different kinds of sugar monomers. • They play structural as well as storage role in living organisms.
- 21. Stroage Polysaccharides • The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. • These molecules are heavily hydrated, because they have many exposed hydroxyl groups available to hydrogen-bond with water. • Strach is a kind of homopolysaccharide, containing D-glucose monomers and are of two types: • Amylose ---- long, unbranched chains of D-glucose residues connected by (α1→4) linkages. • Amylopectin----- highly branched chains of D-glucose connected linearly by (α1→4) linkages and branch points are (α1→6) linkages.
- 22. Starch
- 23. Stroage Polysaccharides • Glycogen is the main storage polysaccharide of animal cells • It is a polymer of (α1→4)-linked subunits of glucose, with (α1→6)-linked branches. • It is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. • Glycogen is especially abundant in the liver. • When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. • Degradative enzymes act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides.
- 24. Stroage Polysaccharides • Dextrans are bacterial and yeast polysaccharides made up of (α1→6)-linked poly-D-glucose; all have(α1→3) branches, and some also have (α1→2) or (α1→4) branches. • Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans.
- 25. Structural Polysaccharide • Cellulose, a fibrous, tough, water-insoluble substance, is found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. • Cotton is almost pure cellulose. • It is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units. • Glucose residues have the β-configuration whereas in amylose, amylopectin, and glycogen the glucose is in the α- configuration. • The glucose residues in cellulose are linked by (β1→4) glycosidic bonds, in contrast to the (α1→4) bonds of amylose, starch, and glycogen.
- 26. Structural Polysaccharide • Chitin is a linear homopolysaccharide composed of N- acetylglucosamine residues in(β1→4) linkage. • The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. • Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. • Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropod and is probably the second most abundant polysaccharide, next to cellulose, in nature.
- 27. Structure of Bacterial cell wall • It is a heteropolymer of alternating (β1→4) -linked N-acetylglucosamine and N- acetylmuramic acid residues. • The linear polymers lie side by side in the cell wall, cross linked by short peptides. • This makes strong sheath that envelops the entire cell and prevents cellular swelling and lysis due to the osmotic entry of water.
- 28. Antibiotic action • The enzyme lysozyme kills bacteria by hydrolyzing the (β1→4) glycosidic bond between N-acetylglucosamine and N- acetylmuramic acid. • Penicillin and related antibiotics kill bacteria by preventing synthesis of the cross-links, leaving the cell wall too weak to resist osmotic lysis.
- 29. Structure of Algal cell wall • It contains agar, a mixture of sulfated heteropolysaccharides made up of D-galactose and an L- galactose derivative ether- linked between C-3 and C-6. The two major components of agar are • The unbranched polymer agarose (Mr ~120,000) • A branched component, agaropectin. The remarkable gel-forming property of agarose makes it useful in the biochemistry laboratory.
- 30. Importance of Agarose • When a suspension of agarose in water is heated and cooled, the agarose forms a double helix: two molecules in parallel orientation twist together with a helix repeat of three residues; water molecules are trapped in the central cavity. • These structures in turn associate with each other to form a gel— a three-dimensional matrix that traps large amounts of water. • Agarose gels are used as inert supports for the electrophoretic separation of nucleic acids, an essential part of the DNA sequencing process. • Agar is also used to form a surface for the growth of bacterial colonies.
- 31. • Another commercial use of agar is for the capsules in which some vitamins and drugs are packaged; the dried agar material dissolves readily in the stomach and is metabolically inert.