CARBOHYDRATES
Carbohydrates have the general molecular formula (CH2O)n, and thus were once thought to represent "hydrated carbon". All sugars are very soluble in water because of their many hydroxyl groups. They are called sugars because they are sweet in taste & crystalline in nature. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells. Carbohydrates provide the bulk of the calories (4 kcal/gram)
1)Definition:
a)Hydrates of carbon Cn(H2O)n{exceptions: Acetic acid, C2H4O2 & lactic Acid, C3H6O3 Formaldehyde, CH2Oare not carbohydrates but Rahnose,C6H12O5 is.}
b)carbohydrates are polyhydroxy aldehydes or ketones or compounds, which yields these on hydrolysis.
a)Monosaccharides:simple sugars that cannot be hydrolyzed to yield Carbohydrates
Name | Aldose | Ketose | Formula |
Trioses | Glyceraldehyde | Dihydroxy acetone | C3H6O3 |
Tetroses | Erythrose | Erytrulose | C4H8O4 |
Pentoses | Ribose | Ribulose | C5H10O5 |
Hexoses | Glucose,Mannose, Galactose | Fructose | C6H12O6 |
Heptoses | Sedoheptulose | C7H14O7 | |
Nonoses | Sialic acid or Neuramic acid | C9H18O9 |
b)Disaccharides: yields two molecules of Monosaccharides on hydrolysis
Name | Monosaccharides | Formula | Sources | Properties |
Maltose | Glucose + Glucose | C3H6O3 | Malt sugar, starch | Reducing |
Lactose | Glucose + Galactose | C3H6O3 | Milk Sugar | Reducing |
Sucrose | Glucose + Fructose | C3H6O3 | Cane sugar | Non-Reducing |
Sucrose: prevalent in sugar cane and sugar beets, is composed of glucose and fructose through an a-(1,2)b-glycosidic bond.
Lactose: is found exclusively in the milk of mammals and consists of galactose and glucose in a b-(1,4) glycosidic bond.
Maltose: the major degradation product of starch, is composed of 2 glucose monomers in an a-(1,4) glycosidic bond.
c)Oligosaccharides: yields 3 to 10 Monosaccharides on Hydrolysis.
Maltotriose | Glucose + Glucose + Glucose |
Raffinose | A trisaccharide that occurs in sugar beets and cotton seeds and cereals |
Stachyose | A tetrasaccharide found in the tubers of the Chinese artichoke |
d)Polisaccharides: yields more than 10 Monosaccharides on hydrolysis. They are of two types.
i)Homopolysaccharides (Homoglycans) Polymer of a single type of Monosaccharide.
Name | Monosacc. | Sources | Importance |
Starch | Glucose | Potato,tapioca,rice,wheat,&grains | Plant source of Energy |
Amylose | Glucose | Un branched starch similar to cellulose, containing alpha(1->4) linkages | |
Amylopectin | Glucose | Branched starch containing both 1,4 and 1,6 linkages. | |
Glycogen | Glucose | 5% of liver mass> branched & compact | Animal storage of Glucose |
Inulin | Fructose | various bulbs&tubers(onion,garlic) | Renal clearance studies. |
Dextrins | Glucose | Product. ofstarch digestion | White dextrin is used in pharmaceutical preparations |
Chitin | N-Ac-Glucosamine | Exoskeleton of Insects |
ii)Hetroplysaccharides (Hetroglycans): polymer of two or more types of sugar residues
Agar | Glu +Gal + other | Obtained from Sea weeds |
A colloidal extract of algae; used especially in culture media and as a gelling agent in foods (undigested by bacteria, dissolves at 100 oC and upon cooling sets into a Gel) |
iii)MUCOPOLYSACCHARIDES (MPS) or GLYCOSAMINOGLYCANS (GAG)
Are Carbohydrates containing Uronic Acid & Amino Sugars.
Name | Functions |
Hyaluronic acid | A viscous mucopolysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humours of the eye; a cementing and protective substance |
Heparin | A polysaccharide produced in basophils (especially in the lung and liver) and that inhibit the activity of thrombin in coagulation of the blood. |
Chondroitin SO4 | Inground substance of connective tissues (cartilage, tendon, bone) |
Keratan SO4 | In cornea & tendons. |
e)Isomerism
i)D & L Isomerism
oThe configuration of the asymmetric carbon atom ( carbon atom whose 4 valencies are filled up by 4 different groups)farthest from the aldehydic or ketonic group
oIf –OH group is attached to the right then 'D' series & left then 'L' series.
ii)Pyranose - Furanose ring Structure
oWhen ring structure were suggested by Haworth (projections)
oFive member ring structure is calledFuranose and Six number as Pyranose
iii)Alpha & Beta Anomerism
oThe anomeric carbon (which originally was a part of the aldo or keto group) forms a hemiacetal or hemiketal in the ring structure generating a new asymmetric centre.
oIf hydroxyl gp (-OH) of the anomeric carbon is to the right then it is -form and if to the left it -form.
iv)Epimerism
oEpimers differ in the position of the Hydroxyl gp at only one of their assymetric C-atom.
oD-Glucose & D-Galactsoe are C-4 Epimers
oD-Glucose & D-Mannose are C-2 epimers
v)Enantiomers
oStereo isomers that are mirror images of each other
oEg: D-Glyceraldehyde & L-Glyceraldehyde.
vi)Mutarotation
oWhen D-Glucose is crystallized at RT and fresh solution is prepared, its specific rotation of polarized light is +112 o; but after few hrs it changes to +52.5 o. This change in rotation is called Mutarotation. This b'se D-Glucose has two anomers & forms. (-D-Glucose has specific rotation of +112 o and -D-Glucose has +19 o at equilibrium 1/3 is form and 2/3 is form to get a specific rotation of +52.5 o
3)Other Physiologically important carbohydrates.
Sugar | Present where | Importance&clinical Significance |
D-Ribose | Nucleic acids | Nucleic acid co-enzymes. |
D-Ribulose | Metabolic Product | Pentose-PO4 pathway intermediate. |
D-Arabinose | Gum arabic, Plum & Cherry gum | Constituent of Glycoproteins |
D-Xylose | Wood gums, GAG's | Constituent of Glycoproteins |
D-Xylulose | Intermediate in Uronic acid pathway | Found in the Urine of patients with Essential pentosuria |
4)CHEMISTRY
a)Reactions
i)Benedict's Reaction
Contains Na2CO3, Na-Citrte,CuSO4
In alkaline medium (Na2CO3), the copper remains as cupric hydroxide in presence of sodium citrate
5 ml Benedict's reagent+ 0.5 ml Urine boil for 2 minutes. If sugar present copper is reduced to produce green, yellow, orange, red precipitate depending on the concentration of the sugar.
SugarEnediolCu++(cupric) CuSO 4(Reagent)
Sugar AcidCu+(cuprous) 2Cu(OH)[Yellow]Cu 2O [Red]
Chemical propertie | ||||||
1 | Reactions of aldehyde / keto group | |||||
Reactions | Reagents | Result | ||||
a. | Osazone formation | Sugar+Phenyl Hydrazine HCl+ Sodium Acetate (boil+cool) | Glucosazone-needle shapedLactosazone - Powder puff Maltosazone - Sunflower | |||
b. | Reduction → Alcohol | Sodium amalgum / Hydrogen | Glucose - sorbitolFructose - Manitol | |||
c. | Oxidation | |||||
I. | Mild (CHO → COOH) | Br2 H2O | Glucose - gluconic acid | |||
ii. | Srong (CHO & CH2OH to COOH | HNO3 | glucose - Glucosaccharic acid | |||
iii. | Oxdn of CH2OH alone | Enzymatic (living cells only) | Glucose to glucuronic acid | |||
d. | Tautomerisation( Inter conversions) | Weak alkaliBa (OH)2, Ca(OH)2 | Glucose ↔ fructose ↔ Mannose | |||
e. | Reduction Reactions | CuSO4, alkaline medium( cupric -> cuprous) | Benedicts, Fehlings & Barfoeds Test( Cuprous oxide- Red ppt) | |||
2 | Reactions of Alcoholic group | |||||
a. | Glycosides | 1→4 &1→6 bond in glycosidic bonds in glycogen | ||||
I. | Cardiac | Derivatives of DIGITALIS, | Drugs in cardiac insufficiency | |||
ii. | Oubin | strophanthus sp. | Sodium pump inhibitor (cardiac) | |||
iii. | Phlorbizin | Root/bark of apple tree | block transport of sugar | |||
iv. | Streptomycine | produced by the actinomycete Streptomyces griseus | Antibioticused to treat tuberculosis | |||
b. | Acetilation/Ester formation | Alcohol + acids | phosphates ( glucose -1-phosphate) | |||
c. | Dehydratin( Molisch, Selivanoffs, Bials tests) | Strong acids (Con.H2SO4) | Pentose→ Furfurals,Hexose→ Hydroxy methyl Furfural. | |||
TESTS | ||||||
Name of test | Substrate | Reagents | Result | |||
1 | Molisch's Test | Sugar | 3d Alfa-Naphthol in Methanol +H2SO4 (side) | Violet color in the Junction. | ||
2 | Iodine test | Starch | Dil. Iodine | Deep Blue Colour | ||
3 | Benedict's | Reducing Sugar(0.5 ml/ 8 dps) | CuSO4+Na2CO3+NaCitrate(5 ml) | Cuprous Oxide (Red Precipitate) | ||
4 | Fehling's Test | Reducing Sugar | CuSO4+KOH+NaK Tartrate | Cuprous Oxide (Red Precipitate) | ||
5 | Barfoed's test | Monosaccharide. | CuAcetate+Dil.AceticAcid | (Red Precipitate) in < 5' | ||
6 | Bial's Test | Pentose | Orcinol Hydrochloride (Boil) | Green | ||
7 | SelivanoffsTest | Ketoses | Dil. Resorcinol in Dil.HCl (heat) | Cherry red | ||
8 | Tollent's Test | Reducing Sugar | Ammoniacal Silver nitrate | Metalic Siver Mirror |
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