Lect 5

• Carbohydrates are polyhydroxy carbonyl compounds and their derivatives (hemiacetals/hemiketals/acetals/ketals)
• The molecular formula of carbohydrates is Cx(H2O)y
• Major form of stored energy in organisms and the metabolic precursors of virtually all other biomolecules
• Monosaccharides are the simplest carbohydrates, containing a single sugar unit and are composed of carbon, hydrogen, and oxygen
• Monosaccharides can be named based on functional groups: a monosaccharide with a ketone group is a ketose, whereas an aldehyde group is called an aldose
• Monosaccharides can exist in several isomeric forms, so it's essential to provide their unique names and indicate the sugar configuration (D-glucose and D-fructose)
• Monosaccharides can cyclize in two ways: aldopentose in a furanose form is more stable, while aldohexose in a pyranose form is more stable
• Derivative forms of monosaccharides include sugar acids (oxidation of sugars), sugar alcohols (reduction of carbonyl groups), deoxy sugars, phosphate esters, sugar esters, amino sugars, acetals, ketals, and glycosides
• Glucose, an aldohexose, is the most vital sugar in the human body, found in foods like dextrose, grape sugar, and blood sugar
• Normal blood glucose levels are 3.9 - 5.6 mmol/L, with the highest concentrations appearing after a meal
• Fructose, or levulose and fruit sugar, is the sweetest sugar, found in honey, corn syrup, and sweet fruits
• Cyclization of fructose produces α- and β-fructose, with the reaction between the C-2 keto group and the C-5 hydroxyl group producing an intramolecular hemiketal
• Fructose forms a five-membered ring structure

  
• Galactose is a crucial hexose found in biological systems as a component of the disaccharide lactose, or milk sugar
• Galactose forms a six-membered ring structure
• In humans, the genetic disease galactosaemia is caused by the absence of the enzymes needed for the conversion of galactose
• Ribose is a component of many biologically important molecules, including RNA and various coenzymes required by enzymes in biochemical reactions
• DNA contains 2-deoxyribose, where the OH group at C-2 has been replaced by an H
• Disaccharides are the simplest oligosaccharides
• Maltose (glucose-α-1,4-glucose) is a component of malt and is necessary in the brewing of beer
• Lactose (galactose-β-1,4-glucose) is the main carbohydrate in milk, critical for nutrition in the early stages of mammals
• Some people with lactose intolerance lack lactase, leading to undigested lactose causing cramping and diarrhoea
• Sucrose (glucose-α-1, 2-fructose) is produced by many higher plants through photosynthesis and is commonly known as table sugar
• Sucrose cannot be absorbed directly and is hydrolyzed by sucrase in the human intestine into glucose and fructose
• Sucrose is a vital carbohydrate in plants, water-soluble, easily transported, and used as a preservative due to its high osmotic pressure
• Polysaccharides are glycans that can be homopolysaccharides or heteropolysaccharides
• Starch is a polysaccharide that stores energy, with amylose and amylopectin as its components
• Amylase digests starch, with salivary α-amylase in animals acting as an endoamylase in the mouth
• Glycogen is the key form of stored polysaccharide in animals, mainly found in the liver and skeletal muscle
• 1. xylose, D-lyxose, D-gulose, and D-talose are aldohexoses, having six carbon atoms and an aldehyde group, differing in the arrangement of hydroxyl groups on the carbon atoms

  
• Ketoses, like dihydroxyacetone, erythrulose, and ribulose, are sugars containing a ketone functional group, classified by the number of carbon atoms they contain

  
• The image shows the two cyclic forms of D-ribose, a sugar molecule, with the pyranose form on the left and the furanose form on the right, demonstrating how the two forms can interconvert

  
• The chemical structure of beta-D-glucosamine and beta-D-galactosamine, two isomers of the amino sugar, is depicted in the image

  
• Cellulose, a structural polysaccharide found in nearly all plant cell walls, is a homopolymer of D-glucose with β(1→4) glycosidic linkages, hydrolysed by cellulase and β-glucosidase
• Cellulose, the most abundant organic molecule in the world, is a polymer of β-D-glucose units linked by β(1→4) glycosidic bonds, typically containing about 3000 glucose units
• Peptidoglycan is a strong, protective peptide-polysaccharide layer of bacteria, composed of L-Ala, D-Glu, L-Lys, and D-Ala
• Chitin, found in the cell walls of fungi and exoskeletons of crustaceans and insects, consists of β(1→4) N-acetyl-D-glucosamines
• Proteoglycans, a family of glycoproteins, predominantly contain glycosaminoglycans linked to Ser-Gly, interacting with other molecules through their glycosaminoglycan components
• Molisch's test is a general test for carbohydrates, where carbohydrates react with conc H2SO4 to form furfural and its derivatives, producing a purple coloured complex when reacted with sulfonated α-naphthol
• Benedict's test is for reducing sugars, changing the colour of Benedict's reagent from green to dark red or rusty brown, indicating the presence and type of sugar through a reduction reaction