Carbohydrates: Monosaccharides, Reactions, and Applications PDF

1. Monosaccharides and Their Structural Characteristics Pentose: Ribose is a pentose (5-carbon sugar). Ketose: Fructose is a ketose (contains a ketone group). Glucose & Galactose: Cyclic Forms with 6-Membered Ring: Glucose and galactose form 6-membered rings. Fru...

1. Monosaccharides and Their Structural Characteristics Pentose: Ribose is a pentose (5-carbon sugar). Ketose: Fructose is a ketose (contains a ketone group). Glucose & Galactose: Cyclic Forms with 6-Membered Ring: Glucose and galactose form 6-membered rings. Fructose: Two Carbon Atoms Outside the Ring: Fructose has two carbon atoms outside its cyclic ring. 2. Key Reactions of Monosaccharides Oxidation Oxidation to acidic sugars - The redox chemistry of monosaccharides is closely linked to the alcohol and aldehyde functional groups present in them. Three different types of acidic sugars ○ Weak oxidizing agents such as Tollens and Benedict’s solutions oxidize the aldehyde end to give an aldonic acid. ○ Reducing sugar is a carbohydrate that gives a positive test with Tollens and Benedict’s solutions. ○ Aldaric Acid (Polyhydroxy dicarboxylic acid): Formed by strong oxidizing agents that oxidize both ends of the monosaccharide. Alduronic Acid - Produced when enzymatic oxidation of the primary alcohol end of an aldosterone such as glucose, without oxidation of the aldehyde group Sugar Alcohols Reduction to sugar alcohols: The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent. ○ The product is the corresponding polyhydroxy alcohol - sugar alcohol. Sorbitol - used as moisturizing agents in foods and cosmetics and as a sweetening agent in chewing gum - is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae. - It is about 60 % as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics. Many “sugar-free” products contain alcohols, such as sorbitol and xylitol Glycoside Glycoside formation: Cyclic forms of monosaccharides are hemiacetals, they react with alcohols to form acetals: ○ Monosaccharide acetals are called glycoside A glycoside is an acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon —OH group with an —OR group: ○ A glycoside produced from glucose - glucoside ○ A glycoside produced from galactose – galactoside ○ Glycosides exist in both Alpha and Beta forms 3. Blood Types and Monosaccharides - Human blood types (A, B, AB, O) are determined by specific monosaccharides on red blood cells (D-galactose and its derivatives). - A transfusion of the wrong blood type can cause the blood cells to form clumps - a potentially fatal reaction. - Type O is a universal donor; type AB is a universal recipient. - The biochemical basis for the various blood types involves monosaccharides present on plasma membranes of red blood cells. - The monosaccharides responsible for blood groups are D-galactose and its derivatives. 4. Phosphate Esters and Amino Sugars Phosphate Ester Formation - Hydroxyl groups of monosaccharides react with inorganic acids to form stable phosphate esters, important in carbohydrate metabolism. Amino Sugar Formation - Involves replacing a hydroxyl group with an amino group, vital in forming polysaccharides like chitin. Two monosaccharides can react to form disaccharide  One monosaccharide act as a hemiacetal and other as alcohol (alcohol and ether attached to the same carbon. 5. Disaccharides Cellobiose - Intermediate in cellulose hydrolysis, containing two β-D-glucose units linked via β(1-4) bonds. Maltose - Digested by humans; contains α(1-4) linkages. Lactose - principal carbohydrate in milk - Made of β-D-galactose and β-D-glucose units; lactose intolerance occurs due to a lack of lactase enzyme. - Human - 7%–8% lactose - cow’s milk - 4%–5% lactose - Lactose intolerance: a condition in which people lack the enzyme lactase needed to hydrolyze lactose to galactose and glucose. - Lactase enzyme hydrolyzes β(1-4) glycosidic linkages. - Deficiency of lactase can be caused by a genetic defect, physiological decline with age, or by injuries to intestinal mucosa. - When lactose is undigested it attracts water causing fullness, discomfort, cramping, nausea, and diarrhea. Bacterial fermentation of the lactose further along the intestinal tract produces acid (lactic acid) and gas, adding to the discomfort. “Human milk contains more lactose than does cow’s milk. Lactose intolerance is a condition In which people lack the enzyme lactase, which is needed to hydrolyze lactose to galactose and glucose” Sucrose (Table sugar) - Composed of glucose and fructose; not a reducing sugar due to its “head-to-head” glycosidic linkage. - The most abundant of all disaccharides and found in plants. - It is produced commercially from the juice of sugar cane and sugar beets.  - Sugar cane contains up to 20% by mass sucrose - Sugar beets contain up to 17% by mass sucrose The Polymer Chain Polymers Many monosaccharide units bonded with glycosidic linkages Two types: Linear and branched, homo- and hetero-polysaccharides 6. Polysaccharides - are not sweet and do not show positive tests with Tollen’s and Benedict’s solutions whereas monosaccharides are sweet and show positive tests - Limited water solubility - Examples: - Cellulose, starch in plants - Glycogen in animals - Chitin in arthropods Starch A storage polysaccharide is a polysaccharide that is a storage form for monosaccharides and is used as an energy source in cells. Starch: ○ Glucose is the monomeric unit ○ Storage polysaccharide in plants ○ Two types of polysaccharidse isolated from starch: Amylose: Straight chain polymer - 15 - 20% of the starch and has α (1 4) glycosidic bonds Molecular Mass: 50,000 (up to 1000 glucose units)  Amylopectin: ○ Branched chain polymer - 80 - 85 % of the starch α (14) glycosidic bond for straight chain and α (16) for branch ○ Molecular Mass: 300,000 (up to 100,000 glucose units) - higher than amylose ○ Human can hydrolyze alpha linkage but not beta linkage Storage polysaccharide in plants, made of amylose (straight-chain) and amylopectin (branched chain). Glycogen - Storage polysaccharide in animals, highly branched, stored in liver and muscles. - Contains only glucose units - Branched chain polymer – a (14) glycosidic bonds in straight chains and a (16) in branches - Molecular Mass: 3,000,000 (up to 1,000,000 glucose units) - Three times more highly branched than amylopectin in starch - Excess glucose in blood stored in the form of glycogen Cellulose - Linear homopolysaccharide with β(1-4) glycosidic bond linkages; indigestible by humans but serves as dietary fiber. - Animals can digest cellulose because they have bacteria in their guts to hydrolyze cellulose - Dietary fiber in food- easily absorbs water and results in softer stools - 20-35 g of dietary fiber is desired everyday Chitin - Similar to cellulose, with N-acetyl amino derivative of glucose, providing rigidity to arthropod exoskeletons such as crabs, lobsters,insects,shrimp. Acidic Polysaccharides polysaccharides with a repeating disaccharide unit containing an amino sugar and a sugar with a negative charge due to a sulfate or a carboxyl group. Structural polysaccharide present in connective tissue associated with joints, cartilage, synovial fluids in animals and humans ○ Primary function is lubrication necessary for joint movement ○ These are heteropolysaccharides - have more than one type of monosaccharide monomer present. Examples: ○ Hyaluronic acid ○ Heparin - Found in connective tissues, such as hyaluronic acid (joint lubrication) and heparin (anticoagulant). Hyaluronic acid: - Alternating residues of N-acetyl-b-D-glucosamine and D-glucuronic acid. - Highly viscous - serve as lubricants in the fluid of joints and part vitreous humor of the eye. Heparin: - An anticoagulant-prevents blood clots. - Polysaccharide with 15–90 disaccharide residues per chain. A glycolipid is a lipid molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it. A glycoprotein is a protein molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it. 7. Nutrition and Glycemic Index - Carbohydrates make up over 50% of most diets. 60% in a balanced dietary food - They can be classified into: a. Simple Carbs:Dietary Monosaccharides or disaccharides, typically sweet (e.g., sucrose). b. Complex Carbs: Dietary Polysaccharides like starch and cellulose, not sweet. Glycemic Foods - Glycemic Index (GI): Measures how quickly carbs are digested, how high blood glucose rises, and how quickly levels return to normal. Foods are rated based on their GI.