The Carbohydrates: Sugars, Starches, and Fibers Photosynthesis Carbohydrates 2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2 Carb-o-hydrates = sugars The Carbohydrates: Sugars, Starches, and Fibers © 2008 Thomson - Wadsworth

Ribose (aldo-pentose) Carbohydrates Carbohydrates are made of carbon, hydrogen and oxygen atoms. Polyhydroxy-aldehydes and polyhydroxy-ketones Fischer Projection Ribose (aldo-pentose) Aldo = aldehyde Pent = 5 carbons ose = suffix for sugar ketohexose keto = ketone Hex = 6 carbons ose = suffix for sugar

Classification: Carbohydrates More general classification scheme where sugars are broken into groups Monosaccharides, Disaccharides and Polysaccharides Monosaccharides are single sugars (most are hexoses). Glucose serves as the essential energy source, and is commonly known as blood sugar or dextrose. Fructose is the sweetest, occurs naturally in honey and fruits, and is added to many foods in the form of high-fructose corn syrup. Galactose rarely occurs naturally as a single sugar. Disaccharides are formed by a linkage between a glucose and another monosaccharide Sucrose is a glucose and a fructose linked together Maltose has 2 glucose linked together Lactose is glucose and galactose Polysaccharides are made by joining many monosaccharides together Starch is a polysaccharide made from glucose Cellulose is a polysaccharide made from glucose also

Open Chain and Ring Structures Monosaccharides exist in rings more than the open chain form Happens because the aldehyde can react with a neighboring hydroxy (OH) group Glucose Open Chain

© 2008 Thomson - Wadsworth

Physical Properties Carbohydrates are poly alcohols – they form hydrogen bonds in water Therefore all monosaccharides and disaccharides are soluble in water Some polysaccharides are soluble in water and insoluble in organic solvents Many carbohydrates are sweet to taste Sweetness is a function of their structure Saccharin (artificial sweetener) sucrose

Reactivity: oxidation Oxidation: The open-chain form of glucose (< 1%) is an aldehyde and can be oxidized Carbohydrates that undergo oxidation are called reducing sugars All monosaccharides are reducing sugars because they either have an aldehyde group or they have an alpha hydroxy ketone and can be converted to an aldehyde

Reactivity: oxidation Fehling’s Reagent Used to test for a reducing sugar A red precipitate signals that you have a reducing sugar Barfoed’s Reagent Mild oxidizing agent that can distinguish between reducing monosaccharides and reducing disaccharides Reducing disaccharides react slowly if at all Reducing monosaccharides react quickly A red precipitate signals that you have a reducing sugar Cu2O(s) Cu2+(aq)

Reactivity: Dehydration of Sugars Carbohydrates can undergo dehydration (-H2O) reactions with non-oxidizing acids Saliwanoff’s Reagent Used to distinguish between aldoses and ketoses Ketoses with resorcinol in 6M HCl goes cherry red Aldoses react much more slowly

Reaction with Iodine: Starch Test Starches (polysaccharides) react with iodine to make a blue/black solution Only large starches give a positive test with I2 not say dextrins Amylose Amlyopectin

Reactivity: Hydrolysis of Sugars Polysaccharides and Disaccharides can be broken down with an acid Disaccharides are broken down completely Polysaccharides (like starch) are partially broken down to make dextrins We will test the reducing properties of hydrolyzed sucrose and starch and the ability of the hydrolyzed starch to react with iodine

The Experiment Fehling’s test (oxidation) Substance Observations Reaction? Glucose Fructose Lactose Sucrose Starch Unknown Lactose Fehling’s test (oxidation) Barfoed’s test (mild oxidation) Seliwanoff’s test (dehydration) Iodine test (starch) Hydrolysis of sucrose and starch Starch