Carbohydrates Carbohydrate – (hydrated carbon) Carbohydrates have empirical formula Cx(H2O)y. Most abundant carbohydrate is glucose, C6H12O6. Glucose is a 6 carbon aldehyde sugar and fructose is a 6 carbon ketone sugar. The alcohol side of glucose can react with the aldehyde side to form a six-membered ring. Glucose and Fructose are the main substrate for respiration, releasing energy for all cell processes.

Most glucose molecules are in the ring form. Note the six-membered rings are not planar. Focus on carbon atoms 1 and 5: if the OH groups are on opposite sides of the ring, then we have -glucose; if they are on the same side of the ring, then we have -glucose. The - and - forms of glucose form very different compounds.

Monosaccharides Simplest form of carbohydrates Classified according to number of carbons Triose – 3C’s Pentose – 5C’s Hexose – 6C’s All have 2 or more alcohols and a carbonyl group General Formula – CH2O These simple sugars that cannot be broken down by hydrolysis with aqueous acids.

2 Common Mono- saccharides – both isomers C6H12O6 Each contain many OH groups which make these molecules soluble in water. This are the straight chain form of these sugars. Aldose sugar Ketose sugar

Monosaccharides When these sugars are put in an (aq) solution, they undergo and internal reaction which results in the more familiar ring structure

The ring form of sugars make it possible for another type of isomer.

Disaccharides Disaccharides are sugars formed by the condensation of two monosaccharides. Examples: sucrose (table sugar) and lactose (milk sugar). Glycoside linkage – “ether” bond formed when monosaccharides combine to form disaccharides or polysaccharides (C-O-C). Disaccharides can be converted into monosaccharides by treatment with acid in aqueous solution.

Disaccharides Lactose – glucose + glactose – found in milk Maltose – glucose + glucose – product from starch digestion Sucrose – glucose + fructose – table sugar All have same molecular formula – C12H12O11 glycosidic linkage

Polysaccharides Polysaccharides are formed by condensation of several monosaccharide units. All polysaccharides are insoluble so they are ideal for storage.

Starch Main storage carbohydrate in plants – this is why food from plants are rich sources of starch – Ex. potatoes, rice, flour… Starch is a polymer of -glucose. Made from two polysaccharides – amylose (straight chain) and amylopectin (branched) Starch contains 1,4 and 1,6 linkages.

Starch Amylose – 1,4 linkage of -glucose monomers Amylopectin – 1,4 and 1,6 linkage of -glucose monomers

Glyogen Also a polymer of -glucose. Sometimes called “amimal starch” as it is the main storage carb in animals – found in the liver and muscles Contains many 1,6 branches

Cellulose Cellulose is a structural material in plant cell walls. Cellulose is a polymer of -glucose. It is linear. Every other monomer is upside down – enables to hydroxyl groups to to form hydrogen bonds The 1,4 linkage can be hydrolyzed by cellulase which is absent in most animals, including mammals.

Major Functions of Carbohydrates Energy sources – (glucose) Energy reserves (glycogen) Structure (cellulose) Dietary Fiber – mainly plant material that is not hydrolyzed by enzymes secreted in the human digestive tract. (may be digested by microflora in the gut) Most is excreted intact by the body. Ex. Cellulose, hemicellulose, lignin and pectin Starch and glycogen are easily broken down and absorbed in the body. Importance –as fiber passes through the digestive system it helps stimulate the lining to produce mucus which helps smooth passage of undigested foods. May help prevent diverticulitis, IBS, colorectal cancer, constipation, obesity, Crohn’s disease, hemorrhoids, and diabetes mellitus.