Macromolecule

Carbohydrate

1. Carbohydrates • Sugars and polymers of sugars • Simplest carbohydrates = Monosaccharides (simple sugars) are MONOMERS • 2 monosaccharides joined by covalent bond = Disaccharides • Polysaccharides = Polymers composed of many sugars

Most monosaccharides are multiples of CH2O • Glucose → Most common monosaccharide, C6H12O6 • Key features of sugars – Made of ≥3 C, with O and H – Polar

Monosaccharides names based on length of carbon chain – 6 carbons are hexoses → ex: Glucose, Galactose, Fructose. – 3 carbons are trioses – 5 carbons are pentoses → ex: Ribose and deoxyribose

• In aqueous solutions, glucose and other 5-6 carbon sugars form rings • Aqueous solution → Water is the solvent

• Disaccharide – Two monosaccharides – Joined by a dehydration reaction called GLYCOSIDIC LINKAGE

• Common disaccharides – Maltose = Glucose + Glucose – Sucrose = Glucose + Fructose, also known as Table sugar – Lactose = Glucose + Galactose

Uses of carbohydrates – Storage polysaccharides → Important nutrient for cells – Structural polysaccharides → Building materials • Shape and function of polysaccharide determined by Sugar monomers Positions of glycosidic linkages

1. Storage polysaccharides • Animals and plants store sugar for later use • Plants store sugar as STARCH → Polymer of glucose, as granules within cell • Most animals have enzymes to hydrolyze starch • Potato tubers and grains (wheat, corn, rice) = important parts of human diet Glucose monomers in starch joined by 1-4 glycosidic linkages – Carbon 1 joined to carbon 4, like in maltose • Forms of starch: – Amylose → Simplest, unbranched – Amylopectin → Branched • Animals store carbohydrates as glycogen in Liver, muscle cells • Looks similar to amylopectin, but more branched

2. Structural polysaccharides • Ex: cellulose – Tough – Encloses plant cells – Polymer of glucose (like starch is) but with DIFFERENT GLYCOSIDIC LINKAGES

2 different ring structures for glucose – Alpha → -OH group of first C is BELOW plane of ring – Beta → -OH group of first C is ABOVE plane of ring

• Starch (storage polysaccharide) → All glucose in ALPHA form • Cellulose (structural polysaccharide) → All glucose in BETA form

Different linkages → different structure • Starch is helical • Cellulose – Straight, never branched – Some –OH groups may H bond with other cellulose – OH groups when parallel • Parallel cellulose strands held together by H bonds in plant cell wall = Microfibrils (view image)

• Enzymes that hydrolyze alpha cannot hydrolyze beta linkages • Few organisms can digest cellulose • Animals and humans can’t digest cellulose. → Usually eliminated as waste • Some animals have microorganisms in gut to digest cellulose – ex: cows, termites

• Chitin (a fibrous substance consisting of polysaccharides and forming the major constituent in the exoskeleton of arthropods and the cell walls of fungi) – Arthropod exoskeletons – Cell walls of fungi – Has a nitrogen-containing group