Unit 1: Biochemistry - #2 Structure and Function of Macromolecules: Carbohydrates

Three Types of Carbohydrates

Monosaccharides, Disaccharides and Polysaccharides

Monosaccharides

• “Mono” is single, “Saccharide” is sugar 

• Simplest sugar 

• Have ratio of C:H:O = 1:2:1 

• Distinguished from one another by: 

• Carbonyl group: either aldehyde or ketone 

• Length of Carbon chain

What happens when Carbohydrates dissolve in water?

• Monosaccharides with five or more carbons are linear in dry state, but form rings when dissolved in water 

• Example: When glucose dissolves in water, the -OH group at carbon 5 reacts with the aldehyde group at carbon 1 to form a closed ring 

Beta-Glucose

50% chance the -OH group at Carbon 1 will end up above the plane of the ring

Alpha-Glucose

50% chance the -OH group at Carbon 1 will end up below the plane of the ring

Disaccharides

• Sugars containing two (disaccharides) simple sugars 

• Linked together by a 1-4 glycosidic linkage: A covalent bond between 2 monosaccharides by a condensation reaction (dehydration synthesis) 

  • The hydroxyl group of carbon 1 of the glucose molecule links with the hydroxyl group of carbon 4 of the adjacent molecule:

Disaccharide Chart

Polysaccharides

• Formed by linking monosaccharides (several 100 to several 1000) by glycosidic linkages 

• Can be straight chained, or branched

• Very polar due to many hydroxyl groups 

• Hydrophilic but will not dissolve due to large size 

• Have two important biological functions: 

  • Energy storage (starch and glycogen) 

  • Structural support (cellulose and chitin)

Starch

• Main form of energy storage in plants 

• Plants produce starch by linking excess glucose molecules together

• Two types: Amylose & Amylopectin

Amylose

No branches, all 𝛂-1, 4-glycosidic linkages

Amylopectin

Branched, 𝛂-1, 4-glycosidic linkages for main chain and 𝛂-1, 6-glycosidic linkages for branches

Glycogen

• Storage polymer in animals (muscle and liver) 

• Glycogen stores small; depleted in a day if not replenished 

• Highly branches: (𝛂 1-4) linked glucose main chain with (𝛂1-6) linked branches 

• More branching and more compact than amylopectin (starch)

Cellulose

• Major component of cell walls 

• Straight chain polymer of beta-glucose held together by ꞵ-1-4 glycosidic linkages, where every other glucose molecule is inverted 

• Humans cannot digest cellulose because they lack an enzyme to hydrolyze ꞵ-1-4 linkage (roughage) 

• Provides rich supply of energy for organisms who can break it down