Carbohydrates B.3

Introduction

• most abundant class of biological molecules

• range from simple sugars (glucose) to complex carbohydrates (starch)

Major functions (B.3.4)

• energy source– glucose converted into ATP– breads high in carbohydrates

• energy storage– glycogen stored in the liver

• precursors – necessary for molecules such as DNA

• dietary fiber– “roughage” is good

Describe the structural features of Monosaccharides (B.3.1)

• simplest sugars (single sugars)– all contain the empirical formula (CH2O)– contain a carbonyl group*, C=O

* a more generic term for a carbon double bonded to and oxygen

– contain at least two hydroxyl groups (-OH)

• can be straight chains or cyclic form• two common isomers of monosaccharides

(C6H12O6)•glucose•fructose

Draw the straight-chain and ring structural formulas of glucose and fructose. (B.3.2)

Glucose

α vs. β?OH on first carbon in βis inverted

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

Fructose

Condensation of monosaccharides to form disaccharides and polysaccharides (B.3.3)

• example of a typical condensation reaction

A-OH + B-OH ===> H2O + A-O-B

• requires enzymes• the hydroxyl (-OH) of two monomers are brought

together and the H of one and the OH of the other come together to make H2O

• the remaining O from one of the monomers bonds the two together in a bond called a glycosidic linkage

Disaccharides• double sugar (contains 2 monosaccharides)• three common disaccharides:

1. sucrose - common table sugar glucose + fructose

2. lactose - major sugar in milkglucose + galactose

3. maltose - product of starch digestion glucose + glucose

+

+

Polysaccharides• starch- condensation of many α glucose molecules

• cellulose- condensation of many β glucose molecules

• glycogen- condensation of many α glucose molecules

= OH

Compare structural properties of starch and cellulose (B.3.5)

• longer chains of simple sugars made of glucose• serve principally as food storage and structural

molecules in plants• Two Types of Polysaccharides

1. Starches (plants)– polymers of glucose molecules

– serve as storage depots of glucose

– two forms

• amylopectin- insoluble, branched chains, up to millions of glucoses

• amylose- water soluble, straight chains, thousands of glucoses

glucose polymer with

(14) linkages, and

branches formed by (16) linkages

glucose polymer

with only (14) linkages.

2. Cellulose (plants)• most abundant polysaccharide on Earth• the major structural material of which plants are made

(wood and plant fibers)• insoluble in water, great strength and rigidity as

hydrogen bonding holds chains in a side by side alignment

• only has beta-1,4 linkage

• plant cell walls are among the strongest of biological structures

• most organisms can’t break cellulose down into simple sugars because they don’t have the enzyme cellulase which is necessary to hydrolyze (break bonds using water) the glycosidic linkages

Dietary fiber and its importance B.3.6-7

• dietary fiber is plant material that we ingest but are not able to digest

• passes through the gut relatively intact, as we do not possess cellulase enzymes capable of hydrolysing it– bacteria in our gut can digest it somewhat– example: cellulose

• importance in our diet– helps “bulk” move through the large intestine

more easily• prevent constipation and diverticulosis (bulges in

the colon at weak places leading to pain)

• may prevent irritable bowel syndrome (IBS)

• may prevent hemorrhoids

– may cause a “full feeling” and therefore decrease chance of obesity

– may help prevent Crohn’s disease