Chapter 20: Carbohydrates

K.Dunlap Chem 104

Carbohydrates• Composed of carbon, oxygen, hydrogen

• two-third of the human diet is composed of carbohydrates

• most carbohydrates are produced by the photosynthesis in green plants

• the three elements that make up all carbohydrates are arranged as alcohols, aldehydes, or ketones

•The three main classes of carbohydrates are: monosaccharides, disaccharides, polysaccharides

Carbohydrates

• Monosaccharide:Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.

– Monosaccharides have the general formula CCnnHH2n2nOOnn, where nn varies from 3 to 8.

– Aldose:Aldose: a monosaccharide containing an aldehyde group.

– Ketose:Ketose: a monosaccharide containing a ketone group.

1) Monosaccharides• also known as simple sugars can not be broken down into smaller CHO

• glucose, galactose, fructose, ribose, and deoxyribose

• hexoses and pentoses

aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal forming a ring with an oxygen bridge between 2 carbon atoms. Rings with five carbons are called furanoses and rings with 6 carbons are call pyranoses

Smallest monosaccharides

Monosaccharides have chiral carbons

The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this –OH is on the right of the Fischer projection, this is a

D sugar D-glucose

Glucose

• the most biologically important monosaccharide

• used directly by the body for energy

• oxidized to carbon dioxide and water

• Excess stored as adipose

Galactose is in many plant gums and pectins

• component of the disaccharide lactose

Fructose is the sweetest of all the naturally occurring sugars

• honey, fruits

• component of the disaccharide sucrose

Monosaccharides

Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to the carbon 5: they are either on the opposite sides (α), or the same side (β) The α and β anomers of

glucose.

Other monosaccarides

-components of DNA and RNA

Disaccharides

consist of 2 monosaccharide Sucrose• glucose and fructose• table sugar • 1/4 of total calories

Lactose• glucose and galactose• milk

Maltose• glucose and glucose• germinating grains

Disaccharides• Sucrose (table sugar)

– Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets.

– Glucose and fructose linked together with a -1,2-glycosidic bond

Disaccharides• Lactose

– Lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk.

– Contains glucose and galactose linked together with a -1,4-glycosidic bond

Disaccharides• Maltose

– Present in malt, the juice from sprouted barley and other cereal grains.

– Maltose consists of two units of glucose joined by an -1,4-glycosidic bond.

Physical Properties• Monosaccharides are colorless crystalline

solids, very soluble in water, but only slightly soluble in ethanol– Sweetness relative to sucrose:

Polysaccharides

- a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.

Starch -2/3 of the human diet-Potatoes, rice, wheat, cereal grains-mixture of amylose and amylopectin

Glycogen-only storage for glucose in the body-liver and muscle-similar in structure to amylopectin but, more branched

Cellulose - forms the structural component of the cell walls of plants

-cotton, paper, linen, rayon

-long unbranched chains of glucose (100- 10,000 glucose molecules)

- linkages instead of linkages that are present in starch

-many herbivores have the enzyme to breakdown linkages

Polysaccharides• Starch:Starch: a polymer of D-glucose.

– Starch can be separated into amylose and amylopectin.– Amylose is composed of unbranched chains of up to 4000

glucose units joined by -1,4-glycosidic bonds.– Amylopectin contains chains up to 10,000 D-glucose units

also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds.

Polysaccharides• Figure 20.3 Amylopectin.

Polysaccharides

• GlycogenGlycogen is the energy-reserve carbohydrate for animals.

– Glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4- and -1,6-glycosidic bonds.

– The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

Polysaccharides

• CelluloseCellulose is a linear polysaccharide of D-glucose units joined by -1,4-glycosidic bonds.– It has an average molecular weight of 400,000 g/mol,

corresponding to approximately 2200 glucose units per molecule.

– Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds.

– This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.

– It is also the reason why cellulose is insoluble in water.

Polysaccharides• Cellulose (cont’d)

– Humans and other animals cannot use cellulose as food because our digestive systems do not contain -glucosidases, enzymes that catalyze hydrolysis of -glucosidic bonds.

– Instead, we have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.

– Many bacteria and microorganisms have -glucosidases and can digest cellulose.

– Termites have such bacteria in their intestines and can use wood as their principal food.

– Ruminants (cud-chewing animals) and horses can also digest grasses and hay.

• Figure 20.4 Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by -1,4-glycosidic bonds.

Starch And Cellulose

1. What are the three main classes of carbohydrates?

2. Cellulose is made from glucose, but humans can not digest cellulose. Explain.

3. How does your body store carbohydrates?

4. Indicate whether each of the following is a monosaccharide, disaccharide, or polysaccharide.a) sucroseb) cellulosec) glucosed) lactose

5. Starch is a mixture of two types of polysaccharides. Name them.

6. Name the 2 monosaccharides in each of the following.a) sucroseb) maltosec) lactose