Human Physiology, 14th edition (2016)

34 Chapter 2

Two monosaccharides can be joined covalently to form a

disaccharide, or double sugar. Common disaccharides include

table sugar, or sucrose (composed of glucose and fructose);

milk sugar, or lactose (composed of glucose and galactose);

and malt sugar, or maltose (composed of two glucose mole-

cules). When numerous monosaccharides are joined together,

the resulting molecule is called a polysaccharide.

The major polysaccharides are chains of repeating glu-

cose subunits. Starch is a plant product formed by the bond-

ing together of thousands of glucose subunits into long chains,

and glycogen (sometimes called animal starch) is similar, but

more highly branched ( fig. 2.15 ). Animals have the enzymes

to digest the bonds (chemically called alpha-1,4 glycosidic

bonds) between adjacent glucose subunits of these polysac-

charides. Cellulose (produced by plants) is also a polysaccha-

ride of glucose, but the bonds joining its glucose subunits are

oriented differently (forming beta-1,4 glycosidic bonds) than

those in starch or glycogen. Because of this, our digestive

enzymes cannot hydrolyze cellulose into its glucose subunits.

However, animals such as cows, horses, and sheep—which eat

grasses— can digest cellulose because they have symbiotic

bacteria with the necessary enzymes in their digestive tracts.

Chitin (poly-N-acetylglucosamine) is a polysaccharide similar

to cellulose (with beta-1,4 glycosidic bonds) but with amine-

containing groups in the glucose subunits. Chitin forms the

exoskeleton of arthropods such as insects and crustaceans.

Many cells store carbohydrates for use as an energy source,

as described in chapter 5. If many thousands of separate mono-

saccharide molecules were stored in a cell, however, their high

concentration would draw an excessive amount of water into

the cell, damaging or even killing it. The net movement of

water through membranes is called osmosis, and is discussed

in chapter 6. Cells that store carbohydrates for energy mini-

mize this osmotic damage by instead joining the glucose mol-

ecules together to form the polysaccharides starch or glycogen.

Because there are fewer of these larger molecules, less water is

drawn into the cell by osmosis (see chapter 6).

Dehydration Synthesis and Hydrolysis

In the formation of disaccharides and polysaccharides, the

separate subunits (monosaccharides) are bonded together

covalently by a type of reaction called dehydration synthesis,

or condensation. In this reaction, which requires the par-

ticipation of specific enzymes (chapter 4), a hydrogen atom

is removed from one monosaccharide and a hydroxyl group

(OH) is removed from another. As a covalent bond is formed

between the two monosaccharides, water (H 2 O) is produced.

Dehydration synthesis reactions are illustrated in figure 2.16.

When a person eats disaccharides or polysaccharides, or

when the stored glycogen in the liver and muscles is to be used

by tissue cells, the covalent bonds that join monosaccharides to

form disaccharides and polysaccharides must be broken. These

digestion reactions occur by means of hydrolysis. Hydrolysis

(from the Greek hydro 5 water; lysis 5 break) is the reverse

of dehydration synthesis. When a covalent bond joining two

Figure 2.14 Structural formulas for three hexose
sugars. These are ( a ) glucose, ( b ) galactose, and ( c ) fructose.
All three have the same ratio of atoms—C 6 H 12 O 6. The
representations on the left more clearly show the atoms in each
molecule, while the ring structures on the right more accurately
reflect the way these atoms are arranged.
See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.

O

CH 2 OH

H H

H

H

Glucose

Galactose

Fructose

OH

OH

H

HO OH

O

CH 2 OH

HO H

H

H

OH

OH

H

H OH

HOCH 2

HO

H

H

OH

OH

H CH 2 OH

O

C

C

H

(a)

(b)

(c)

H

OH

O

C

HO H

C

HOHC

C

HOH

H

HOH

C

C

H

H

OH

O

C

HO H

C

HO HC

C

HOH

H

HOH

C

OC

HO H

C

HOHC

C

HOH

H

HOH

C

H

HOH