Biology 12

Chapter 1 The Chemistry of Life • MHR 15

substantial portions of many biological molecules
consist of bonded chains of carbon and hydrogen.

Molecular Isomers

Because carbon can form so many compounds with
so many elements, it is common to encounter
several organic compounds with the same
molecular formula but different structures. Such
compounds are known as isomers. For example,
two isomers of glucose, a six-carbon sugar, are
fructose and galactose. Glucose, fructose, and
galactose all have the same molecular formula
(C 6 H 12 O 6 ). However, they differ in their molecular
structures, as shown in Figure 1.11.

Figure 1.11The different ways in which the same atoms
are arranged in glucose, galactose, and fructose make
them isomers.

There are two main types of isomers. Structural
isomersare two or more compounds with the same
atoms bonded differently. Glucose and fructose, for
example, are structural isomers. Notice that a
glucose molecule contains a ring of five carbon
atoms and an oxygen atom, whereas a fructose
molecule contains a ring of four carbon atoms and
an oxygen atom. Because their structures are
different, glucose and fructose have different
properties, and cells metabolize them differently.
Stereoisomersare two or more compounds with
their atoms bonded in the same way, but with
atoms arranged differently in space. Stereoisomers
may be geometrical or optical. Geometrical isomers
can have very different physical properties (such
as different melting points), but they tend to have
the same chemical properties. Glucose and
galactose are examples of geometrical isomers.
Optical isomers, shown in Figure 1.12, are non-
superimposable mirror images of each other. They

usually have similar chemical and physical

properties, but enzymes or proteins on the cell

membrane can distinguish between them. Usually,

one optical isomer is biologically active and the

other biologically inactive. In some cases however,

this is not always true. For example, sometimes

one optical isomer of a drug is not as effective as

the other or can even cause complications. In the

early 1960s, many pregnant women were prescribed

a drug called thalidomide for morning sickness.

Thalidomide is a mixture of two optical isomers; one

produced the desired effect, but the other caused

major birth defects. As the thalidomide example

demonstrates, organisms can be very sensitive to

minute variations in molecular geometry.

Figure 1.12Optical isomers, such as these molecules of

the amino acid, alanine, are identical in their structures

except that they are non-superimposable mirror images

of each other. Your left and right hands are also

non-superimposable images of each other.

The Functional Groups

Chemical reactions involve breaking or forming

chemical bonds. These processes can transform

simple molecules such as glucose into complex

molecules such as starch or cellulose. Many of

these complex molecules contain groups of atoms

with characteristic chemical properties. These

groups of atoms, known as functional groups,

include hydroxyl, carbonyl, carboxyl, amino,

sulfhydryl, and phosphate groups, as shown in

Figure 1.13 on page 16. Many compounds have

more than one functional group in their structure.

C

OH

OH

C

H

H

C

OH

H

C

HO

H

CO

H

H 2 COH

H 2 COH

H 2 COH H 2 COH

glucose

C

OH

OH

C

H

H

C

OH

C

H

O

fructose

C

OH

OH

C

H

H

C

OH

H

C

H

C

H

OH

O

galactose