Human Physiology, 14th edition (2016)

32 Chapter 2

atoms are arranged three-dimensionally in space. Stereoiso-

mers include (1) those that are designated as cis (Latin for “on

this side”) and trans (meaning “across”), where two functional

groups are located either on the same side ( cis ) or across from

each other ( trans ) in the molecule ( fig. 2.13 a ); and (2) enan-

tiomers (also called optical isomers ) that are mirror images of

each other ( fig. 2.13 b ). Enantiomers are like a left- and right-

hand glove; if the palms are both facing in the same direction,

they cannot be superimposed on each other. By convention,

one enantiomer is often designated the D -isomer (for dextro,

or right-handed) and the other is designated the L -isomer (for

levo, or left-handed).

These subtle differences in structure are extremely impor-

tant biologically. They ensure that enzymes—which interact

one can correctly refer to the molecule as either lactic acid or
lactate.

Stereoisomers

Stereoisomers are molecules that have the same atoms in the
same sequence, but differ from each other in the way their

Figure 2.10 Various functional groups of organic
molecules. The general symbol for any functional group is
R. Specific functional groups are indicated within the gold
rectangles.

Carbonyl (CO)

Carboxyl (COOH)

Hydroxyl (OH)

Sulfhydryl (SH)

Amino (NH 2 )

Phosphate (H 2 PO 4 )

C CCC

O

CC

C

O

CCCC OH

P

O

OH

CCCCCO OH

OH

CCCCC

SH

CCCCC

NH 2

CCCCC

Figure 2.11 Categories of organic molecules based
on functional groups. Acids, alcohols, and other types of organic
molecules are characterized by specific functional groups.

Ketone Organic acid

Aldehyde Alcohol

C

O

CH 3 CH 3 C

O

CH 3 OH

C

H

H

C CH 3 OH

O

CH 3 H

Figure 2.12 The carboxyl group of an organic

acid. This group can ionize to yield a free proton, which is a

hydrogen ion (H^1 ). This process is shown for lactic acid, with the

double arrows indicating that the reaction is reversible.

H CC

H

H

OH

H OH

C

O

H CC

H

H

+ H+

OH

H O–

C

O

Lactic acid Lactate

Figure 2.13 Stereoisomers. ( a ) Stereoisomers include

molecules such as butene, which can have its methyl groups

(CH 3 ) on either the same side ( cis ) or the opposite side ( trans ) of

the molecule. ( b ) Other stereoisomers are enantiomers (optical

isomers), which are mirror images of each other. Using the

molecule glyceraldehyde as a reference, these can be designated

as either D or L isomers.

CC

HH

cis-2-butene trans-2-butene

CH

CH 2 OH

OH

C

H O

CHO

CH 2 OH

H

C

H O

D-glyceraldehyde L-glyceraldehyde

H 3 CCH 3

CC

H

H 3 C H

CH 3

(a)

(b)