Human Physiology, 14th edition (2016)

Chemical Composition of the Body 31

molecules, the carbon atoms are not shown but are understood
to be located at the corners of the ring. Some of these cyclic
molecules have a double bond between 2 adjacent carbon
atoms. Benzene and related molecules are shown as a six-sided
ring with alternating double bonds. Such compounds are called
aromatic. Because all of the carbons in an aromatic ring are
equivalent, double bonds can be shown between any 2 adjacent
carbons in the ring ( fig. 2.9 ), or even as a circle within the hex-
agonal structure of carbons.
The hydrocarbon chain or ring of many organic molecules
provides a relatively inactive molecular “backbone” to which
more reactive groups of atoms are attached. Known as func-
tional groups of the molecule, these reactive groups usually
contain atoms of oxygen, nitrogen, phosphorus, or sulfur. They
are largely responsible for the unique chemical properties of
the molecule ( fig. 2.10 ).
Classes of organic molecules can be named according to their
functional groups. Ketones, for example, have a carbonyl group
within the carbon chain. An organic molecule is an alcohol if it
has a hydroxyl group bound to a hydrocarbon chain. All organic
acids (acetic acid, citric acids, lactic acid, and others) have a car-
boxyl group ( fig. 2.11 ).
A carboxyl group can be abbreviated COOH. This group
is an acid because it can donate its proton (H^1 ) to the solu-
tion. Ionization of the OH part of COOH forms COO^2 and H^1
( fig. 2.12 ). The ionized organic acid is designated with the suf-
fix - ate. For example, when the carboxyl group of lactic acid
ionizes, the molecule is called lactate. Because both ionized
and unionized forms of the molecule exist together in a solu-
tion (the proportion of each depends on the pH of the solution),

Figure 2.8 Single and double covalent bonds. Two carbon atoms may be joined by a single covalent bond ( left ) or a double
covalent bond ( right ). In both cases, each carbon atom shares four pairs of electrons (has four bonds) to complete the 8 electrons
required to fill its outer shell.

1P

1P

1P

6P

6N

1P

1P

6P

6N 1P

6P

6N

1P

6P

6N

1P

1P

1P

C C

H

H

C 2 H 4

CH C

H

H

H

H

H

C 2 H 6

H

H

Ethane (C 2 H 6 ) Ethylene (C 2 H 4 )

Figure 2.9 Different shapes of hydrocarbon

molecules. Hydrocarbon molecules can be ( a ) linear or ( b ) cyclic

or have ( c ) aromatic rings.

HHHHHH

HHHHHH

(a) HCCCCCCHC 6 H 14 (Hexane)

(b)

(c)

C 6 H 12 (Cyclohexane)

C 6 H 6 (Benzene)

H 2 C CH 2

CH 2

H 2 C

CH 2

CH 2

or

or

HH

HH

H

H

C

C

CC

CC