Biology 12

Chapter 1 The Chemistry of Life • MHR 21

1.3 Making and Breaking Macromolecules

Large molecules can be broken down to release
energy. Alternatively, they can be formed to build
cellular structures or store information. In
biological systems there are four major types of
chemical reactions involved in breaking apart and
building molecules:

acid-base or neutralization reactions, which
transfer hydrogen ions between molecules,
redox, or oxidation-reduction reactions, which
transfer electrons between molecules,
hydrolysis reactions, in which molecules react
with H 2 Oto form other molecules, and
condensation reactions, in which molecules react
to form H 2 Oand other molecules.
These types of chemical reactions are described
below.

Acids, Bases, and

Neutralization Reactions

Acids and bases are compounds that may be
inorganic or organic. Hydrochloric acid, found in
the mammalian stomach, is an inorganic acid.
Acetic acid and amino acids are examples of
organic acids. Sodium hydroxide, a key component
of oven cleaners, is an inorganic base. Purines and
pyrimidines, the molecules that form part of the
subunits of nucleic acids, are examples of organic
bases; they are often referred to as nitrogenous
bases, because they include the nitrogen-containing
amine group.
What is it, however, that makes one substance an
acid and another a base? In biology, acids and bases
are understood in relation to their behaviour in
water. Under normal conditions, pure water exists
in the form of H 2 Omolecules. A small number of
these molecules dissociate, which means that they
break up into ions. When a water molecule
dissociates, it forms a positively charged hydrogen
ion, H+, and a negatively charged hydroxide ion,
OH−. Since very few water molecules dissociate,

the concentration of these ions is low. In pure water

at 25°C, the concentration of each of these ions is

the same: 1 × 10 −^7 mol/L. Because hydrogen and

hydroxide ions are very reactive, changes in their

concentrations can drastically affect cells and the

macromolecules within them. Acids and bases, and

more specifically the concentrations of hydrogen

and hydroxide ions within cells, determine how

effectively cellular processes are carried out.

Figure 1.15

Substances that

are acidic have a

pH less than 7,

while substances

that are basic

have a pH

greater than 7.

pH

14

13

12

11

10

9 8 7 6 5 4 3 2 1 0

MORE BASIC

MORE ACIDIC

1 mol/L NaOH (14.0)

lye (sodium hydroxide)

(13.0)

household ammonia

(11.9)

milk of magnesia

(antacid) (10.5)

detergent solution

(approximately 10)

ocean water (7.0–8.3)

blood (7.4)

milk (6.4)

urine (4.8–7.5)

rain water (5.6)

tomatoes (4.2)

vinegar (2.4–3.4)

lemon juice (2.2–2.4)

stomach acid

(mainly HCl) (1.0–3.0)

1 mol/L HCl (0.0)

NEUTRAL

EXPECTATIONS

Identify and describe the four main kinds of biochemical reactions.

Use molecular models to infer the polarity of molecules.

Investigate the structures and properties of macromolecules and functional

groups using models.