Biology 12

Chapter 1 The Chemistry of Life • MHR 27

and some other polysaccharides, are formed by
a condensation reaction, which removes water
from 2 –OH functional groups or neighbouring
monosaccharides. Because of its chemical
composition, cellulose (a polysaccharide found in
all plants) is indigestible for animals. The bonds in
cellulose are difficult to break by normal metabolic
means. In contrast, other polysaccharides, such as
the amylopectin found in potatoes, rice, and wheat,
serve as convenient and accessible forms of stored
energy. The bonds that bind their high-energy
glucose molecules together are easily broken and
easily formed.
In living cells and tissues, polysaccharides and
disaccharides can be broken into smaller units by
the process of hydrolysis. The complete hydrolysis
of most forms of starch produces a form of glucose,
which is a simple sugar that cannot be decomposed
by hydrolysis. In the investigation on page 24, you
can determine the products of hydrolysis reactions.

Nucleotides and Nucleic Acids

Nucleic acidssuch as DNA and RNA are huge
polymers of nucleotides. These are molecules
composed of one, two, or three phosphate groups,
a five-carbon sugar (deoxyribose or ribose), and a
nitrogen-base (see Figure 1.19). DNA contains
genetic information about its own replication and
the order in which amino acids are to be joined to
form a protein. RNA is the intermediary in the
process of protein synthesis, conveying information
from DNA regarding the amino acid sequence in a
protein. There are four different bases in DNA —
adenine, thymine, guanine, and cytosine. In RNA,
uracil replaces thymine as a base. Adenine not only
helps code genetic material and build proteins, but
it also has important metabolic functions. You will
investigate the structure and functions of nucleic
acids further in Unit 3.

Figure 1.19Structure of a nucleotide

ATP, adenosine triphosphate, is composed of

adenosine (adenine joined to ribose, as in RNA)

and three phoshate groups (see Figure 1.20). The

hydrolysis of ATP results in the formation of ADP

and a phosphate (Pi), and in the release of a large

quantity of energy for cellular work. After ATP

breaks down, it can be rebuilt by the addition of

the phosphate to ADP by condensation.

Condensation Synthesis and

Hydrolysis of Proteins

Proteins are important as structural components,

sources of nutrition, and for their role in speeding

up metabolic processes in the cell. Peptide bonds

formed in condensation reactions link amino acids

in proteins (see Figure 1.21). Each amino acid is

composed of a carbon atom bound to a hydrogen

atom and three additional groups — an amino

group, −NH 2 , a carboxyl group, –COOH, and an

R-group that is different in each amino acid. When

two amino acids join, they become a dipeptide. A

chain of amino acids is called a polypeptide. Try the

Thinking Lab to model a polypeptide. Polypeptides

may join to form proteins. The sequence of these

polypeptides, their particular orientations in space,

and their three-dimensional shapes determine the

type of protein they form. Enzymes, essential to

metabolism (as you will see in Chapter 2), are

proteins that are shaped in different ways depending

P

O

pentose sugar

nitrogen-

containing

base

phosphate

adenosine

ATP ADP

triphosphate adenosine diphosphate phosphate

P P P P P + +energy

+HO 2

HO 2

Pi

−

Figure 1.20When cells require energy, ATP undergoes hydrolysis, thereby
producing ADP, a phosphate, and energy.