Chapter 2
Chemistry Comes Alive
55
2
each other. When its terminal high-energy phosphate bonds
are broken (hydrolyzed), the chemical “spring” relaxes and the
molecule as a whole becomes more stable.
Cells tap ATP’s bond energy during coupled reactions by us-
ing enzymes to transfer the terminal phosphate groups from ATP
to other compounds. ±ese newly
phosphorylated
molecules are
said to be “primed” and temporarily become more energetic and
capable of performing some type of cellular work. In the process
of doing their work, they lose the phosphate group. ±e amount
of energy released and transferred during ATP hydrolysis corre-
sponds closely to that needed to drive most biochemical reactions.
As a result, cells are protected from excessive energy release that
might be damaging, and energy squandering is kept to a minimum.
Cleaving the terminal phosphate bond of ATP yields a
molecule with two phosphate groups—
adenosine diphosphate
Comparison of DNA and RNA
CHARACTERISTIC
DNA
RNA
Major cellular site
Nucleus
Cytoplasm (cell area outside the nucleus)
Major functions
Is the genetic material; directs protein synthesis;
replicates itself before cell division
Carries out the genetic instructions for protein synthesis
Sugar
Deoxyribose
Ribose
Bases
Adenine, guanine, cytosine, thymine
Adenine, guanine, cytosine, uracil
Structure
Double strand coiled into a double helix
Single strand, straight or folded
RNA is located chiefly outside the nucleus and can be con-
sidered a “molecular slave” of DNA. ±at is, RNA carries out the
orders for protein synthesis issued by DNA. [Viruses in which
RNA (rather than DNA) is the genetic material are an exception
to this generalization.]
RNA molecules are single strands of nucleotides. RNA bases
include A, G, C, and U (U replaces the T found in DNA), and its
sugar is
ribose
instead of deoxyribose. ±e three major varieties
of RNA (messenger RNA, ribosomal RNA, and transfer RNA)
are distinguished by their relative size and shape, and each has a
specific role to play in carrying out DNA’s instructions for pro-
tein synthesis. In addition to these three RNAs, small RNA mol-
ecules called
microRNAs
(
miRNAs
) appear to control genetic
expression by shutting down genes or altering their expression.
We discuss DNA replication and the relative roles of DNA and
RNA in protein synthesis in Chapter 3.
Check Your Understanding
29.
How do DNA and RNA differ in the bases and sugars they
contain?
30.
What are two important roles of DNA?
For answers, see Appendix H.
Adenosine Triphosphate (ATP)
Explain the role of ATP in cell metabolism.
Glucose is the most important cellular fuel, but none of the
chemical energy contained in its bonds is used directly to power
cellular work. Instead, energy released during glucose catab-
olism is coupled to the synthesis of
adenosine triphosphate
(ATP)
. In other words, some of this energy is captured and
stored as small packets of energy in the bonds of ATP. ATP is the
primary energy-transferring molecule in cells and it provides a
form of energy that is immediately usable by all body cells.
Structurally, ATP is an adenine-containing RNA nucleotide
to which two additional phosphate groups have been added
(Figure 2.23)
. Chemically, the triphosphate tail of ATP can be
compared to a tightly coiled spring ready to uncoil with tre-
mendous energy when the catch is released. Actually, ATP is a
very unstable energy-storing molecule because its three nega-
tively charged phosphate groups are closely packed and repel
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)
Adenosine monophosphate (AMP)
Adenosine
Adenine
Ribose
Phosphate groups
H
H
HO
OH
H
H
CH
2
N
H
H
H
H
O
O
O
O
O
O
O
O
O
O
N
P
P
P
N
N
N
O
High-energy phosphate
bonds can be hydrolyzed
to release energy.
~
~
Figure 2.23
Structure of ATP (adenosine triphosphate).
ATP
is an adenine nucleotide to which two additional phosphate groups
have been attached during breakdown of food fuels. When the
terminal phosphate group is cleaved off, energy is released to do
useful work and ADP (adenosine diphosphate) is formed. When the
terminal phosphate group is cleaved off ADP, a similar amount of
energy is released and AMP (adenosine monophosphate) is formed.
Table 2.4
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