Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 25

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CHAPTE R 14? Entropy and Free Energy

14.6 ? Thermodynamics in Living Systems
Many biochemical reactions have a positive ?G° value, yet they are essential to the maintenance
of life. In living systems, these reactions are coupled to an energetically favorable process, one
that has a negative ?G° value. The principle of coupled reactions is based on a simple concept:
we can use a thermodynamically favorable reaction to drive an unfavorable one. Suppose, for
example, that we want to extract zinc from a zinc sulfide (ZnS). The following reaction will not
work because it has a large positive ?G° value:
Zn(s) + S(s)?????G° = 198.3 kJ/mol

ZnS(s)

On the other hand, the combustion of sulfur to form sulfur dioxide is favored because of its large
negative ?G° value:
S(s) + O2(g)

SO2(g)?????G° = –300.1 kJ/mol

By coupling the two processes, we can bring about the separation of zinc from zinc sulfide. In
practice, this means heating ZnS in air so that the tendency of S to form SO2 will promote the
decomposition of ZnS:
ZnS(s)

Zn(s) + S(s)

?G° = 198.3 kJ/mol

S(s) + O2(g)

SO2(g)

?G° = –300.1 kJ/mol

ZnS(s) + O2(g)

Zn(s) + SO2(g)

?G° = –101.8 kJ/mol

Coupled reactions play a crucial role in our survival. In biological systems, enzymes facilitate a wide variety of nonspontaneous reactions. In the human body, for example, food molecules,
represented by glucose (C6H12O6), are converted to carbon dioxide and water during metabolism,
resulting in a substantial release of free energy:
C6H12O6(s) + 6O2(g)

6CO2(g) + 6H2O(l)?????G° = –2880 kJ/mol

In a living cell, this reaction does not take place in a single step; rather, the glucose molecule is
broken down with the aid of enzymes in a series of steps. Much of the free energy released along
the way is used to synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP)
and phosphoric acid (Figure 14.6):
ADP + H3PO4

ATP + H2O?????G° = 31 kJ/mol

The function of ATP is to store free energy until it is needed by cells. Under appropriate conditions,
ATP undergoes hydrolysis to give ADP and phosphoric acid, with a release of 31 kJ/mol of free
energy, which can be used to drive energetically unfavorable reactions, such as protein synthesis.
Proteins are polymers made of amino acids. The stepwise synthesis of a protein molecule
involves the joining of individual amino acids. Consider the formation of the dipeptide (a unit
composed of two amino acids) alanylglycine from alanine and glycine. This reaction represents
the first step in the synthesis of a protein molecule:
alanine + glycine

alanylglycine?????G° = 29 kJ/mol

Figure 14.6 Structures of ATP and ADP.
N
O
?O

P
?O

O

O
O

P
?O

O

HC

P

C

OCH2

?O

N
H

H
HO
Adenosine triphosphate
(ATP)

bur11184_ch14_570-603.indd 594

NH2

NH2
C
C

N

N

CH
?O

O

P
?O

H
H
OH

O

O

N

O

HC

P

OCH2

?O

C

N
H

C
C

CH
N

O
H

H
HO

N

H
OH

Adenosine diphosphate
(ADP)

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