BioPHYSICAL chemistry

When ions from different salts are dissolved together, the ionic strength
is calculated by combining all of the contributions of the ions.

Adenosine triphosphate

In living organisms, the energy released by the oxidation of nutrients is
stored as adenosine triphosphate(ATP; Figure 6.8). The history of ATP and
its utilization in living cells has been fairly controversial at times. The ATP
molecule was first isolated in 1929 and biochemical studies in the 1940s
showed that ATP was utilized in oxidative phosphorylation. The usefulness
of ATP lies in its ability to convert to adenosine diphosphate(ADP), with the
loss of the terminal phosphate through hydrolysis, producing inorganic
phosphate(Pi):

ATP +H 2 O ↔ADP +Pi+H 3 O+ (6.16)

The reaction is strongly exothermic, with a 30.5 kJ mol−^1 release of energy
under normal biological conditions. The ADP–phosphate bond is some-
times termed a high-energy bond to identify the strong tendency to undergo
the reaction. This term is not meant to imply that the bonds are unstable,
as they represent normal chemical bonds with a bond dissociation energy
of about 500 kJ mol−^1. Rather, the bond is described as a high-energy bond
because chemical reactions in which the bonds are hydrolyzed by water
have a negative energy change, making the process spontaneous. As an
example, the breakdown of foods begins with the oxidation of glucose
into glucose 6-phosphate in an endothermic process (see Chapter 2). This
endothermic process, with ΔG°=+13.8 kJ mol−^1 , can be driven by coupling
it to the highly exergonic ATP reaction, resulting in a favorable overall
Gibbs energy change:

Glucose +ATP ↔glucose 6-phosphate +ATP (6.17)
ΔG°=13.8 −30.5 kJ mol−^1 =−16.7 kJ mol−^1

Figure 6.8The chemical structure of adenosine triphosphate, ATP, with
the terminal phosphate group shaded.

CHAPTER 6 REDOX REACTIONS AND BIOENERGETICS 123

CH 2 O

HH

H

OH OH

Ribose Adenine

H

O

O

OOP

O

O

O P

O

O

O P

N

C

C

HC

CH

N

N

NH 2

N

P P P