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processes within the body, e.g., in the reaction stances is lower than that of ATP, but still rela-
+
–
CO 2 + H 2O HCO 3 + H . In most cases (e.g. tively high.
metabolic pathways, ion gradients), only a The free energy liberated upon hydrolysis of
steady state is reached. Such metabolic path- ATP is used to drive hundreds of reactions
ways are usually irreversible due, for example, within the body, including the active trans-
to excretion of the end products. The thought membrane transport of various substances,
of reversing the “reaction” germ cell ! adult il- protein synthesis, and muscle contraction. Ac-
lustrates just how impossible this is. cording to the laws of thermodynamics, the
At steady state, the rate of a reaction is more expenditure of energy in all of these reactions
important than its equilibrium. The regulation leads to increased order in living cells and,
of body functions is achieved by controlling re- thus, in the organism as a whole. Life is there-
action rates. Some reactions are so slow that it fore characterized by the continuous reduc-
is impossible to achieve a sufficient reaction tion of entropy associated with a correspond-
rate with enzymes or by reducing the concen- ing increase in entropy in the immediate en-
tration of the reaction products. These are vironment and, ultimately, in the universe.
therefore endergonic reactions that require Energy Production and Metabolism
the input of outside energy. This can involve
“activation” of the educt by attachment of a
high-energy phosphate group to raise the Pe.
ATP (adenosine triphosphate) is the univer-
sal carrier and transformer of free enthalpy
within the body. ATP is a nucleotide that
derives its chemical energy from energy-rich
nutrients (! C). Most ATP is produced by oxi-
dation of energy-rich biological molecules
such as glucose. In this case, oxidation means
the removal of electrons from an electron-rich
(reduced) donor which, in this case, is a carbo-
hydrate. CO 2 and H 2O are the end products of
the reaction. In the body, oxidation (or electron
transfer) occurs in several stages, and a portion
of the liberated energy can be simultaneously
used for ATP synthesis. This is therefore a
coupled reaction (! C and p. 17 B). The stan-
dard free enthalpy ∆G of ATP hydrolysis,
0
ATP ADP + P i [1.31]
– 1
is – 30.5 kJ ! mol . According to Eq. 1.27, the
∆G of reaction 1.31 should increase when the
ratio ([ADP] ! [P i)]/[ATP] falls below the equi-
librium constant K eq of ATP hydrolysis. The fact
that a high cellular ATP concentration does
indeed yield a ∆G of approximately – 46 to
– 54 kJ ! mol – 1 shows that this also applies in
practice.
0
Some substances have a much higher∆G of
hydrolysis than ATP, e.g., creatine phosphate
(– 43 kJ ! mol ). These compounds react with
– 1
ADP and P i to form ATP. On the other hand, the
energy of ATP can be used to synthesize other
compounds such as UTP, GTP and glucose-6-
41
phosphate. The energy content of these sub- 41
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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