Page 25 - Color Atlas of Biochemistry

16 Basics

Energetics the amount of matter reacting (in mol).
Although absolute values for free enthalpy G
To obtain a better understanding of the pro- cannot be determined, ∆G can be calculated
cesses involved in energy storage and conver- from the equilibrium constant of the reaction
sion in living cells, it may be useful first to (see p.18).
recall the physical basis for these processes.

B. Energetics and the course of processes
A. Forms of work
Everyday experience shows that water never
There is essentially no difference between flows uphill spontaneously. Whether a partic-
work and energy. Both are measured in joule ular process can occur spontaneously or not
(J = 1 N m). An outdated unit is the calorie depends on whether the potential difference
(1 cal = 4.187 J). Energy is defined as the abil- between the final and the initial state, ∆P=
ity of a system to perform work. There are P 2 –P 1 , is positive or negative. If P 2 is smaller
many different forms of energy—e. g., me- than P 1 ,then ∆Pwill benegative, andthe
chanical, chemical, and radiation energy. process will take place and perform work.
A system is capable of performing work Processesof thistype are called exergonic
when matter is moving along a potential gra- (B1). If there is no potential difference, then
dient. This abstract definition is best under- the system is in equilibrium (B2). In thecaseof
stood by an example involving mechanical endergonic processes, ∆P is positive (B3).
work (A1). Due to the earth’s gravitational Processesof thistype do not proceed sponta-
pull, the mechanical potential energy of an neously.
object is the greater the further the object is Forcing endergonic processes to take place
away from the center of the earth. A potential requires the use of the principle of energetic
difference (∆P) therefore exists between a coupling. This effect can be illustrated by a
higher location and a lower one. In a waterfall, mechanical analogy (B4). When two masses
the water spontaneously follows this poten- M 1 and M 2 are connected by a rope, M 1 will
tial gradient and, in doing so, is able to per- move upward even though this part of the
form work—e. g., turning a mill. processisendergonic. The sum of the two
Work and energy consist of two quantities: potential differences (∆P eff = ∆P 1 + ∆P 2 )is
an intensity factor, which is a measure of the the determining factor in coupled processes.
potential difference—i. e., the “driving force” When ∆P eff is negative, the entire process can
of theprocess—(hereit is the height differ- proceed.
ence) and a capacity factor,which is a mea- Energetic coupling makes it possible to
sure of the quantity of the substance being convert different forms of work and energy
transported (here it is the weight of the into one another. For example, in a flashlight,
water). In the case of electrical work (A2), an exergonic chemical reaction provides an
the intensity factor is the voltage—i. e., the electrical voltage that can then be used for
electrical potential difference between the the endergonic generation of light energy. In
source of the electrical current and the the luminescent organs of various animals, it
“ground,” while the capacity factor is the is a chemical reaction that produces the light.
amount of charge that is flowing. In the musculature (see p. 336), chemical en-
Chemical work and chemical energy are ergy is converted into mechanical work and
defined in an analogous way. The intensity heat energy. A form of storage for chemical
factor here is the chemical potential of a mol- energy that is used in all forms of life is aden-
ecule or combination of molecules. This is osine triphosphate (ATP; see p.122). Ender-
stated as free enthalpy G (also known as gonic processes are usually driven by cou-
“Gibbs free energy”). When molecules spon- pling to the strongly exergonic breakdown
taneously react with one another, the result is of ATP (see p.122).
products at lower potential. The difference in
the chemical potentials of the educts and
products (the change in free enthalpy, 'G)is
ameasure of the “driving force” of the reac-
tion. The capacity factor in chemical work is

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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