Page 51 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Energy Production and Metabolism Heat is transferred in all chemical reactions.
The amount of heat produced upon conversion
Energy is the ability of a system to perform of a given substance into product X is the same,
work; both are expressed in joules (J). A poten- regardless of the reaction pathway or whether
tial difference (potential gradient) is the so- the system is closed or open, as in a biological
called driving “force” that mobilizes the matter system. For caloric values, see p. 228.
involved in the work. Water falling from height Enthalpy change (∆H) is the heat gained or
X (in meters) onto a power generator, for ex- lost by a system at constant pressure and is re-
ample, represents the potential gradient in
lated to work, pressure, and volume (∆H = ∆U
Fundamentals and Cell Physiology formed can be determined by multiplying the ∆H is positive in endothermic reactions. The
+ p ! ∆V). Heat is lost and ∆H is negative in ex-
mechanical work. In electrical and chemical
work, potential gradients are provided respec-
othermic reactions, while heat is gained and
tively by voltage (V) and a change in free en-
– 1
thalpy ∆G (J ! mol ). The amount of work per-
second law of thermodynamics states that the
total disorder (randomness) or entropy (S) of a
potential difference (intensity factor) by the
closed system increases in any spontaneous
process, i.e., entropy change (∆S) # 0. This
corresponding capacity factor. In the case of
must be taken into consideration when at-
the water fall, the work equals the height the
tempting to determine how much of ∆H is
water falls (m) times the force of the falling
water (in N). In the other examples, the
freely available. This free energy or free en-
times the amount of charge (C). Chemical work
chemical reaction. The heat produced in the
performed = ∆G times the amount of sub-
process is the product of absolute temperature
1 amount work performed equals the voltage (V) thalpy (∆G) can be used, for example, to drive a
and entropy change (T · ∆S).
stance (mol).
Living organisms cannot survive without an Free enthalpy (∆G) can be calculated using
adequate supply of energy. Plants utilize solar the Gibbs-Helmholtz equation:
energy to convert atmospheric CO 2 into oxy- ∆G ! ∆H – T · ∆S. [1.24]
gen and various organic compounds. These, in ∆G and ∆H are approximately equal when ∆S
turn, are used to fill the energy needs of approaches zero. The maximum chemical
humans and animals. This illustrates how work of glucose in the body can therefore be
energy can be converted from one form into determined based on heat transfer, ∆H,
another. If we consider such a transformation measured during the combustion of glucose in
taking place in a closed system (exchange of a calorimeter (see p. 228 for caloric values).
energy, but not of matter, with the environ- Equation 1.24 also defines the conditions
ment), energy can neither appear nor disap- under which chemical reactions can occur. Ex-
pear spontaneously. In other words, when ergonic reactions (∆G $ 0) are characterized
energy is converted in a closed system, the by the release of energy and can proceed spon-
total energy content remains constant. This is taneously, whereas endergonic reactions (∆G
described in the first law of thermodynamics, # 0) require the absorption of energy and are
which states that the change of internal energy not spontaneous. An endothermic reaction
(= change of energy content, ∆U) of a system (∆H # 0) can also be exergonic (∆G $ 0) when
(e.g. of a chemical reaction) equals the sum of the entropy change ∆S is so large that ∆H–
the work absorbed (+W) or performed (–W) by T · ∆S becomes negative. This occurs, for ex-
a system and the heat lost (–Q) or gained (+Q) ample, in the endothermic dissolution of crys-
by the system. This is described as: talline NaCl in water.
∆U ! heat gained (Q) " work performed Free enthalpy, ∆G, is a concentration-de-
(W) [J] and [1.22] pendent variable that can be calculated from
∆U ! work absorbed (W) " heat lost the change in standard free enthalpy (∆G ) and
0
(Q) [J]. [1.23] the prevailing concentrations of the sub-
0
(By definition, the signs indicate the direction stances in question. ∆G is calculated assum-
of flow with respect to the system under con- ing for all reaction partners that concentration
38 sideration.) = 1 mol/L, pH = 7.0, T = 298 K, and p = 1013 hPa.
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.