HEAT OF REACTION:
1.
2.
Heats of reaction for a chemical reaction A  B may be calculated by using
the heats of formation, heats of combustion, Gibbs free energy or any
other consistent thermodynamic data for A and B since the calculation is
dependent on reactant and product and not the route of the reaction.
Biological systems follow the same thermodynamic laws. The reaction for
the biological system is:
Carbon Source(s) + Energy Source(s) + Nutrient(s)
=
Biomass + End Products
In theory, if the heats of formation, heats of combustion, Gibbs free
energy or any other consistent thermodynamic data for these
Reactants and Products are available, then calculating the heat of
reaction would be a routine calculation.
However there are a number of “complicating factors”:
a. Heats of formation for biomass is difficult (if not impossible to
estimate) and /or not available routinely (since, it is defined as
the heat released or absorbed (enthalpy change) during the
formation of a pure substance from its elements, at constant
pressure).
b. Thermodynamic data often is restricted to pure substances and
biological reactions do not utilize pure substances but rather
dilute mixtures of substances. Biological databanks do exist but
they are not widely available for substances used in typical
biological reactions and in the concentrations ranges normally
encountered.
c. Group contribution methods have been used to estimate the
heats of combustion
d. Heat of mixing an be used in conjunction with other
thermodynamic data to estimate the heat of combustion at the
concentrations normally experienced in biological systems.
3. Heat of combustion is the commonly used thermodynamic property for
these calculations, since it is possible to use a calorimeter to measure
the heat of combustion of biomass and all other carbon sources,
energy sources, nutrient sources and products at any concentration.
4. The heat of combustion of a cell is about 19-22 kJ/ g
5. There exists a relationship between the specific oxygen uptake rate
and the heat evolved during a biochemical reaction. This
relationship is very important, since:
a. We are able to calculate (from previous lectures) both the
catabolic oxygen uptake and the anabolic uptake rate for
biochemical reactions involving oxygen uptake and the
“equivalent specific oxygen uptake rate” for biochemical
reactions involving other electron acceptors. This allows us to
use the relationship in all biochemical reactions.
b. The relationship should been seen as consistent with our
previous studies on metabolism since:
i. If oxygen is uptaken, then NAD(H) is being regenerated
to NAD+ (QO2 =  QNAD(H) ) 
ii. If NAD(H) is used then ATP is being made by some
stoichiometry (QNAD(H) =  QATP)
iii. ATP is utilized by the cell at an almost constant efficiency
of 30-32% (QATP(used for cell growth) =  QATP)
iv. Hence, the remaining ATP is wasted as heat (QATP(used
for heat generation) = QHEAT = (1-) (QATP))
v. Consequently, we would expect there to be a relationship
between specific oxygen uptake and heat evolution (=
QO2 *( QNAD(H) / QO2)*(  QATP/ QNAD(H))* (1-)*QATP
=
QO2 * * * (1-)
=
QHEAT
Or QO2 = constant * QHEAT