Modern Engineering Thermodynamics

17.4 Energy Conversion Efficiency of Biological Systems 699
Most of the various cellular processes that require energy
use adenosine triphosphate (ATP) as the energy source.
This compound has about 33 MJ/kgmole of energy
stored in each of two phosphate bonds. When these
bonds are split by enzyme action to form adenosine
diphosphate (ADP), their energy is then made available
for other uses. The cell contains many enzymes that can
catalyze the splitting of the ATP bonds and utilize the
liberated energy.
Energystoragereactionswithinthecell,ontheother
hand, are limited to two basic types: photosynthetic
(in plant cells), wherein incoming light is used as the
energy source, and metabolism (in animal cells),
wherein the food brought into the cell (generally glucose
and molecular oxygen) is utilized to reconstitute
ATP from ADP, with the production of carbon dioxide
and water waste products, which must be expelled
from the cell. Figure 17.3 shows how these two energy
transport mechanisms are linked together in the life
cycle, and Figure 17.4 illustrates the ATP–ADP cycle.
An open system entropy rate balance applied to a
living cell gives
_Q
+∑ _ms −∑
T b in out
_ms + _S P =
dS
dt
where T b is the temperature of the cell boundary
(assumed isothermal here). Because the metabolic
heat must leave the cell for it to survive, we know that
_Q < 0: Also, _S P > 0 due to the irreversibilities of the life
cell
process within the cell. Since food products are brought into the cell and waste products expelled,
∑ out
_me >∑ in
_me (as these two flow streams are at the same temperature and the molecular order of the waste
material is less than that of the food). For a cell to grow and continue to maintain its elaborate internal molecular
order, we must have ðdS/dtÞ cell < 0, or
∑ _ms −∑ _ms + _Q/T
b
out
≥ _S P
in
Photosynthesis
Carbon dioxide
and water
Carbohydrate
Oxygen
Aerobic
metabolism
FIGURE 17.3
Energy transport mechanisms in living systems.
Oxygen and
glucose
Respiration
Carbon dioxide
and water
FIGURE 17.4
The ATP–ADP cycle.
which is perfectly reasonable so long as the cell remains alive (i.e., _Q < 0 and ∑ in
_me