Modern Engineering Thermodynamics

17.3 Thermodynamics of Biological Cells 695
17.3 THERMODYNAMICS OF BIOLOGICAL CELLS
It is unlikely that a single energy source was directly responsible for the synthesis of all the organic molecules on
the newly formed Earth. In recent decades, laboratory experiments with the elements of carbon, hydrogen, oxygen,
and nitrogen have shown that basic organic compounds can be synthesized by a variety of energy sources
under early Earth conditions. Table 17.1 lists an estimate of the energy rate per unit area available on the surface
of the primitive Earth. Though solar radiation was clearly the largest source of energy, the energy contained in
long-wavelength (150 to 200 nm) ultraviolet light is so strong that it decomposes absorbing molecules rather
than building them. However, the water of the primitive Earth’s oceans protected complex organic molecules
from disruptive ultraviolet radiation until the Earth’s ozone layer developed. It was not until this protective
atmospheric layer had developed that life forms could leave the oceans and populate the dry land.
The most widely used source of energy for the synthesis of primitive organic compounds in the laboratory is an
electrical discharge in a mixture of gases. The most common compounds produced by this technique are amino
acids, with yields as high as 5%.
As concentrations of organic compounds built up in the primitive oceans, biological life processes began to
synthesize and replicate molecules. Enzymes and genetic molecules evolved, but reaction rates were limited by
the comparatively low concentrations of these essential building blocks. Specialized molecular barriers then
evolved that completely enclosed small volumes of fluid containing complex molecular machinery. These barriers
are called membranes and the resulting enclosure is called a cell. The purpose of biological membranes is to
maintain concentration differences that would be advantageous to the molecular operation of the cell. To do
this, the membrane must be able to transport certain ions against the concentration gradient (this is called active
transport). This requires that the membrane operate as an energy converter, with some of the internal energy of
the cell being used to maintain the various concentration gradients across the membrane. Table 17.2 lists some
ion concentrations inside and outside common human cells.
Table 17.1 Estimates of Energy Rates Available for the Formation of Simple Organic
Compounds, Averaged over the Surface Area of Primitive Earth
Source
Electric discharge (lightning, etc.) 170
Solar radiation in the 0−150 nm range 71
Thermal quenching of hot gases from
Shock waves from meteors and lightning 46
Volcanoes 5.4
Highly ionizing radiation from
Radioactivity 1.0 km deep in the Earth 33
Solar wind 8.4
Cosmic rays 0.1
Energy Rates
per Unit Area [KJ/(m 2 · a)]
Source: Material drawn from Oró, J., Miller, S. L., Urey, H. C. Energy conversion in the context of the origin of life. In: Buvet, R., Allen, M. J.,
Massué, J.-P. (Eds.), Living Systems as Energy Converters. North-Holland Publishing, 1977, pp. 7–19, New York. Reprinted by permission of
Elsevier Science Publishers (Biomedical Division), Amsterdam, and the authors.
Table 17.2 Approximate Ion Concentration Inside and Outside Human Cells
Concentration in Osmoles per cm 3 of Water
Ion
Outside the Cell
Inside the Cell
Na + 144 14.0
K + 4.1 140
Mg 2+ 1.5 31
Cl – 107 4.00
HCO − 3 27.7 10.0
SO 2 −
4
0.5 1
HPO 2 −
4
,H 2 PO − 4
2.0 11
Note: One osmole is the number of gram moles of the substance that do not diffuse or dissociate in solution. Also, pH outside = 7:4 and
pH inside = 7:0, where the concentration of hydrogen ions ðH + Þ in gmoles/L is10 −pH :