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Modern Engineering Thermodynamics

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17.3 <strong>Thermodynamics</strong> of Biological Cells 695<br />

17.3 THERMODYNAMICS OF BIOLOGICAL CELLS<br />

It is unlikely that a single energy source was directly responsible for the synthesis of all the organic molecules on<br />

the newly formed Earth. In recent decades, laboratory experiments with the elements of carbon, hydrogen, oxygen,<br />

and nitrogen have shown that basic organic compounds can be synthesized by a variety of energy sources<br />

under early Earth conditions. Table 17.1 lists an estimate of the energy rate per unit area available on the surface<br />

of the primitive Earth. Though solar radiation was clearly the largest source of energy, the energy contained in<br />

long-wavelength (150 to 200 nm) ultraviolet light is so strong that it decomposes absorbing molecules rather<br />

than building them. However, the water of the primitive Earth’s oceans protected complex organic molecules<br />

from disruptive ultraviolet radiation until the Earth’s ozone layer developed. It was not until this protective<br />

atmospheric layer had developed that life forms could leave the oceans and populate the dry land.<br />

The most widely used source of energy for the synthesis of primitive organic compounds in the laboratory is an<br />

electrical discharge in a mixture of gases. The most common compounds produced by this technique are amino<br />

acids, with yields as high as 5%.<br />

As concentrations of organic compounds built up in the primitive oceans, biological life processes began to<br />

synthesize and replicate molecules. Enzymes and genetic molecules evolved, but reaction rates were limited by<br />

the comparatively low concentrations of these essential building blocks. Specialized molecular barriers then<br />

evolved that completely enclosed small volumes of fluid containing complex molecular machinery. These barriers<br />

are called membranes and the resulting enclosure is called a cell. The purpose of biological membranes is to<br />

maintain concentration differences that would be advantageous to the molecular operation of the cell. To do<br />

this, the membrane must be able to transport certain ions against the concentration gradient (this is called active<br />

transport). This requires that the membrane operate as an energy converter, with some of the internal energy of<br />

the cell being used to maintain the various concentration gradients across the membrane. Table 17.2 lists some<br />

ion concentrations inside and outside common human cells.<br />

Table 17.1 Estimates of Energy Rates Available for the Formation of Simple Organic<br />

Compounds, Averaged over the Surface Area of Primitive Earth<br />

Source<br />

Electric discharge (lightning, etc.) 170<br />

Solar radiation in the 0−150 nm range 71<br />

Thermal quenching of hot gases from<br />

Shock waves from meteors and lightning 46<br />

Volcanoes 5.4<br />

Highly ionizing radiation from<br />

Radioactivity 1.0 km deep in the Earth 33<br />

Solar wind 8.4<br />

Cosmic rays 0.1<br />

Energy Rates<br />

per Unit Area [KJ/(m 2 · a)]<br />

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.,<br />

Massué, J.-P. (Eds.), Living Systems as Energy Converters. North-Holland Publishing, 1977, pp. 7–19, New York. Reprinted by permission of<br />

Elsevier Science Publishers (Biomedical Division), Amsterdam, and the authors.<br />

Table 17.2 Approximate Ion Concentration Inside and Outside Human Cells<br />

Concentration in Osmoles per cm 3 of Water<br />

Ion<br />

Outside the Cell<br />

Inside the Cell<br />

Na + 144 14.0<br />

K + 4.1 140<br />

Mg 2+ 1.5 31<br />

Cl – 107 4.00<br />

HCO − 3 27.7 10.0<br />

SO 2 −<br />

4<br />

0.5 1<br />

HPO 2 −<br />

4<br />

,H 2 PO − 4<br />

2.0 11<br />

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<br />

pH inside = 7:0, where the concentration of hydrogen ions ðH + Þ in gmoles/L is10 −pH :

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