Modern Engineering Thermodynamics

694 CHAPTER 17: Thermodynamics of Biological Systems
an unusual phenomenon. Fairly complex living microscopic creatures existed on Earth within a few hundred
million years of its formation.
There is no clear-cut understanding of the scientific concept of life. Living systems have six recognized characteristics:
(1) molecular organization, (2) metabolism, (3) growth, (4) adaptation, (5) response to stimuli, and (6)
reproduction. But perhaps the best definition available today is that a system is said to be “living” if it sustains
its low-entropy molecular complexity (i.e., DNA) that contains hereditary information transmitted to offspring
on reproduction, by a metabolic energy transport from a high-energy source (food) to a low-energy sink (waste)
via catalytic macromolecules, called enzymes.
The different living systems on Earth have numerous items in common. For example, they all use the same class
of molecules for energy storage, the nucleotide phosphates. Also, of the billions of chemically possible organic
compounds, only about 1500 are actually used by living systems. And all these 1500 compounds are made with
less than 50 simpler molecular building blocks, utilizing no more than 24 of the available elements. Hydrogen
atoms make up 63% of all the atoms in the human body. Oxygen accounts for 25.5%, carbon 9.50%, nitrogen
1.40%, and the 20 remaining elements essential for mammalian life account for only about 0.60%. Only 3 of
the 24 elements known to be essential to life on this planet have atomic numbers greater than 34, and these 3
are needed in only trace amounts. Thus, since living systems are made up of the simplest atomic elements, they
can be expected to develop early on any planet that has the proper environmental conditions.
Living systems are organized around a cell structure of some kind. A cell is like a small factory whose main function
is to carry out its metabolic process, and the cell boundaries appear to exist to provide the high enzyme concentration
necessary for efficient metabolism. The smallest free-living cell known today is a pleuropneumonialike
organism that has a mass of only about 5 × 10 –19 kg and has a diameter of only about 10 –7 m (about one fifth of
the wavelength of visible light). This cell contains about 100 enzymes and can be seen only with a high-powered
electron microscope. The human body contains about 10 14 cells with an average diameter of 10 –5 m. Each cell
typically consists of a central nucleus, with the remaining material being the cytoplasm (see Figure 17.1). The chemical
activity inside the cell is very high, with each enzyme entering into the synthesis of about 100 molecules per
second.
The oldest remnants of life on Earth are cellular microfossils that are over 3.5 billion years old. Since the age of
the Earth is only about 4.5 billion years, the thermal and chemical requirements for the evolution of living systems
must have developed remarkably fast. All known living systems on Earth are water based and therefore
cannot exist far outside the temperature range from 0 to 100°C. It is amazing that the surface of the Earth had
regions in this temperature range for at least 80% of its existence.
WHAT IS METABOLISM?
Metabolism is the name given to the processes of breakdown and synthesis of large macromolecules within a cell.
Nucleus
(contains chromosomes)
Cell membrane
Vacuoles
Mitochondria
Lysosome
FIGURE 17.1
Schematic of a typical living cell.