BioPHYSICAL chemistry

The sudden decrease in pressure causes
a rapid expansion of the volume of the
gas. Since the expansion is competing
against the attraction of the gas mole-
cules for each other, energy is lost by the
expansion. This energy loss effectively
decreases the temperature of the gas
molecules. For most situations, the tem-
peratures at which gases of other mole-
cules become liquid are much lower
than room temperature, hence repeated
cycles of expansion must be used. Once
a liquid gas is achieved, it serves as a
valuable low-temperature bath that can
be used to cool samples; for example,
the temperatures of 77 and 4.2 K are
obtained with the use of liquid nitrogen
and helium, respectively, at 1 atm. The
availability of an experimental means to
poise samples at low temperatures was
key to the development of many of the
spectroscopic techniques used to charac-
terize biological samples.

Molecular basis for life

Living organisms are composed of many different kinds of molecules. These
molecules are used to extract energy from the environment and the energy
is used to build intricate structures and perform various types of work.
Unlike inanimate objects, living organisms can perform self-replication and
self-assembly. These functions are performed by components that contribute
to the entire ensemble as part of a larger, coordinated program for repro-
duction and perpetuation. The complexity of the components is usually
reflective of the domain to which an organism belongs, with prokaryotic cells
not possessing the complex architecture found in eukaryotic cells.
For all cells, energy is a key aspect, as the ability to grow and reproduce
depends upon a constant supply of energy. Thermodynamics provides the
framework for understanding how the energy in the form of sunlight or
nutrients can be utilized by the cell to perform mechanical work, syn-
thesize compounds, or be converted into heat. In some cases, a process
may be energetically favorable but too slow to be useful, as can be under-
stood from a consideration of the factors that control the kinetics. Since
many reactions involve bond rearrangements or the transfer of electrons,
the atomic properties influence the reactions as explained by quantum

CHAPTER 1 BASIC THERMODYNAMIC AND BIOCHEMICAL CONCEPTS 13

Heat

exchanger

Compressor

Gas input

Liquid out

Liquefied gas

Throttle

Returning gas

Figure 1.4Gases

can be liquefied

by use of pressure

changes and heat

exchangers.