Other common units of pressure are the bar and Torr. The bar is equal
to 100,000 Pa and so is very close to 1 atm:
1 atm =101.325 kPa =1.01325 bar (1.9)
The Torr represents the pressure required to push a column of mercury
up by 1 mm and can be converted to Pascals using:
1 Torr =133.325 Pa (1.10)
Due to the range of units available, standard pressure can be expressed
as any of several equivalent values and units (Table 1.1).
Scientists make use of a certain terminology when expressing measures
of pressure. Atmospheric pressure is the pressure of the air surrounding
us. Barometric pressure is the same as atmospheric pressure but this term
is used in reference to the use of a barometer to measure the atmospheric
pressure. The value of pressure can be expressed using different references.
The gauge pressure is the pressure measured relative to the atmospheric
pressure. Alternatively, the absolute pressure is relative to a complete
vacuum, or zero pressure. Hence, the absolute pressure is equal to the sum
of the gauge pressure and the barometric pressure.
For biological systems, the only relevant pressure is normally 1 atm,
so why would you be concerned about pressure? Whereas most living
systems are found on the surface of the planet, life is also found at the
bottom of the oceans centered around deep-sea vents. The initial discovery
was made in 1977 by scientists who were examining the hydrothermal
fluids of seafloor vents using the submersible ALVIN. Deep-sea vents are
regions where there is volcanic activity. Water can seep into cracks and
reach temperatures as high as 400°C. The hydrothermal vent carries
dissolved minerals and is cooled rapidly upon emergence from the vent.
4 CHAPTER 1 BASIC THERMODYNAMIC AND BIOCHEMICAL CONCEPTS
Table 1.1
Standard pressure in various units.
1.000 atmosphere (atm)
33.899 feet of water
14.696 pounds/square inch absolute (psia)
29.92 inches of mercury (inHg)
760.0 millimeters of mercury (mmHg)
1.013 × 105 Pascals (Pa)
1.013 × 105 Newton/square meters (N m−^2 )
1.01325 bar
