Water Vapor and
Other Gases
3
Terrestrial organisms live in a gaseous medium composed mostly of nitrogen and oxygen. Water vapor is present in varying amounts, and carbon
dioxide and other gases, in trace amounts. Organisms exchange oxygen,
carbon dioxide, and water vapor with their surroundings. Carbon dioxide
is a substrate for photosynthesis and oxygen is a product; while oxygen
is a substrate for respiration and carbon dioxide is a product. Exchange
of these gases with the environment is therefore a requirement for life.
Water vapor almost always moves from the organism to the environment.
The humidity of the organism is near 100 percent, while the surroundings are nearly always much drier. The organism must remain in a highly
hydrated state in order for biochemical reactions to occur, so the constant
loss of water is a threat to survival, and frequent access to liquid water is
a necessity for most terrestrial organisms. The intake of liquid water and
the loss of water vapor to the environment are usually the most important
components of the water budget of an organism.
Water loss is generally viewed as detrimental to the organism, though
it may have some benefit in the circulation system of plants. It does,
however, have a definite benefit when we consider the energy balance of
the organism. Environmental temperatures are often higher than can be
tolerated by biological systems. If there were not some mechanism for
cooling the organism, it would perish. As water evaporates, roughly 44
kilojoules of energy are required to convert each mole to the vapor state.
This is called the latent heat of vaporization. It is 580 times the energy
required to change the temperature of one mole of water by one Celsius
degree, and therefore represents an enormous sink for energy in the organism environment. Evaporative cooling is a natural way of controlling
temperatures of organisms in hot environments. The amount of cooling
available to the organism depends on the concentrations of water vapor
at the organism surface and in the environment, and on the conductance
to water vapor of the organism surface and boundary layer. This chapter discusses terminology for specifying gas concentrations and provides
information about concentrations of gases in terrestrial environments.
Other Gases
3
Terrestrial organisms live in a gaseous medium composed mostly of nitrogen and oxygen. Water vapor is present in varying amounts, and carbon
dioxide and other gases, in trace amounts. Organisms exchange oxygen,
carbon dioxide, and water vapor with their surroundings. Carbon dioxide
is a substrate for photosynthesis and oxygen is a product; while oxygen
is a substrate for respiration and carbon dioxide is a product. Exchange
of these gases with the environment is therefore a requirement for life.
Water vapor almost always moves from the organism to the environment.
The humidity of the organism is near 100 percent, while the surroundings are nearly always much drier. The organism must remain in a highly
hydrated state in order for biochemical reactions to occur, so the constant
loss of water is a threat to survival, and frequent access to liquid water is
a necessity for most terrestrial organisms. The intake of liquid water and
the loss of water vapor to the environment are usually the most important
components of the water budget of an organism.
Water loss is generally viewed as detrimental to the organism, though
it may have some benefit in the circulation system of plants. It does,
however, have a definite benefit when we consider the energy balance of
the organism. Environmental temperatures are often higher than can be
tolerated by biological systems. If there were not some mechanism for
cooling the organism, it would perish. As water evaporates, roughly 44
kilojoules of energy are required to convert each mole to the vapor state.
This is called the latent heat of vaporization. It is 580 times the energy
required to change the temperature of one mole of water by one Celsius
degree, and therefore represents an enormous sink for energy in the organism environment. Evaporative cooling is a natural way of controlling
temperatures of organisms in hot environments. The amount of cooling
available to the organism depends on the concentrations of water vapor
at the organism surface and in the environment, and on the conductance
to water vapor of the organism surface and boundary layer. This chapter discusses terminology for specifying gas concentrations and provides
information about concentrations of gases in terrestrial environments.