Liquid Water in
Organisms and
their Environment 4
Almost all of the water in living organisms is liquid, rather than vapor.
In addition, water is taken up from the organism environment mainly in
the liquid phase. Good physical descriptions of water in the liquid phase
are necessary to understand liquid-phase water exchange and organism
response. The energy state of liquid water can also affect the vapor pressure and concentration of water at evaporating surfaces. Vapor exchange
is therefore also influenced by the state of the liquid water.
4.1 Water Potential and Water Content
Two types of variables are required to describe the state of matter or
energy. One describes the amount, while the other describes the quality
or intensity. For example, the thermal state of a substance is described
in terms of its heat content and its temperature. While the two variables
are related (the higher the temperature of a substance, the higher the heat
content), they are not equivalent. The heat content depends on the mass,
specific heat, and temperature of the substance, and gives no indication of
which direction heat will flow ifthe object is placed in contact with another
object. The temperature of an object specifies the intensity or quality of
the heat, and temperature differences relate directly to direction and rate
of heat flow.
Variables like temperature, which describe intensity of quality are
called intensive variables, while variables describing amount are called
extensive variables. Temperature is intensive while heat is extensive. To
refer to thermal time as "heat units" is a confusion of intensive and
extensive variables.
Describing the state of water in a system also requires the use of intensive and extensive variables. The extensive variable is familiar to most,
and is called the water content. The intensive variable, called the water
potential, is less familiar. Like temperature, it determines the direction
and rate of water flow. As with temperature, there is often a relationship
between the water content and the water potential of a substance, though
it is always much more complicated than is the case for temperature. The
important thing to realize is that water in soil, or in the tissues of living
Organisms and
their Environment 4
Almost all of the water in living organisms is liquid, rather than vapor.
In addition, water is taken up from the organism environment mainly in
the liquid phase. Good physical descriptions of water in the liquid phase
are necessary to understand liquid-phase water exchange and organism
response. The energy state of liquid water can also affect the vapor pressure and concentration of water at evaporating surfaces. Vapor exchange
is therefore also influenced by the state of the liquid water.
4.1 Water Potential and Water Content
Two types of variables are required to describe the state of matter or
energy. One describes the amount, while the other describes the quality
or intensity. For example, the thermal state of a substance is described
in terms of its heat content and its temperature. While the two variables
are related (the higher the temperature of a substance, the higher the heat
content), they are not equivalent. The heat content depends on the mass,
specific heat, and temperature of the substance, and gives no indication of
which direction heat will flow ifthe object is placed in contact with another
object. The temperature of an object specifies the intensity or quality of
the heat, and temperature differences relate directly to direction and rate
of heat flow.
Variables like temperature, which describe intensity of quality are
called intensive variables, while variables describing amount are called
extensive variables. Temperature is intensive while heat is extensive. To
refer to thermal time as "heat units" is a confusion of intensive and
extensive variables.
Describing the state of water in a system also requires the use of intensive and extensive variables. The extensive variable is familiar to most,
and is called the water content. The intensive variable, called the water
potential, is less familiar. Like temperature, it determines the direction
and rate of water flow. As with temperature, there is often a relationship
between the water content and the water potential of a substance, though
it is always much more complicated than is the case for temperature. The
important thing to realize is that water in soil, or in the tissues of living