of water molecules is normally in a steady state and a living organism will
only be stressed if there is a net flow out of the cytoplasm, leading to
plasmolysis, or a net flow into the cell leading to rupture of the mem-
brane, and the latter is normally prevented by the presence of a cell wall
in the bacteria and fungi.
In our everyday lives we think of water as existing in its liquid state
between its freezing point (0 1 C) and boiling point (100 1 C) and we might
expect that this would limit the minimum and maximum temperatures at
which growth could possibly occur. But, of course, the freezing point of
water can be depressed by the presence of solutes and there are a number of
micro-organisms which can actively grow at subzero temperatures because
their cytoplasm contains one or more compounds, such as a polyol, which
act as an antifreeze. Similarly the boiling point of water can be elevated by
increased hydrostatic pressure and, in nature, very high pressures exist
at the bottom of the deep oceans. Under these circumstances the temper-
ature of liquid water may be well above 100 1 C and the relatively recent
exploration of submarine volcanic vents has uncovered some remarkable
bacteria which can indeed grow at such high temperatures.
Although the cytoplasm must be in the liquid phase for active growth
(and it is important not to confuse growth and survival, for many
microorganisms can survive but not grow when their cytoplasm has
been completely dried), water in the environment of the living organism
may be present, not only in the liquid phase as pure water or a solution,
but also in the atmosphere in the gaseous phase, or associated with what
would be described macroscopically as the solid phase (Figure 3.6).
Figure 3.6 A cell in equilibrium with liquid, solid and gaseous phases, each of these being in
equilibrium with each other
Chapter 3 37