My understanding is that bound water is the water that associates with
cellular components so tightly that it is difficult if not nearly impossible
to remove. These waters are strongly hydrogen bonded to the various
nitrogen and oxygen components in the cells. This bond is so tight as to
make the water appear immobile (essentially frozen). To remove this water
from the cellular component you would have to heat it, but this water can be
bound so tightly that the amount of heat you would have to use can threaten
the denaturation of the component itself.
This process is common in chemistry and serves as one of my favorite
examples of entropy. When you add acid to water the water heats up. The
reason that this happens is that the water molecules bind very tightly to
the H+ ions released by the acid. The water becomes so tightly held that it
essentially *frozen* in place. In order to freeze water (for real) you must
remove heat. When the water *freezes* to the H+ ion it must also release
heat and it is this heat that warms up the water. The entire process is
driven by entropy. You are essentially taking liquid water and turning into
solid water (ice) by removing heat. The same sort of processes are occuring
in the cell, and it takes a lot of heat to remove this bound water once it
has frozen itself to a cellular component.
This, of course, differs markedly from the behaviour of pure water or water
with low cellular or salt content. This water is essentially free to roam
about as a liquid unbound to anything but other water molecules. It is this
freedom to move about that marks the greatest difference between bound and
unbound water.
Steven Fawl
Chemistry Dept.
Napa Valley College