On Wed, 15 Feb 1995 ctfaulkn at UTKVX.UTCC.UTK.EDU wrote:
>> I've never had any problem with the operational definition of
> parasitisim as an obligate relationship where one organism is metabolically
> dependent on another. However, I have had problems operationalizing
> the fine-line distinction between commensalism and parasitism. It seems
> that those using parasite always equate it with harm to the host. Is this
> necessarily true? Just wondering.
The issue raised by these last comments are, I believe, quite
important to the discussion. The concept of taking from the net
resources of the host does not specify energy (ie, ATP or ADP) but maybe
it should. However, permit me a brief digression. Most text books on
the subject indicate that the field of parasitology deals with protozoans
and helminths (worms). Many texts include chapters on insects of medical
importance (ie, vectors) and occasionally things like stinging animals
which range from bees, wasps and scorpions to jellyfish. They then go to
say this is completely arbitrary and there is no logical basis to exclude
intracellular bacteria and viruses (not to mention fungi). <end of
digresion>.
I have tried since my first undergraduate course in parasitology
to think philosophically on this question. I have decided that energy
and fossil fuels are interconvertible to the economy of human beings. We
first and foremost require food for engery and all the rest of things
filling biochemistry courses. We work or use fossil fuels to make things
that allow us to earn money to buy foods and thus we arbitrarily convert
the values of the two. Remember that the cost of transporting beef to
market increased so much after the Arab oil embargo of 1973 that we could
no longer afford to buy fetal calf serum for research because many
farmers could not afford the increased costs of keeping their cows. This
then led me to the belief that parasites make demands on their host not
necessarily in the form of robbing ATP in the direct sense, but in the
indirect sense that even having to physically carry these intruders
around assuming they could be photosynthetic and make all their own
nutrients, they were parasites. The best example of this that I can
think of is a plant indigenous to the southern part of the US known as
Spanish Moss. All it wants from its host is to be held up to the
sunlight. However, after a time, the parasite overshadows the leaves and
host does in fact, die. Also, Spanish moss can live on telephone wires
and do just as well. Telephone wires aren't hosts <are they?>.
Anyway, I believe that any movement of energy as it pertains to
the three laws of thermodynamics that is influenced in one organism by
another (read that diverted from one to another) is a parasite. The
question raised by Mr. Faulkner about commensalism and its practioners is
only a matter of degree. We say that the equation in this case is that
commensals do not have a net negative effect on the energy economy of the
host (the example of organisms in the termite intestine). What we really
mean is that the typical host parasite system is not deleterious but this
begs the question because the host still expends its energy carrying
around the commensal population. Even if the host derives the use of
cellulose breakdown products the host expended its ATP currency to do so.
I realize this does not answer the question but my demand to wax
philosophical has been fulfilled for tonight.
Steve Kayes, parasite of my parents
