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From: lonetto@phri.UUCP (Michael Lonetto)
Newsgroups: talk.origins,net.bio
Subject: Re: What's this LIFE stuff?
Message-ID: <2446@phri.UUCP>
Date: Wed, 1-Oct-86 16:03:21 EDT
Article-I.D.: phri.2446
Posted: Wed Oct  1 16:03:21 1986
Date-Received: Fri, 3-Oct-86 05:28:55 EDT
References: <45500088@uiucdcs> <7670@tekecs.UUCP> <829@jplgodo.UUCP>
Reply-To: lonetto@phri.UUCP (Michael Lonetto)
Organization: Public Health Research Inst. (NY, NY)
Lines: 76
Keywords: assimilation, energy conversion
Xref: linus talk.origins:121 net.bio:493
Summary: Why, metabolism of course.

> = Steve Schlaifer 
>> = Michael Sellers 
>>(quoting Steve)
>> There are some general common to all those things we say are alive (though
>> most if not all of them have a few notable exceptions):
>> 
>>   IT MUST METABOLIZE.  All living things use material from their environment
>> that, when broken down, is used for the maintenance of the thing itself (or
>> for growth or reproduction, etc.).
>>   IT MUST REPRODUCE.
>>   IT MUST RESPOND TO ITS ENVIRONMENT.
>>   IT MUST BE AN INDEPENDENT ENTITY,
>>   There may be more to this list, but I believe it is fairly complete.
>
>A problem I have had with definitions like this is there are many things
>we don't consider to be living that slip through and some we do consider
>living that fail to pass.  For the most part, a mule doesn't reproduce
>(there are reports of rare exceptions).  A solar powered refrigerator with
>automatic sun-tracking and a motorized carriage, on the other hand, satisfies
>the other three conditions above as well as the mule does.  Seems to me that
>this is a VERY complicated question that cannot be simply defined.

It seems to me that part of the problem is that the definition is TOO
complicated already.  In order for something to be alive it need not be
reproductively competent (my grandmother doesn't look at all dead yet,
even though she's well past menopause).  Obligate parasites are still
considered alive (many protozoan parasites are only found outside their
hosts under laboratory conditions).  This leaves only metabolism and
response to the environment.  Broadly defined, metabolism and response
to the environment overlap, and the important events from the viewpoint
of life vs nonlife all fall under metabolism.  

The question thus becomes:  What is this metabolism stuff??  

Metabolism is a SELF SUSTAINING, SELF REGULATING network of chemical
reactions capable of maintaining HOMEOSTASIS.  Homeostasis is a "steady
state", ie: changes in the rates and directions of the reactions are
kept within limits, as is the overall chemical activity of the organism.
This is not the same as stasis (the lack of change), but implies that
the changes tend toward a specific rate.  This is a process of
considerable complexity.  Metabolism is responsible for ASSIMILATION,
which isthe conversion of environmentally obtained chemicals to
substances which can be used as part of the organism, and their
subsequent utilization.  Note that the substances taken in from the
environment are chemically distinct from the substances produced from
them by the organism.  This is in marked contrast to the growth of a
crystal, where each molecule or ion which is added is identical to those
already there, and no chemical change occurs (even in the case of doping
semiconductors this second condition holds true).  So while an organism
may take in protein and DNA and make protein and DNA, the proteins and
DNA taken in have different structures than those produced.  In the case
of proteins, even if identical proteins are available, the organism will
digest them to amino acids before reutilizing them (I'll discuss
exceptions for DNA if any interest is expressed).  Metabolism is also
responsible for ENERGY CONVERSION.  In order to carry out assimilation
energy is required (amongst other reasons is the never ending battle
with entropy: life is entropically very unfavorable, and thus requires a
great deal of energy for maintainance).  Living beings convert chemical
energy to different forms (e.g. burning sugar and converting the energy
to high energy phospate bonds) and sometimes convert sunlight to
chemical energy (photosynthesis converts C02 and water to sugar, sugar
can be oxidized to produce energy).  

Unfortunately I can only give examples from life as we know it, but the
principles of assimilation and energy conversion should hold for
anything to be considered living.  By this definition viruses are not
living, though they are clearly borderline.  Other parasites, with more
complete metabolisms, would be considered living.  Note that this
discussion leaves out entirely the more complex issues which sometimes
arise with complex organisms: is a brain dead person who's heart is
still beating alive?  By this criteria he is, though clearly the person
we used to know is no longer with us.  But that's all there is time for
so till the next time I can't let something by.

Michael Lonetto UUCP:(allegra!phri!lonetto) 
USMAIL: Public Health Research Institute, 455 1st Ave, NY, NY 10016