Re: Can a single-celled organism 'run away' from possible danger?

First of all we need to define the "Fight-or-Flight Response". 

Definition:  the reaction that occurs in the body when faced by a sudden,
unexpected threat or source of stress. The name of this reaction comes from
the fact that an animal experiencing this reaction almost immediately
decides to fight or to run. In this reaction there is a sudden release of
the hormones epinephrine and norepinephrine, which increase blood flow to
the muscles, increase arterial blood pressure, improve muscle strength and
mental ability, as well as increase blood glucose concentration. Through
these actions the body prepares for a confrontation or a fast escape.

-As single celled organisms do not possess any form of endocrine systems
from which hormones such as epinephrine or norepinephrine are produced.
These hormones also affect the ciculatory system and the nervous system,
which single celled organisms do not possess. 

-Many single celled organisms such as viruses and bacteria do not possess
the able to fight or run. In the case of viruses, which are simply a
protein coat encapsulating a strain of RNA or DNA, the organism is incapable
of nearly any action at all. Bacteria can response to various environmental
conditions, but this is more a form of adaptation then an instinctive or
hormonal response.

-Protozoa do possess the able to "run away" from a potential danger. But
whether or not the organism can recognize it is in danger or remains
oblivious to it is a different matter. But this response is not influenced
by a "fear" reaction but simply a matter of moving towards a more favorably
surrounding.

So while some single celled organisms may respond to environmental
conditions, this response is quite different from the "fight-or-flight"
system that multicellular animals.

 http://stress.about.com/cs/cortisol/g/def_fightflight.htm