Introductory material on metabolism often includes some chatter about the
response of the cell when the level of ATP goes up or down, or notes the
role of ATP as a regulator molecule. _Does_ the level of ATP go up or
down? I recall seeing data some time ago showing a passably substational
change in the level of ATP in working and resting muscle. I've also heard
that the level of ATP holds steady. I'm trying to cultivate a (crude)
mental picture of pathway integration. It's easy to imagine 'tight'
metabolic rigidity, in which reactions are tightly in train and levels of
things don't change, only flux does. It's also easy to imagine 'loose'
pathway integration in which there is a certain amount of sloshing among
reactions. Which is more accurate? Or perhaps more pertinently, is there a
framework for assigning reactions or pathways to one group or the other?