Re: MFW journal club - lifting weights makes you thin, cures diabetes?

On Tue, 12 Feb 2008, Andrzej Rosa wrote:

Dnia 2008-02-11 Tom Anderson napisa?(a):
On Mon, 11 Feb 2008, Andrzej Rosa wrote:

Tom Anderson wrote:

No. That only works if the process is near equilibrium.

Which means, that there should be enough time for changes of temperature
to not result in changes of pressure. It works like that by default in
condensed phase.

Or do you mean, that a cell can't be considered a closed system?

That.

Well, actually, i meant something else entirely, but i've realised i was
talking nonsense, so i'm going with that instead.

For fast reactions it shouldn't matter, and in bulk of muscle mass it
shouldn't matter at all.

On the contrary! A *lot* of heat escapes the cell.

And enters another one, doesn't it?

It may do, but it gets to the bloodstream, and is then transported to your skin. At the cell and tissue level, heat is flowing out.

For this process, the amount of energy 'lost' to entropy (really,
the amount of energy given up to drive the reactions forward quickly)
is fixed by the stoichiometries of the enzymes, which is what i looked at.
Except i didn't account for the 'clutch slip' by leakage of protons across
the mitochondrial membrane, which can be a significant amount.

I managed to never learn biochemistry on this level (which wasn't all that
easy), so I can't comment here. (But your wording regarding entropy, energy
and kinetics makes me cringe a bit. ;-))

My apologies! What's wrong with it?

You don't lose energy to entropy. Entropy is an information about
energy distribution in the system, so one doesn't drive the other.

So what's that T - delta - S term in the formula for Gibbs free energy, then?

Entropy is heat. And as i'm sure you're aware, heat is work and work is heat.

You don't lose heat to "drive the reactions fast" either. While one can speed up a reaction with heat, it works fairly slowly. Twice faster for every 10 degrees (iirc), by which time a human would be well done, so my guess is that we lose heat to actually prevent this scenario, not to drive our reactions really fast (though one could argue, that we already do by walking so close to the temperature of cooking proteins).

No, that's not what i'm talking about. I don't mean the effect of temperature on reaction rate (which is more complicated in biological systems anyway, as temperature affects protein folding, even small changes, which affects the catalysis, which counteracts the increase in rate from increased thermal motion of the molecules). The temperature of the cells doesn't vary much, i think.

What i mean is that if there isn't energy given up to heat, ie if chemical potential energy is conserved across the reaction (eg if you put 100 kJ of glucose in, you get 100 kJ of ATP out), then a reaction won't go forward - it is, by definition, at equilibrium. It's *only* when a reaction loses energy to heat that it proceeds. It doesn't have to be a lot, but the more it loses, the faster it will tend to go. The apparent inefficiency of metabolism is there because we'd rather have less energy more quickly.

I appreciate that i'm explaining this really badly. I did thermodynamics about this time in 1999, so i'm rusty on the terminology and nuances. I was a biochemist, so i came away with what i needed to know for biochemistry, which was not a lot more than some rules of thumb.

tom

--
quick good
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