bravoghost

So I think I've hit a roadblock. I see that in a lot of biochem enzymatic reactions, two substances are held in equilibrium (for example in glycolysis, DHAP and GAP are in equilibrium through an enzymatic reaction). However, I've also read that an enzyme will convert a substrate into a product and that the process is irreversible (E + S <---> ES ---> EP <---> E + P, where the middle step is irreversible). So my question is how can both be true? How can a substrate and product be converted back and forth by an enzyme (to establish equilibrium) while at the same time the form of an enzymatic reaction is irreversible?

Would the energy of a phase change (let's say going from a gas to a liquid) be equal to the energy of all newly formed intermolecular bonds being formed as the gas transitions to liquid?

HOLY GOD. IT MAKES SENSE.

I suddenly realized the friction thing after the first post in this thread. It was a 'Eureka!' moment.

This is what's holding me up. I'm imagining a pure solvent in a flask, with a vacuum above it. There is enough space above the liquid for... let's say... a trillion gaseous solvent molecules. So in pure solvent, there will be a trillion molecules that escape the liquid and enter the gas phase. Once they reach that trillion number, the gas and liquid enter equilibrium. Those trillion gas molecules exert a certain pressure on the container - its vapor pressure. Now let's say it is a solution instead. There is still space above the solution for a trillion solvent molecules - so the solvent molecules still can escape the solvent (even though it is at a slower rate because there are fewer spaces on the surface area for the solvent to escape). The solvent molecules still escape the liquid, albeit at a lower rate, until about a trillion gaseous molecules are above the liquid. At that trillion number, it would establish equilibrium again. It seems that if there is space for a trillion gaseous molecules, then there should be a trillion gaseous molecules.

So I started connecting some dots, and got lost. Work=Force x Distance. Of course. Force = mass x acceleration. Of course. Acceleration must be non-zero in order for there to be a force present. So imagine you're moving a box across the floor. You get the box moving, and from that point on you move at a constant velocity. The constant velocity indicates there is 0 acceleration, if there is 0 acceleration then 0 force is being applied. But that would mean that there is no work being done. But there must be work being done since the box is moving over a distance. How am I tripping up?

That part makes sense... lower rate of coming and going - but why does the rate determine pressure? Shouldn't the number of molecules above the liquid determine pressure??

I can't conceptualize this very well. I understand the principle - a solute in a solvent will decrease the surface area that the solvent has. This means fewer molecules of the solvent can escape. This means that at equilibrium, fewer molecules can enter the liquid and escape from it (it's a one-in, one-out situation). What I don't understand is how this decreases vapor pressure. In my mind, the surface area only determines the rate of establishing equilibrium. The rate of molecules escaping the solvent is lower, but that just means it should take longer to reach equilibrium - not change the pressure. A change in pressure would insinuate that fewer molecules are in the air above the liquid, right? Fewer molecules = fewer collisions = lower pressure. So why can't the same number of molecules occupy the space above the liquid regardless of whether or not it is a solution? See what I'm saying? Or am I not explaining myself well?

Ah-HA! I get it. It reminds me of the deformation of a baseball on a bat... http://webusers.npl.illinois.edu/~a-nathan/pob/pics/ballbat2.jpg

Here's my question: imagine a ball rolling towards a brick wall. It hits the brick wall, and stops. It has lost its kinetic energy, right? And change in kinetic energy is equal to Work. But work also relies upon force being applied over a distance. So what I'm thinking is that there has been a nonzero change in kinetic energy, but because no change in distance occurred, no work was done. But that would violate the idea that the change in kinetic energy doesn't equal the work done. How is that possible?

I assume cofactors & allosteric control are closely related. Are all cofactors a method of allosteric control? And is all allosteric control based on cofactors? Thank you greatly.