# Q & A: energy, heat, ATP

Q:
The whole subject of heat confuses me. The answers I find sound circular. Compression of molecules increases motion and hence collisions; this leads to increased temperature. Here, it sounds like increased motion led to increased heat. Then I read increasing the temperature increases motion. Is the inherent motion of atoms the same thing as Brownian Motion, and if so, is Brownian Motion caused by heat or vice versa? I read in some places that heat is energy; I read in others that it is the transfer of energy. Can something be both things? I can understand that it can be transferred down a temperature gradient, but I cannot understand how energy = the transfer of energy. Last of all, I am totally mystified how energy is 'transferred'. When ATP is hydrolyzed, specifically how is the energy from that hydrolysis translated into say the Na+/K+ pump working? I can understand how you could use the energy of flowing water to turn a water wheel which in turn moved a series interlocking devices to ultimately turn a wheat grinder to make flour. But for the life of me, I don't know how the energy from ATP accomplishes it many feats. Thank you for any input.
- Judy (age 68)
Atlanta, Ga, USA
A:

Great questions. Your confusion about whether heat is a type of energy transfer or a form of energy  means that you've been listening carefully enough to know that the word is used in at least two different ways, not consistently. When we're being very careful, we say that heat is energy transferred due to temperature differences. The amount of energy that an object has due to not being at absolute zero temperature is called "the amount of thermal energy". In sloppier speech we often use "heat" in pace of "thermal energy". The reason that can lead to confusion is that thermal energy can get into something via other ways than heat flow. You metion a good example- the heating of a gas by the work done compressing it. So an object has a well-defined amount of thermal energy in it, but not a well-defined amount of heat in the careful sense of the word.

Brownian motion is something that particles that have some thermal energy in a fluid do as they move around, bouncing off other particles. That type of motion is only one of the forms of thermal energy. I don't know what it would mean to say that Brownian motion "caused" thermal energy or vice versa. It's a little like asking whether trees cause forests or vice versa.

On how ATP drives little engines, e.g. those ion pumps, people do know, but not us! I'll try to get an answer from a biophysicist colleague and then update this. Now that you've raised the issue, one of the most interesting questions is how many altogether different evolutionary paths there were for those ATP engines (linear muscle movements, circular rotors, ion pumps,...) and how many branched of from a smaller number of different early versions. Meanwile, here's a Wikipedia article (https://en.wikipedia.org/wiki/Na%2B/K%2B-ATPase) that discusses the ATP-driven ion pump, but without a nice picture of the comparative free energies of the various intermediate states.

Mike W.

(published on 01/26/2016)