second law of thermodynamics

Question

When you exercise your body gets hotter. Stored chemical energy in the form of glucose and muscle glycogen is burned to provide your muscles with ATP so they can move. How is this an example of the second law of thermodynamics? Is going from glycogen to glucose a dehydration reaction or a hydrolysis reaction?

Answer 1

Hi Ashley,

 

1. Is going from glycogen to glucose a dehydration reaction or a hydrolysis reaction?  Neither.  It is an example of phosphorolysis, which is kind of like hydrolysis, but instead of using water (hydro-) to break apart the molecule (lys), the process uses phosphate.
2. How is this an example of the second law of thermodynamics? This is kind of a complicated question - not as straightforward as your question seems to assume.  First the second law of dynamics states: "in a natural thermodynamic process, there is an increase in the sum of the entropies of the participating systems."  To fully appreciate that law, we should define entropy.  Many textbooks and many teachers will define entropy as an increase in disorder, but that definition is a little vague and in some ways kind of inaccurate. The reason teachers will use it is because it is a little easier to grasp than the real definition of entropy.  But if you understand the real definition, everything else is much easier to understand, so let us make the effort.  Entropy is an increase in the number of possible micro states that a system can experience.  What this means is that a system in which the individual components can be arranged in more ways is preferable to a system in which they can be arranged in fewer ways.  For example: the second law of entropy hates high school.  In high school, everybody belongs to a clique.  You have your jocks, your cheerleaders, your rabble rousers, your nerds, etc... People sit at tables that define some aspects of their groups (if memories serve me right and if teenage romance movies are to be trusted).  But when you get into the real world a good deal of that falls away.  If you want to sit with someone outside of your social grouping, that is perfectly acceptable.  So post-high school life allows for an increase in entropy.  In the same way, if I were to take a bunch of big molecules (each of which is really just a collection of atoms) and break them apart into smaller molecules, that would allow for a larger number of ways in which each atom in the set could be arranged.  In other words, a carbon that is attached to a glycogen molecule is kind of a fixed in one way, but if I free a glucose from that glycogen, that carbon has a greater amount of latitude in how it can interact with the environment around it.  This is an example of an entropic move.  And if I take that glycogen and break it down into individual components of carbon dioxide (which is what happens to glucose as we go through glycolysis, pyruvate decarboxylation, and the Kreb's cycle in a process that will eventually create ATP), then I have further increased the number of ways in which that carbon can interact with the universe around him.  At the same time, the very creation of ATP from ADP takes a phosphate and forces it into a relationship with a molecule that is clearly more restrictive to the phosphate than had been true before.  So there is a trade off... one thing (glucose) become more entropically favored and another (that phosphate) becomes less.  Nevertheless, if we look at all of the steps that take us from glycogen to ATP and then from ATP to muscle usage (at which point the ATP is returned to an ADP state), we will find that overall our entropy goes up.  I hope this explanation helps.

Warm regards,

 

Jim O.