[Blogstress note: this article is not similar to that in AARR recently mentioned here]
In the background here I've been doing a lot of thinking and writing on thermodynamics and how the body uses food to produce internal energy to "do stuff". In the end, we "burn" the macronutrient molecules in our food to release the chemical energy stored in those molecules, and ultimately we use that energy to do "work".
I've said we can mostly ignore entropy in the context of the human body, but in chemical thermodynamics we have the concept of Gibbs free energy: G. Most importantly in chemistry, we are concerned with the change in free energy of reactions:
ΔGrxn = ΔHrxn - TΔSrxn
ΔHrxn = change in enthalpy (energy) ΔSrxn = change in entropy
ΔGrxn = change in free energy
Free energy is the energy available to do work. For example to move things such as the piston in an engine, an electron through a wire, or to move an ion from one side of a cell membrane to another. In the human body, we are ultimately using energy derived from molecules in our food to support life by "powering" other chemical reactions (synthesis, etc.), and things like moving food through our digestive tract, generating nerve impulses, and moving our body parts to sit, stand, walk, run, etc.etc. A sticking point for a lot of people with thermodynamics relates specifically to the context in which most first learn about entropy: the combustion engine. [You are invited to read some of my other posts on thermodynamics here: Of Thermodynamics, Chemistry, Biology and Biochemistry, Of Thermodynamics, Complexity, Closed Systems & Equilibrium and A Fein(man), Fine Mess of Thermodynamics.] In the human body, most of the "work" done directly by the chemical energy is electrochemical in nature. Still, eventually we do somehow translate that into physical movement, aka mechanical work.
Some analogies can be drawn between a combustion engine and our electrochemical "engine", but others cannot. A major distinction is that in the combustion engine, all of the chemical potential energy is released as heat, which creates pressure due to the gasseous products wanting to expand. This pressure (force) acting on the piston moves it, thereby performing mechanical work. In the human engine, the macromolecules are broken down and mostly converge at the point of a molecule called acetyl-CoA after which point the rest of the chemical "workings" of the engine are the same regardless of the original fuel source.
In All Roads Lead Through Krebs, I included the graphic at right from Marks' Basic Medical Biochemistry. This is the Tricarboxylic Acid Cycle or Krebs Cycle, and I've copied it here for reference. Please refer to the post for other ways that some amino acids enter this cycle.
Molecules often referred to as "reducing equivalents" (NADH, FADH2) are generated in this cycle, and these, along with reducing equivalents generated in fatty acid oxidation and glycolysis, are "fed into" what is called the Electron Transport Chain, or ETC. Whatever chemical energy that is lost as heat along the way has already been lost at this point, so we are dealing with a predictable number of these reducing equivalents per acetyl-CoA entering the TCA/Krebs. Unless you are a small animal such as a rodent, the ultimate outcome of the ETC is to use the electrochemical energy to power the phosphorylation of ADP to ATP. In times when heat is needed, this process is diverted or "uncoupled" from ATP generation, but that is a topic for another day.
Moving Off the Grid ~ (analogy time again)
Let's say you want to move away from civilization and rely solely on energy you can harness on your property. You find an ideal location with a strong running stream, frequent bursts of wind, and an open field. Oh ... and you decide that rather than use powered exercise equipment you're going to harness the energy from those as well. Only you hate cardio and learn that Tony Little has moved to the 100 acres next door, so you invite him to come over and use your Gazelle.
All of these are hooked up to a giant battery in your basement and everything in your home is run off of this Main Battery: heat, light, appliances, etc. [Note: this configuration is for the purposes of an analogy only.] So the water wheel basically charges the battery continually, the wind energy in short random intervals and the solar panel in cyclic daily fashion. Tony, well, he just adds a little extra to make him feel good :-) When you turn on your light bulb, it will illuminate if there is enough charge in your Main Battery. Whatever wattage your bulb is will determine how much energy it drains from the battery over the time that it is used. If you are using an inefficient old incandescent bulb to light your room, it drains more of the battery and gives off heat. If you are using an energy efficient LED, it provides the same light while draining less energy and giving off virtually no heat.
The light bulb draws energy from the Main Battery. It needs a certain amount to be illuminated. Your light bulb doesn't care where the energy stored in the battery came from.
The Main Battery provides energy based on the "load" that is placed on it. Your battery doesn't care what it supplies energy for, or how efficiently that device uses the energy.
Adenosine Tri-Phosphate (ATP) ~ Your Main Battery
More than two thirds of all ATP generated is produced through the TCA/Krebs Cycle. The vast majority of cellular activities is powered directly by ATP. It would be fair to say that ATP is the equivalent of the "free energy available to do work". It is also true that all ATP are alike regardless of where they came from. Therefore, from the point of ATP forward, all ATP-powered "work" and the thermodynamics, the energetics, whether or not entropy losses are involved, etc., all of it is the same.
ATP is like the Main Battery in the previous example, only there is no central depot for this internal energy supply, but rather it is contained locally "on site" of every cell. I suppose an analogy could be made for our house that rather than a main battery, there would be some battery network at every socket or directly in the appliances and such. I don't think this is necessary to elaborate, however, in order to draw the following corollaries to the above arguments:
Walking requires ATP to do the mechanical work of muscle contraction. A certain amount of ATP is needed to produce the necessary muscle contractions, etc. The muscle cells involved just use ATP and can't distinguish where it came from.
The amount of ATP required to perform a task is based on the "activity load" (speed, incline, etc.). Each ATP provides the same amount of energy regardless of what activity it is fueling.
So perhaps the water wheel is analogous to fatty acids continually cycling in your blood. And perhaps the windmill is analogous to carbohydrates and the solar panel to amino acids. Tony? Why he's alcohol of course! Pops by to add some energy during the week, perhaps he comes by more than once a day on the weekend. :-)
Dat Entropy ...
I recorded and put three short animations/tutorials available online here (Chapter 10) that shows how muscles contract and the role of ATP. Direct link to my video.
Towards the end of the video when the ATP is attached to the myosin head, is broken down to ADP+P and transfers energy to the myosin head, and the subsequent movement that would be mechanical work is the "equivalent" of the expanding gasses in a combustion engine. How much heat is lost in this process I don't know, but that wouldn't be entropy. Are there entropy considerations so that the full energy stored in the ADP-P bond is not available to do the mechanical work? Probably. Does it matter? Probably not. Not as far as tracing it back to any difference between macronutrient sources. It would be the same for each ATP used in each specific application.
- All macros are broken down and converge on acetyl-CoA. (Some amino acids feed directly into Krebs at different points)
- The difference along the way is in some direct ATP formation, and the formation of different amounts of different reducing equivalents (NADH, FADH2).
- From acetyl-CoA on down the energy production line through TCA/Krebs and the ETC, all reactions are the same in the production of ATP.
- Once energy is stored in the ATP "batteries", no cell knows the macronutrient origin of that energy.
- All ATP does whatever work -- chemical, electrochemical -- it does through the same reactions and mechanisms.
- Energy use or any losses associated with converting ATP chemical potential energy to other forms is the same regardless of the source of that ATP.