Science In Real Life

Today is, of course, the American holiday of Thanksgiving, where friends and families all across the nation gather to celebrate that most favored pastime of this great land: eating absurd quantities of food. And it is in that spirit that I devote this Science In Real Life essay to the chemical process by which our body extracts and uses energy from the foods we eat. From poultry to pumpkin pie, from giblets to gravy, all of the food we eat stores chemical energy in the form of carbohydrates or fatty acids, and all multicellular animals use the same biochemical processes to function off of those fuels.

First, though, we need to understand how it is that a molecule can carry energy. Consider for a moment the Empire State Building. In this corner, weighing in at about 370,000 short tons (that is, tons of the 2000 lbs each variety), we have an edifice with most of its mass lifted up from the surface of the Earth. But gravity attracts everything – how is it not a heap of rubble? Because the structural arrangement is elegantly designed to resist exactly that process. There is energy contained in that building in a form called Gravitational Potential Energy; it’s Potential because the energy has the potential to be released if the building falls down. A quick back of the envelope calculation suggests that at current levels of American energy usage, the gravitational potential energy in the ESB is enough to fully power 20 households for a year. If the current energy crisis gets particularly unbearable and some clever chap designs a way to extract energy from falling buildings, we could blow up skyscrapers to heat our homes.

In this way do molecules contain potential energy – only this time it’s not gravitational, it’s electromagnetic. Molecules are made of atoms are made of charged particles, and a negatively charged electron in a chemical bond in a molecular arrangement contains some level of energy by being restrained “above” its positive counterparts. Left entirely to their own devices, molecules would all “fall down” into the lowest energy arrangements. But we are fortunate enough to have a source of energy which is as abundant as the sun. Sunlight becomes sugar, thanks to photosynthesis, and we ingest the sugar. You see, unlike today’s tech companies, Momma Nature has actually implemented a design credo based on interchangeable components. Glucose (C6H12O6) is the currency of choice for 99% of lifeforms on this planet; the rest accept VISA and Mastercard.

So you’ve wolfed down some glucose (or maybe some more complex sugars, or even a carbohydrate polymer or two – your body will have broken it down to glucose within minutes). Then what? Enter glycolysis, an ancient and venerated metabolic pathway. Since before time was counted, since before the sun rose over anything with limbs, even before prime time television (gasp!) there was glycolysis. Now, the actual biochemical process that is glycolysis is more complicated than I can explain here. But the upshot is exactly what I described above – the tight chemical bonds of glucose are broken, and the energy from that is used to form new molecules. In this case, the relevant products are substances called Pyruvate (C3H4O3) and a molecule called ADP, for Adenosine DiPhosphate. If glucose is the incoming mail for your body, ADP (and its relatives A-Tri-P and A-Mono-P) are the internal memorandums. They are used to transfer energy in and around the different parts of every single one of your cells.

But we still have to deal with this pyruvate business. Here is where your body really kicks it into high gear with an engine called the Krebs Cycle. This is … complicated. Here, scope this out:

( Image credit: Wikimedia commons )

Did your eyes just pop out of your head? Mine did. Doctors and biologists know this cycle like the proverbial back of the proverbial hand. I am neither of those things, but the nice thing about being a physicist is I get to cheat – the twin principles of conservation of energy and increasing entropy are always at work, throwing badly behaved misconceptions out of our heads like bouncers at an exclusive nightclub. We know, therefore, that energy comes in with the pyruvate and leaves with everything else. We also know that because the energy is going from concentrated (one molecule) to dispersed (many molecules) there are transferences of energy taking place. Where energy disperses, entropy increases. And where entropy increases, heat increases. Congratulations, you now have one warm-blooded mammalian body, ready for use. This cycle, ladies and gentlemen, is where some of your body heat comes from.

The rest of it merely happens further downstream. Those sunburst molecules of ATP and GTP (energy carriers) float away from where the Krebs cycle happens (that’d be your mitochondria) and into the rest of the cell, where they are picked up like dollar bills by so many hopeful entrepreneurs, and used to conduct the normal operations of your cell, like repairing structures, moving muscles around, reproducing, et cetera. Those processes also generate “waste” heat, which goes into warming you up. At a rough estimate, 60% of the energy you ingest goes into keeping you warm, and only 40% to the energy available for voluntary actions like moving, thinking, and writing science essays. Now if you’ll excuse me, I have to go consume large amounts of high-energy chemical bonds. Happy Thanksgiving!