Jon Webb's Blog

Friday, August 31, 2007

Zombie Physiology

An interesting question regarding zombie physiology is: how do zombies move? They do not breathe (or at least their lungs do not supply oxygen to their muscles, since they have no circulatory system). While they consume flesh, they do not digest. Where does their energy come from?
Putting aside for the purposes of this article the fact that zombies do not actually exist, we can find the source of zombie energy in the basic chemical reaction that powers the muscles of the living, which is the Krebs citric acid cycle, and its alternate which takes place when insufficient oxygen is present, glycolysis. Zombies must be using glycolysis to power their muscles.
Glycolysis is an inefficient mechanism for transforming glucose into energy. It is, in fact about 14 times less efficient than the Krebs cycle used in cellular respiration (in other words, a 6-carbon glucose molecule is oxydised to produce 2 molecules of ATP. The Krebs cycle produces about 28 molecules of ATP per oxydised glucose molecule). And it produces as a by product lactic acid. What effects does this have on zombies?
First, the loss of energy due to depending purely on glycolysis is dramatic. Given that zombies do not consume, or at least do not digest, food, they must depend purely on stored body reserves of fat for the duration of their existence. A rough estimate of the reserves is given by the "rule of three": a person can survive 3 minutes without air, 3 days without water, 3 weeks without food. By this rule, zombies should "survive" only a day and a half since they burn up their reserves 14 times less efficiently? How, then, do they "survive" for years?
The answer lies in the fact that zombies are not warm-blooded and do not have to expend energy maintaining most organs. All they have to do is control their muscles and move. The other organs in their body are useless. So a day and a half supply of energy goes much further. In fact, the organs they are not using can be broken down and used to supply their muscles with energy as well.
It should also be pointed out that the anaerobic (and acidic, because of the lactic acid) environment inside a zombie's environment partly explains why zombies do not suffer decay due to the action of bacteria, for example those found in the gut. The inside of a zombie's body would likely be sterile following zombification.
It should also be noted that the production of acid by the zombie's muscles must eventually lead to decay of body parts and may account in part for the traditional horrifying appearance of zombies (aside from the fact that they exist only to eat the flesh of the living, of course).
A zombie that is not moving need consume very little energy, only that needed to keep itself aware of the environment. In such a situation a zombie could last for years, using only its initial store of body fat, especially if it started out obese.
The information this article is based on comes from the useful Zombie Survival Guide by Max Brooks.


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