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Whether this way of storing energy is efficient is an interesting question, because it illuminates a constraint imposed by the Second Law. We can measure how much energy is released when an ordered glucose molecule is subsequently disordered. It turns out to be:

The energy released is in the form of heat (as when a log is burned), and it represents about 36 percent of the energy initially captured as light.3

We can now state two thermodynamic principles that are important to our understanding of physiol ogy, whether it occurs inside or outside an organism. The first concerns the forms energy can take. In the glucose examples there were two transformations of energy: light energy was transformed into a potential energy stored in the bonds of a glucose molecule, and that energy was then recovered as heat when the glucose molecule was broken up again. More transformations are possible: indeed, there is only one intrinsic limit (which we will explore in the next chapter) on the number of transformations a parcel of energy can undergo. The only constraint is that whatever transformation energy undergoes, the total quantity of energy cannot change. In constructing a glucose molecule, for example, 1.4 X 10-17 joules of energy were put in on the left side of the equation, and there must be 1.4 X 10-17 joules coming out on the right side. In other words, energy is conserved. This is the First Law of Thermodynamics (which shall henceforth be referred to simply as the "First Law").

Conservation of energy implies that as much energy should be recovered from destroying glucose as was put into creating it. But that obviously isn't true: burning glucose yields only about 36 percent of the energy put in. This leads us to our second important principle: no transformation of energy that involves work is ever perfectly efficient. In the photosynthetic reaction for glucose presented above, for example, I did not give enough detail concerning the fate of the energy. The complete equation is thus:

48 photons + 6CO2 + 6H2O ^ C6H12O6 1.4 X 10-17J = 4.8 X 10-

energy in

= energy in glucose

6O, heat

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