Archive for the ‘Science’ Category

Life, Death, and the Second Law

Thursday, February 28th, 2008

I think a lot of people have the wrong idea about the Second Law of Thermodynamics. I don’t mean about what the law says, though the concept of entropy is harder to grasp than that of energy, which at least seems to be less abstract until you get to know it better. No, I mean the idea that it’s really unfortunate that everything tends to even out to sameness over time and a downright shame that we and our devices are forced to work so hard just to keep our bodies and our society running against the inexorable dissolution ordained by the law of entropy increase.

Indeed, some may see the second law as a defect in the universe. Is it an argument agaist the existence of God for Creation to be running down? Or should we view it in Manichean terms—the Good God of energy conservation struggling valiantly in a lost cause against the Evil One of entropy increase?

There is in fact something strange and subtle about the connection between life and the second law, something that goes beyond the idea that living organisms are able to maintain their highly ordered selves against the tide of universal disorder by squeezing out order at the expense of the rest of the universe. The subtlety of the connection has to do with the way in which living systems accomplish that feat.

Chemists, including biochemists, have an infallible way of determining how chemical reactions proceed within a complex mixture of chemicals not yet in equilibrium: the reactions will take place in such a way as to decrease a physical quantity, which can be calculated, called the free energy. This is under the assumption, which is pretty good in a cell, that the reactions are taking place at constant temperature and pressure. This free energy is the energy in the chemical system theoretically available for useful work, say for causing a heart to beat in an organism. In order for a certain reaction to take place that increases the free energy of the participants in the reaction, it must occur along with a second reaction in which the free energy decrease more than offsets the increase of the first one. That the free energy must always decrease, sounds a lot like the second law of thermodynamics, and in fact the decrease of the free energy in a process at constant temperature and pressure is equivalent to a net increase in the entropy of the universe.

Living organisms have the ability to utilize chemicals from their environment to both sustain their own existing chemical structures and components and to extract useful energy to do the work of living. How does the blind science that rules the organism determine whether it needs to combine molecules A and B into a new one AB or needs instead to start splitting AB molecules apart? Whether the organism is breaking large molecules into smaller ones, combining smaller into larger, moving from molecules with free energy the organism can’t utilize to others with a lesser amount of free energy it can use, or using energy stored in chemical bonds for mechanical work, or dong all in one cell at the same time, the tally sheet of free energy for all the reactions taking place has to show a net decrease.

This means that the incredibly complex and dynamic chemistry of living cells—that involved in nerve signal transmission, DNA replication, digestion, photosynthesis, and the whole network of biochemical processes—all that chemistry is regulated by the second law of thermodynamics. The “self-regulation” of living systems comes through their submission to the second law of thermodynamics. The iron law that says all things decay and that we must all one day die also allows us our brief time of life and consciousness! Nature is deep, very deep, and it is wonderful to think that the very processes that enable our thoughts and understanding of it depend upon the same law that leads inevitably to their end.