MadSci Network: Evolution

Re: How does the orderly process of life go against entropy?

Area: Evolution
Posted By: Michael Onken, WashU
Date: Fri Mar 21 16:35:12 1997

Far from going against entropy, life absolutely requires entropy to function!

But first, it is important to define what entropy is and what it is not. Entropy is not Chaos. Entropy is a thermodynamic state function that relates the number of possible chemical states a substance can occupy to the energy in the substance. T he relationship between entropy and "usable" energy is defined by the Second Law of Thermodynamics, which states that: for reversible processes, the change in entropy around the entire cycle (from A to B to A) is zero (DS = 0); and for spontaneous processes, the change in entropy is always positive (DS > 0). This means that the amount of work that can be extracted from a spontaneous process is less than the change in us able energy of the process - some of the energy becomes unusable, i.e. entropy. Because all processes can be classified as reversible or spontaneous, the total change in entropy for all processes is positive (DSr + DSs = DSt > 0), or put another way, universal entropy is always increasing.

But, what are spontaneous processes? Since entropy is the number of possible chemical states a substance can occupy, an increase in entropy means that the substance can occupy more states. So entropy is the driving force behind evaporation, expansion, diffusion, and even surface tension and hydrophobicity. Respiration is a good example of a life process that is driven by entropy. Entropy makes the air rush into the lungs when the chest expands. Entropy makes oxygen diffuse from the lungs into the blood (in fact, the surface tension of water in the lungs is so great, that specialized lipid membrane structures, called surfactants, are required to reduce the surface tension, allowing gas exchange to occur*). When the oxygen-rich blood reaches the capillaries, entropy makes the oxygen diffuse into the oxygen-hungry tissues. In the cell, entropy allows the intake of oxygen and sugar to drive the production of energy by the cell. Other processes like nerve i mpulses, digestion of food, kidney function, signalling by hormones; in fact, all enzymatic processes rely on an entropic energy component to function.

The order in life, in a chemical and thus entropic sense, involves the sequestering of materials and the formation of gradients. Both processes are reversible, so the negative change in entropy is balanced by a positive change when the reverse process oc curs. Even so, the entropic drive toward equilibrium is so great, that huge amounts of energy are required to generate gradients and isolate materials from mixed substances. This is why living organisms must constantly consume food and absorb energy to live. This is also why life cannot exist on planets where there is insufficient solar of thermal energy. Entropy existed long before the origins of life, so life must obey the same thermodynamic laws as the rest of the universe.

* Water molecules can occupy many more states in solution than in contact with an insoluble material like air or oil, because it is forced by its polarity to form pseudocrystalline lattices around these materials to maximize its potential for form ing hydrogen bonds in solution. So entropy forces the water to minimize its contact with these materials, generating surface tension. This can best be seen when you shake a mixture of oil and water and see bubbles of oil in the water, or when water is r eleased in a spaceship and forms spherical globs (a sphere offers the maximum volume with the minimum surface area for maximum entropy).

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