Laws of Thermodynamics

Laws of Thermodynamics

© Eric R. Pianka

Let us briefly review some fundamentals of thermodynamics. All organisms require energy to persist and to replace themselves, and the ultimate source of practically all Earth's energy is the Sun. One can think of our Sun as "feeding" the earth via its radiant energy. But 99 percent or more of this incident solar radiation goes unused by organisms and is lost as heat and heat of evaporation; only about 1 percent is actually captured by plants in photosynthesis and stored as chemical energy. Moreover, energy available from sunlight varies widely over the earth's surface both in space and in time.

Physics and chemistry have produced two basic laws of thermodynamics that are obeyed by all forms of matter and energy, including living organisms.

The first law is that of "conservation of matter and energy," which states that matter and energy cannot be created or destroyed. Matter and energy can be transformed, and energy can be converted from one form into another, but the total of the equivalent amounts of both must always remain constant. Light can be changed into heat, kinetic energy, and/or potential energy. Whenever energy is converted from one form into another, some of it is given off as heat, which is the most random form of energy. Indeed, the only energy conversion that is 100 percent efficient is conversion to heat, or burning. Aliquots of dried organisms can be burned in "bomb calorimeters" to determine how much energy is stored in their tissues. Energy can be measured in a variety of different units such as ergs and joules, but heat energy or calories is the common denominator.

The second law of thermodynamics states that energy of all sorts, whether it be light, potential, chemical, kinetic, or whatever, tends to change itself spontaneously into a more dispersed, random, or less organized, form. This law is sometimes stated as "entropy increases" -- entropy being random, unavailable energy. Suppose you heat a skillet to cook an egg, and after finishing you leave it on the stove. At first, heat energy is concentrated near the skillet, which is, relative to the rest of the room, hot and quite nonrandom. But by the next morning the skillet has cooled to air temperature, and the heat energy has radiated throughout the room. That heat energy is now dispersed and unavailable for cooking; the system of the skillet, the room, and the heat has gone toward equilibrium, become more random, and entropy has increased. Unless an outside source of energy such as a stove, with fuel or electricity, is continually at work to maintain a non-equilibrium state, dispersion of heat results in a random equilibrium state. The same is true for all kinds of energy. According to this law, our solar system and presumably the entire universe should theoretically become a completely random overdispersed array of molecules and heat in the far distant future.

Life has sometimes been called "reverse entropy" (negentropy) because organisms maintain complex organized non-random states compared to their surroundings. But they must obey the second law just as any other system of matter and energy; all organisms must work continually to build and maintain nonrandom assemblages of matter and energy locally. This process requires energy, and organisms use the energy of the decaying sun (which, of course, also obeys the second law of thermodynamics and tends toward decreasing concentration of energy -- A Cosmic Perspective) to "oppose" the second law within their own tissues by concentrating energy in their own bodies. Wherever there is a live plant or animal, there must be an energy source. Without a continued influx of energy, no organism can survive for very long. Again, this "reverse entropy" occurs only within each organism, and the overall energy relations of the entire solar system are in accord with the second law of thermodynamics, with the overall system continually becoming more and more random.

Almost all life on Earth depends on photo-synthesis, the capture of solar energy by plants. Remnants of ancient photosynthetic prokaryotes (bacteria-like organisms) long ago became incorporated into eukaryotic cells of all higher plants. Known as chloro-plasts, these tiny green engines house chlorophyll and other molecular machinery that enables plants to convert solar energy into the chemical energy on which all life on Earth depends.

Only a small fraction of the plant food on land is actually harvested by animals; most products of primary production are consumed by decomposers. Transfer of energy from one trophic level to the next higher trophic level is defined as ecological efficiency. Such efficiencies of transfer of energy from one trophic level to the next are low, generally only about 5 to 10 percent.

Consider complex networks of interacting species such as those that occur in natural ecological communities. Are these "designed" for orderly and efficient function? Is there a natural "balance of nature?" Although it may be tempting, it is dangerously misleading to view entire ecosystems as having been "designed" for orderly and efficient function. Natural selection does not operate for the "good of the species" but works by differential reproductive success of individual organisms. Antagonistic interactions at the level of individuals and populations are widespread: for example, these include competition, predation, and parasitism. Even the two parties engaged in a mutualism experience conflicts of interest because costs and benefits differ for each participant. Such negative interactions must frequently impair some aspects of ecosystem performance.

Natural selection operating on individual prey organisms favors escape ability, which in turn reduces the rate of flow of matter and energy through that trophic level, decreasing ecological efficiency but simultaneously increasing community stability. In contrast, predators evolve so as to be better able to capture their prey, which increases the efficiency of flow of energy through trophic levels but reduces a system's stability.

In the coevolution of a predator and its prey, to avoid extinction, the prey must remain a step ahead of its predator. As a corollary, community-level properties of ecological efficiency and community stability may in fact be inversely related because natural selection operates at the level of individual predators and prey. Moreover, the apparent constancy and observed low levels of ecological efficiency are probably a result of this compromise that must be reached between prey and their predators.


Second Law of Thermodynamics

Storm Smith -- Thermodynamics

Storm Smith -- Thermodynamics

Storm Smith -- Nuclear Power

The World is Flat

The Energy Shadow of a Dollar