The Evolution of Obesity

6 Energy, Metabolism, and the Thermodynamics of Life

136 CHAPTER SIX Energy, Metabolism, and the Thermodynamics of Life Obesity, at its core, results from a sustained period of positive energy balance. More energy is ingested than is expended in the processes of life. The excess energy is stored on the body, primarily as fat. The increase in adipose tissue is central to the metabolic cascades that will lead to a lessening of health. The simplistic and generally unrealistic answer is to reverse the process; expend more calories than are consumed. This is very easy to say; extremely hard to accomplish. In this chapter we explore the concept of energy as it pertains to biological systems. Energy is a powerful, subtle, and sometimes confusing concept. It is central to modern science. The term energy was first used in the modern sense by Robert Young of England in 1807; he substituted it for the Latin term vis viva used by Newton and Liebnitz to refer to what we now call kinetic energy. The principles of thermodynamics require the modern concept of energy. Indeed the first law of thermodynamics is that energy is conserved; that energy is neither created nor destroyed. Energy and the laws of thermodynamics are central to understanding metabolism, the collection of biochemical processes involved in living organisms. What is energy? First of all energy is not a physical thing; rather it is a physical quantity that can be calculated for a system. This fact may have played a substantial role in the slow acceptance of the modern concept of energy. In the late 1700s, Antoine Lavoisier, considered by many to be the father of modern chemistry, made a number of key research findings that are central to our understanding of chemistry and metabolism. He showed that mass is conserved in chemical reactions; in other words, in a chemical reaction the total mass of the products is always equal to the mass of the reactants. He also showed that what we would now call heat energy is ENERGY, METABOLISM, AND THE THERMODYNAMICS OF LIFE 137 conserved. From this came the caloric theory of heat. The caloric theory of heat proposed that heat was an indestructible fluid that flowed between objects, from hot to cold. Heat was a thing. Just as mass was conserved, caloric was conserved. This was certainly a start toward the modern conception of energy, but it lacked the fundamental property that energy can be transformed among many manifestations without loss. Energy is a powerful concept that can also be abstract and confusing. What is elegant and pleasing to some can be mind-numbing and sleep inducing to others. Potential energy, kinetic energy, the ability to do work, the energy in photons, the energy in electron orbitals, the energy in chemical bonds, the energy in electric and magnetic fields, the mechanical energy in a spring, all of these are different conceptions of what is ultimately the same quantity. And all of the above forms of energy are used by living things. But energy is not a thing; it is a scalar quantity that can be calculated for a system. If the system is closed, such that nothing enters or leaves, then that calculated quantity called energy will remain constant, no matter how much other characteristics of that system change. The physics Nobel laureate Richard Feynman (1964) perhaps expressed it best: “There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law—it is exact so far [as] we know. The law is called conservation of energy; it states that there is a certain quantity, which we call energy that does not change in manifold changes that nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same.” Hermann von Helmholtz may have been the first scientist to apply the principles of conservation of energy to physiology. He argued forcefully that physiology should be founded on the principles of physics and chemistry, and he rejected the notion of vital forces somehow separate from the nonliving world (Helmholtz, 1847). He showed that the conservation of kinetic energy was a mathematical consequence of the...