We can easily recognize different kinds of energy around us like heat, light, electrical, chemical, nuclear, sound, etc., and you probably know that energy is measurable (calories, joules, volts, decibels, quanta, photons…). Even mass is a form of energy, as you may recall from Albert Einstein’s famous e=mc2 equation (the law of relativity).
The problem in thinking about thermodynamics is that the universe is big and there are too many kinds of energy to contemplate at once! To simplify, imagine only two kinds of energy in the universe: potential energy and kinetic energy. A helpful example is a dam. The water above the dam has potential energy. As the water flows over (or through) the dam, its potential energy is released as kinetic energy. In the old days the kinetic energy of flowing water could be used to power (i.e., turn) a millstone to grind wheat or other grains into flour. These days, water is more likely to flow through a hydroelectric dam where kinetic energy is converted (transduced) to electricity. In this simple view, heat (molecular motion), electricity (a current of electrons), sound (waves), and light (waves OR moving ‘particles’) are different forms of kinetic energy. The energy of mass or its position in the universe is potential energy. Thus chemical energy, for example the energy in a mole of ATP, is potential energy. Physicists talk a lot about potential energy and about kinetic energy flow and conversion.
An equally simple way to conceptualize energy is as useful vs. useless. This concept led directly to the arithmetic formulation of the thermodynamic laws. In this utilitarian way of thinking about energy, useless energy is entropy, while useful energy can be any of the other forms of energy (potential or kinetic).
A key to understanding bioenergetics is recognizing the difference between closed and open systems in the universe. Systems such as biochemical reactions in a test tube, reach equilibrium. Such systems are considered closed. Closed systems are artificial, possible only in the laboratory, where one can restrict and measure the amount of energy and mass getting into or escaping the system. Cells on the other hand (in fact every reaction or event in the rest of the universe) are open systems. Open systems readily exchange energy and mass with their surroundings.
With this brief introduction, we can imagine ourselves to be early scientists trying to understand energy flow in the universe, asking how the Laws of Thermodynamics apply to living systems (bioenergetics). We’ll see that the Laws can be demonstrated because all kinds of energy can be measured (as heat in calories or joules, electricity in volts, light in quanta, matter in units of mass, etc.).
138 Different Kinds of Energy; Chemical Equilibrium