4.2: Energy has two forms: kinetic and potential.

“Batteries not included.” For a child, those are pretty depressing words. We know that many of the toys and electronic gadgets that make our lives fun or useful (or both) need energy—usually in the form of batteries. Generating ringtones, lights, and movement requires energy. The same is true for humans, plants, and all other living organisms: they need energy for their activities, from moving to reproducing to thinking.

Energy is the capacity to do work. And work is anything that involves moving matter against an opposing force. In the study of living things, we encounter two types of energy: kinetic and potential. Kinetic energy is the energy of motion. The kinetic energy of an object is the energy that it has due to its motion. Legs pushing bike pedals and birds flapping wings are examples of kinetic energy (FIGURE 4-3). Heat, which results from lots of molecules moving rapidly, is another form of kinetic energy. Because it comes from the movement of high-energy particles, light is also a form of kinetic energy—probably the most important form of kinetic energy on earth. (When we look at photosynthesis later in this chapter, we explore how sunlight is harnessed for producing food molecules.)

An object does not have to be moving to have the capacity to do work; it may have potential energy, which is stored energy that results from an object’s location or position. Water behind a dam, for example, has potential energy. If a hole is opened in the dam, the water can flow through, and perhaps spin a waterwheel or turbine. A concentration gradient, which we discussed in Chapter 3, also has potential energy: if the molecules in an area of high concentration move toward an area of lower concentration, the potential energy of the gradient is converted to the kinetic energy of molecular movement, and this kinetic energy can do work. Chemical energy, the storage of energy in chemical bonds, is also a type of potential energy.

Because potential energy doesn’t involve movement, it is a less obvious form of energy than kinetic energy. An apple has potential energy, as does any other type of food (FIGURE 4-4). Why? Because, during cellular respiration, the chemical energy stored in the chemical bonds making up the food can be released (when those bonds are broken and lower-energy bonds are re-formed), enabling you to run, play, and work. We explore cellular respiration, the energy-releasing breakdown of molecules, later in this chapter. But first we need to know more about the nature of energy.