Life can exist only where molecules and cells remain organized. All cells need energy to maintain organization. Physicists define energy as the ability to do work; in this case, the work is the continuation of life itself.
The behavior of energy has been expressed in terms of reliable observations known as the laws of thermodynamics. There are two such laws. The first law of thermodynamics states that energy can neither be created nor destroyed. This law implies that the total amount of energy in a closed system (for example, the universe) remains constant. Energy neither enters nor leaves a closed system.
Within a closed system, energy can change, however. For instance, the chemical energy in gasoline is released when the fuel combines with oxygen and a spark ignites the mixture within a car's engine. The gasoline's chemical energy is changed into heat energy, sound energy, and the energy of motion.
The second law of thermodynamics states that the amount of available energy in a closed system is decreasing constantly. Energy becomes unavailable for use by living things because of entropy, which is the degree of disorder or randomness of a system. The entropy of any closed system is constantly increasing. In essence, any closed system tends toward disorganization.
Unfortunately, the transfers of energy in living systems are never completely efficient. Every body movement, every thought, and every chemical reaction in the cells involves a shift of energy and a measurable loss of energy in the process. For this reason, considerably more energy must be taken into the system than is necessary to carry out the actions of life.