To understand the dependence of life on energy we need to examine two of the fundamental laws of physics - the First and Second Laws of thermodynamics. We are not concerned with the mathematical formalism of thermodynamics, but rather want to gain a qualitative view of these two laws. Thermodynamics can be thought of as the rules of energy and order. Thermodynamics governs how energy and matter can be transformed.
Energy is defined as the capacity to do work. It comes in many forms: gravitational, kinetic, heat, electrical, chemical, osmotic, light, mass, and others. For virtually all life on Earth the primary source of energy is light from the Sun. Photosynthetic organisms transform light energy into chemical energy. Cells that are photosynthetic must import chemical energy to survive. In this context we can think of a cell as a factory that uses energy to do work.
First Law of Thermodynamics
Energy is conserved
This means that energy is neither created nor destroyed. It can only be changed from one form to another.
Intuitively we know more about energy than is expressed by the First Law. For example, if we consider a mouse in a closed box in which no energy can enter or leave, we know that in time the mouse will die. This will happen even though no energy leaves the box. The reason the mouse dies is that things tend to decay if left alone. This concept is expressed in the Second Law of thermodynamics as follows:
Second Law of Thermodynamics
The disorder of a closed system remains constant or increases
This means that if there is activity, disorder increases. The disorder of a system is measured by a term called entropy. Another way the second law is stated is that for a closed system the entropy stays the same or increases.
The First and Second Laws of Thermodynamics help us to understand the rules that organisms must obey in using energy. In practical terms, the second law says that if you want to maintain a complex system that does work, like a cell, then you must add energy to it. Furthermore, when you consider the source of the energy together with the cell, the entropy of the entire system must increase.