To this point we have been looking at static objects, the various large and small molecules that are the raw materials of life. Now me turn to a more fundamental study, the examination of patterns. One of the most characteristic and sustaining aspects of life is the pattern of continual energy flow. The molecules that a living organism ingests, more often than not, are valued more for the energy that they contain than for their atoms. Whenever this energy flow is interrupted, life ceases. We shall be concerned with the two most important patterns of energy flow: the breakdown of glucose to yield useful energy (respiration), and the tapping of solar radiation to synthesize glucose for future needs (photosynthesis). These are the dual mainsprings of life on our planet.

Both plants and animals burn their foods with oxygen to produce energy, carbon dioxide, and water. Only plants can use energy from the sun to combine carbon dioxide and water into sugars, releasing oxygen in the process. Thus animals are dependent on plants as primary sources of food and as restorers of oxygen to the atmosphere. The relationship in a way is parasitic; we cannot get along without plants but they can get along without us.

This chapter inevitably will appear complicated, because the machinery that has developed for efficient energy management during the 3.5 billion years is complicated, with many moving parts. The important thing, however, is to see patterns and understand principles rather than to memorize molecules. The goal is not to learn the structure of pyruvic acid, for example, but to understand how energy is managed.