For reasons that will be outlined in Chapter 26, we believe that procaryotic life evolved on Earth around 3.5 billion years ago. The "invention" of the more efficient and more versatile eucaryotes took place 2 billion to 1.5 billion years ago; so the first half of life on Earth was procaryotic. Most of the life that we see around us is eucaryotic, and there is a tendency to accept this pattern of life as the norm. This chapter deals mainly with the chemistry of eucaryotes. Bacterial chemistry is much more varied, and one has the feeling that eucaryotes settled upon only one among many possible metabolic choices. Bacterial chemistry can become an exercise in chemical archaeology; many of the fascinating alternative ways of doing things that eucaryotes have uniformly abandoned have been retained in one species of bacteria or another. Some of these alternative chemical schemes are very important and will be discussed in Chapter 26.

We cannot talk meaningfully about these bacterial exceptions before we understand the chemistry of the mainstream, which means the eucaryotes. Two questions will be asked in this chapter:

1. How do eucaryotes break down glucose and other high-free-energy molecules and store the energy for their own use?

2. How do photosynthetic eucaryotes tap solar radiation as a source for synthesizing high-free-energy compounds?