Introduction

Living organisms are extraordinarily diverse — yet a simple question sits underneath that diversity: are all living things built from the same chemical elements? Chemistry gives us a way to answer this through elemental analysis, a technique you have already met in your chemistry course. If elemental analysis is run on a sample of plant tissue, animal tissue, or a microbial paste, it returns a list of elements — carbon, hydrogen, oxygen and several others — along with how much of each is present per unit mass of the tissue.

Now compare that with the same analysis run on a piece of the earth's crust, a natural example of non-living matter. At first glance the two lists look alike: essentially every element found in the earth's crust also turns up in living tissue, so in absolute terms there is no clear line separating a 'living' list of elements from a 'non-living' one.

Look more closely, though, and a real pattern emerges. Carbon and hydrogen make up a much larger share of a living organism's composition, relative to the other elements present, than they do in the earth's crust. This single observation — that living matter is disproportionately rich in carbon and hydrogen compared to non-living matter — is the starting point for everything this chapter builds on, and it sets up the natural next question this chapter opens with: what kinds of organic compounds actually account for that difference, and how do we go about finding them?