To return to the quote from Richard Feynman that opened the book, “It is very easy to answer many of these fundamental biological questions; you just look at the thing!” Now that you have looked at many bacterial and archaeal cells, we hope you are beginning to appreciate the structure that underlies their complex lives.
It is important to keep in mind, though, that we have only highlighted a small fraction of the components of these cells. Look at any tomogram in the book and notice all the dark spots that are not labeled; each is a macromolecule, or larger complex, with a function for the cell. From illustrations of a cellular machine like the flagellar motor, it is easy to imagine it acting in isolation. In fact, though, its environment looks more like this beautiful illustration of an Escherichia coli cell by David Goodsell [95]. It is simply a part of the crowded life of the cell. And each of these parts, their functions and their interactions have been shaped, and will continue to be shaped, by evolution. Biology is very much a work in progress.
Our knowledge of biology is also a work in progress, and we are at an early stage. Many of the structures you saw in this book were characterized only in the past decade, and most are not yet fully characterized. Other structures have been observed inside cells, but we do not yet know what they are, or what function they may serve (e.g. [96]). You saw examples highlighted in white throughout the book.
Finally, the cells we have imaged represent only a fraction of the full diversity of bacteria and archaea. For every bacterial species that has been cultured in a lab, many more exist in nature, unseen and unknown. Archaea, which do not attract human attention with pathogenic functions, are even less studied. Some of the most tantalizing species, which promise to bring us closer to an understanding of how eukaryotes first evolved, have been identified from genetic fragments but have not yet made it into microscopes in the lab. Many questions remain unanswered, and much work remains to be done. We hope you will keep looking.
