374.1
We also tend to consider either the core elements used in life's chemistry or its solvent in isolation. However, perhaps altering both would result in a viable system of life. The reactive nature of silicon (when not in silicates) might be mitigated by a silicon chemistry that evolved in cold environments, such as in a liquid nitrogen. Could a silicon-based life form evolve in liquid nitrogen? Although such ideas are intriguing, altering both the fundamental chemistry and the solvent of life takes us even further into unknown chemical territory at the current time. However, these concepts remind us to keep an open mind.
4.12 The Structure of Life and Habitability
Astrobiologists are very interested in looking for habitable conditions on other planets, a theme that crops up throughout this book. Habitability can simply be explained as conditions suitable for life (or at least one known organism, since our concept of habitability is necessarily hemmed in by the biology that we know). This explains why the discussions in this chapter are so important, because the minimum conditions required by life ultimately determine what we define to be habitable conditions and the assessment we make of the habitability of other worlds. We need to be clear about what we think is the plausible minimum set of chemical elements and compounds as well as physical and chemical conditions required to make life.
So far, the discussions in this chapter permit us to list at least two requirements for an environment to be habitable. We need to have: (i) an environment that provides the basic elements for life (CHNOPS) and (ii) an environment where physical and chemical conditions support the existence of a suitable solvent, which at the current time we assume is liquid water. We see in Chapter 6 that we also require an energy supply. In Chapter 7, we discuss in more detail the physical and chemical limits to life on Earth, which also define the limits of habitable conditions.
4.13 Conclusions
Despite the huge variety of elements in the Periodic Table, the selection used by all life is very small. Six of them (CHNOPS) are used to construct the major molecules of life, with other elements commandeered by organisms for specific purposes. The chemical versatility of carbon manifested in its ability to form a wide diversity of covalent bonds with elements that have comparable bond strengths accounts for the millions of compounds it can form and the capacity to generate the molecular complexity required by life. Although the diversity of carbon molecules is vast in life, they broadly fit into some major groups, including proteins, carbohydrates, lipids, and nucleic acids. Proteins and sugars exhibit a chiral preference for L and D forms, respectively. For a long time, there has been speculation about alternative chemistries: silicon as the most favored alternative to carbon, ammonia as a possible alternative to water as the solvent in which biochemistry might occur. Although we should not rule out life based on these different chemistries, we have seen that there are good chemical reasons for carbon- and water-based life. With this knowledge, we are now able to see how these molecules come together to build living systems. This is the focus of Chapter 5.
Questions for Review and Reflection
1 Discuss the common molecular features of proteins, carbohydrates, and nucleic acids. How might such features be used to search for life elsewhere?
2 Life requires CHNOPS elements, but most living things also require some other elements. Describe the use of two other elements in living things.
3 Explain in simple terms how information is encoded in DNA and how the structure of that molecule allows for the information to be replicated.
4 Contrast the chemical steps in the assembly of proteins and carbohydrates. What are the similarities and differences?
5 We are composed mainly of L-amino acids and D-sugars. Why do you think life has this chiral preference? Use your knowledge of chemistry and astrochemistry to discuss whether you think this arrangement would be universal in all life (you might like to return to this question after you have read Chapter 10).
6 Describe two atomic features of carbon that make it a good element from which to build complex chained molecules in life. Contrast these features to silicon.
7 “Silicates have such astonishing molecular diversity that I see no reason why living things could not be made from minerals and rocks.” Discuss this point of view.
8 “Water is chemically fine-tuned to be perfect for living things.” Critique this statement.
Bibliography
Books
1 Grinspoon, D. (2004). Lonely Planets. New York: Harper Collins.
2 Schulze-Makuch, D. and Irwin, L. (2019). Life in the Universe. Heidelberg: Springer.
3 Ward, P. (2005). Life as We Do Not Know It. New York: Penguin.
Papers
1 Bains, W. (2004). Many chemistries could be used to build living systems. Astrobiology 4: 137–167.
2 Ball, P. (2017). Water is an active matrix of life for cell and molecular biology. Proceedings of the National Academy of Sciences of the United States of America 114: 13327–13335.
3 Benner, S.A., Ricardo, A., and Carrigan, M.A. (2004). Is there a common chemical model for life in the Universe? Current Opinions in Chemical Biology 8: 672–689.
4 Conrad, P.G. and Nealson, K.H. (2001). A non-Earth-centric approach to life detection. Astrobiology 1: 15–24.
5 Deamer, D. (2017). The role of lipid membranes in life's origin. Life 7: 5.
6 Firsoff, V.A. (1965). Possible alternative chemistries of life. Spaceflight 7: 132–136.
7 Georgiou, C.D. and Deamer, D.W. (2014). Lipids as universal biomarkers of extraterrestrial life. Astrobiology 14: 541–549.
8 Johnson, S.S., Anslyn, E.V., Graham, H.V. et al. (2018). Fingerprinting non-terran biosignatures. Astrobiology 18: 915–922.
9 Kamerlin, S.C., Sharma, P.K., Prasad, R.B. et al. (2013). Why nature really chose phosphate. Quarterly Reviews in Biophysics 46: 1–132.
10 McKay, C.P. and Smith, H.D. (2005). Possibilities for methanogenic life in liquid methane on the surface of Titan. Icarus 178: 274–276.
11 Morawietz, T., Singraber, A., Dellago, C. et al. (2016). How van der Waals interactions determine the unique properties of water. Proceedings
