Читать книгу Astrobiology - Charles S. Cockell - Страница 94

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.