Читать книгу Astrobiology - Charles S. Cockell - Страница 75

The elements of life come together in combinations to form molecules. The most common elemental backbone of these molecules is carbon, for example in the simplest amino acid, glycine, shown in Figure 4.2, in which two carbon atoms form the core of this molecule. Most of these atoms, as shown for glycine, are bound together covalently. The abundance of carbon in biological molecules means that we often refer to life on Earth as “carbon-based.”

Figure 4.2 The organic molecule glycine, the simplest amino acid.

There is a variety of reasons why carbon is the basic atomic building block of life. It forms stable bonds with many other elements, such as H, N, O, S, and P. Some of these bonds are more common than others. CH bonds are ubiquitous throughout life, but CS are rarer (they are found in the amino acid cysteine and used to make disulfide bridges in proteins). CP bonds are rare but can be found in alkyl phosphonates. P is more commonly found in phosphodiester bonds, such as in adenosine triphosphate (ATP), where the P is linked to O atoms that are themselves linked to carbon. Bonds between carbon and other atoms are stable, but not so strong to make it necessary to use large quantities of energy to break them (if that were the case, it would be difficult for life to break down and metabolize compounds). The energy it takes to break bonds with H (413 kJ mol⁻¹), N (308 kJ mol⁻¹), O (360 kJ mol⁻¹), S (272 kJ mol⁻¹), P (264 kJ mol⁻¹), and other carbon atoms (347 kJ mol⁻¹) is quite similar, which means that carbon can interchange between these atoms without much energy being required or released. This gives carbon versatility in being involved in the breaking down and forming of new complex molecules. Furthermore, carbon forms stable carbon–carbon double and triple bonds, which further increases the diversity of possible compounds.

Molecules containing carbon range in structure from chains to rings. The simplest carbon molecules are alkanes [with the formula C_nH_(2n+2)]. If n = 1 then the molecule is CH₄ or methane, which is very common on Saturn's moon Titan, and underground on Earth where it is produced by microbes called methanogens. If n = 2 the molecule is C₂H₆, which is ethane, another common organic molecule in the Universe. The substitution of hydrogen with other atoms results in functional groups of wide use in different biochemical functions. For example, esters have the general formula –COO–R (where R is an alkyl group; an alkyl group is any group with the general formula C_nH_2n+1). These turn up in the membranes of cells. Amino (–NH₂) and carboxyl (–COOH) groups attach to carbon to form amino acids, the units used to make proteins. The phosphate group, –PO₄, is attached to, or incorporated into, a whole variety of molecules including membrane lipids, DNA, and many enzymes. Alcohols are carbon compounds with an –OH group, used by microorganisms in energy-yielding reactions. And the list goes on, a vast array of compounds made possible by the covalent bonding of carbon to the CHNOPS elements, which includes other carbon atoms.