Читать книгу Astrobiology - Charles S. Cockell - Страница 89

One solvent that meets these needs, as life on Earth shows, is water, or H₂O.

Of the characteristics that make water a particularly suitable solvent for life is its dipole moment or polarity (measured as permittivity or the dielectric constant). The dipole moment of water (6.2 × 10⁻³⁰ C m [coulomb meter] or 1.85 debye) is such that the molecule readily dissolves both ions and small organic molecules (Figure 4.15). Both cations (positively charged ions such as K⁺, Na⁺, Fe²⁺) and anions (negatively charged ions such as Cl⁻) play a role in a variety of functions such as stabilizing membranes or as sources of electrons. The dissolution of ions contributes to the ability of water to act as a medium for chemical reactions that require charged species. The polarity of water also allows for the dissolution of small organic compounds, such as amino acids. This property allows water to act as a mediator of the organic polymerization reactions discussed in previous sections.

Figure 4.15 Water dissolves a range of substances, including salts, sugars, and small organic compounds such as amino acids.

The polarity of water allows water molecules to bond together through hydrogen bonding. This property accounts for the wide temperature range of water, temperatures that overlap with environmental conditions on the surface and in the subsurface of a variety of planetary bodies (obviously including Earth) such as the interior of the Jovian moon Europa or the Saturnian moon Enceladus. Without the capacity for hydrogen bonding, the small molecular weight of water would result in a much smaller range of temperature conditions in the liquid state.

Water is not merely a solvent for life; it also plays fundamental roles in biochemical reactions. For example, it can act as a proton wire, conducting protons through its hydrogen-bonded network. This has been demonstrated in bacteriorhodopsin, a molecule involved in bacterial photosynthesis. In some proteins, water is found within the active site of enzymes where more than merely being part of the solvent, the water molecules there play a role in binding to the substrate as it is aligned within the active site to take part in chemical reactions. In proteins, water plays a role in binding to the outside of molecules and contributing to the fine balance between the flexibility and rigidity of the molecules. Thus, water is very much part of the structure of life, not merely a passive solvent in which reactants move.

Other properties of water account for many of its beneficial uses as a biological solvent. Water has a high heat of vaporization (in other words, it takes quite a lot of energy to get it to vaporize), which promotes a stable liquid phase inside organisms and stabilizes temperatures within them, enhancing the ability of organisms to cope with fluctuating environmental temperature regimens. A high heat of vaporization also implies a high energy loss during evaporation, which is used by multicellular organisms to achieve evaporative cooling against high temperatures in the environment.

Perhaps one of the most discussed properties of water that has been implicated in its biological usefulness is the lower density of ice than water, which we discussed in Chapter 3. As ice floats on water, organisms can remain protected in the liquid water beneath it. There is little doubt in saying that this property is beneficial for fish and many other aquatic organisms. However, it is worth considering briefly organisms that can tolerate freezing. Many microorganisms can resist freezing, and the wood frog (Lithobates sylvatica; Figure 4.16), in a similar way to other North American frogs that hibernate close to the surface in soil or leaf litter, can tolerate freezing temperatures. The frog transforms glycogen in its blood to the sugar glucose in response to internal ice formation at the beginning of winter. These molecules act as protectants against damaging ice crystal formation. Frogs can survive freezing during winter if no more than about 65% of the total body water freezes. Although the wood frog is an unusual example, it is clear that evolutionary strategies do exist to tolerate freezing, and although the physical attribute that ice has to float on water may appear to favor life underneath, it does not seem to be a fundamental requirement for life to exist. Life on Earth does not depend on having liquid water under ice to be able to persist. Nevertheless, the lower density of ice compared to liquid water has other important implications. Ice formed on the surface of a water body tends to trap energy underneath and thus maintain a liquid state over long time periods. If water ice sank, lakes, rivers, and other water bodies would freeze completely from the bottom up, which could mean a greater energy requirement to melt the ice seasonally. From a physical point of view, the property is important for understanding aspects of planetary habitability.

Figure 4.16 The wood frog (Lithobates sylvatica). The frog can tolerate freezing temperatures by using glucose as an antifreeze protectant in its tissues.

Source: Reproduced with permission of Brian Gratwicke, https://en.wikipedia.org/wiki/Wood_frog#/media/File:Lithobates_sylvaticus_(Woodfrog).jpg.

The property of floating ice and its tendency to trap warmer water with its benefits for life raises a more fundamental question. Are the properties of water fine-tuned for life? Some people take this point of view, but it invites the problematic idea that someone or something has produced a solvent ideal for producing biology. A more evolutionary point of view is to see this from the opposite standpoint: Is life fine-tuned to use the properties of water? For example, the use of water as a proton wire and as part of the active site of enzymes can be understood as life evolving to use the beneficial properties of the water molecule that confer selective advantage, rather than water having an uncanny set of properties “just right” for life.

It is also worth pointing out that water does have some properties that are not always conducive to life. In certain situations, it can be a reactive solvent, causing the dissociation of molecules. For example, it can be disruptive to the hydrogen bonding between amino acids in proteins, making it sometimes unconducive to protein folding. Water provides a medium for the reactive deamination of nucleobases in nucleic acids, meaning that organisms must constantly repair their DNA against this chemical damage.

Furthermore, as we have seen, the formation of some biological molecules, such as proteins, involves condensation reactions where water is removed from a bond, eliminating it from the chemistry. This is not a deleterious property of water as such, but it does show that chemistry conducive to forming the macromolecules of life involves the removal of water from molecules. We should be careful not to view water as a perfect solvent. However, of all possibilities, and despite some biochemically harmful properties in certain situations, water might be the best solvent for carrying out a large variety of chemical reactions in carbon-based life. We can revisit this question when we discuss alternative solvents later in the chapter.