enough so that the two strands can be unzipped when they need to be pulled apart for replication of the DNA molecule when cells divide, without the requirement for a large amount of energy to be expended. You can imagine that if the two strands were linked at every base pair with covalent bonds, it would require much more energy to pull apart the two DNA strands. In Chapter 4, we investigate the structure and functions of DNA in more detail. For now, let us note how the evolutionary process tends to select bonding types at the atomic level that are optimized to do particular tasks. Hydrogen bonding is ideal for a situation where molecular stability is required, but where the regular opening of molecular bonds means that it is also optimal to have a bonding type where energy requirements to break the bonds are minimized. You might like to ask yourself: Could you imagine a genetic material where two complementary strands are held together using van der Waals interactions that might offer a similar compromise?
3.11 An Astrobiological Perspective
How much of the preceding discussion on bonding types is applicable on a universal scale? We could probably confidently say that the major bonding types we have discussed are universal. They are shaped by fundamental interactions between electrons of different ions, atoms, and molecules. As we have already agreed that fundamental atomic structures are universal, then the bonding types available to construct life are also likely to be universal. However, how much of the detail on how these bonding types are used in different molecular structures is universal? Are we confident that ionic interactions between amino acids to make proteins are universal? Must all life contain proteins? Would all material involved in storing information about life (genetic material or its equivalent) use hydrogen bonding to hold together molecules that need to split in two and replicate? We have already explored some ideas about why life on Earth does not use metallic structures. Are those proposed reasons universal, or could we find aliens with steel bars for bones and skulls made of titanium? (Here I am referring to the process of natural selection, not technological augmentation, which, as we all know, can be used to put steel bars in bones.) As we discuss the major macromolecules of life in Chapter 4, continue to consider the question of the universality of the structure of life.
In the meantime, let us now continue to consider the basic behavior of matter.
Discussion Point: Is the Structure of Life Universal?
Some people say that astrobiologists are narrow-minded and that alien life forms, if they exist, will be constructed in ways unimaginable to us and possibly very differently from life on Earth. Do you agree with this? As you progress through this and later chapters, you might like to consider at what level of hierarchy this statement may or may not be true. Hydrogen bonding, for example, would be expected to be the same anywhere in the Universe, as it is determined by the interactions between atoms in the universal Periodic Table. So too with all the forms of bonding explored in this chapter. Surely, therefore, are we are on safe ground to say that if life exists elsewhere it would use the same types of bonding to hold its atoms, ions, and molecules together? In Chapter 4, we look at how molecules in terrestrial life are put together. Are these molecules universal structures? Would we expect cells to be put together using the same basic molecules? We then progress in Chapter 8 to look at the evolutionary relationships between whole organisms. Are these universal? Consider at what scale in the structure of life from atoms to communities of organisms you would be confident to say that all life in the Universe would share identical characteristics. In other words, at what level of the hierarchy of life's structure is biology deterministic and where can contingency, or chance, play a role? If you could run evolution again, what structures of life could you confidently predict would reappear?
3.12 The Equation of State Describes the Relationship Between Different Types of Matter
Having investigated how matter is held together, we now look at different types of matter, their interrelationships, and some of the important consequences for life.
Matter can be found in different forms. The major forms are solid, liquid, gas, and plasma. Which one of these states of matter exists in any given environment is determined by the pressure and temperature conditions. The relationship that describes the state of matter under any given conditions is called the equation of state.
3.12.1 Phase Diagrams
One very common way to show the equation of state is to focus on just the pressure and temperature (although we could also look at relationships between pressure and volume or temperature and volume, but these tend to be less interesting and give us less information). This depiction of the state of matter as a function of pressure and temperature is called a phase diagram. You can see a phase diagram for water in Figure 3.17.
Figure 3.17 A phase diagram for water. The axes are not drawn to a fixed scaling, but they are drawn to exaggerate values of important features of the diagram.
Let's examine some of the main features of this diagram. On the x axis is temperature and on the y axis pressure. Follow the horizontal line shown on the figure from the left to the right. This line shows the change in the form of water at a pressure of 1 atm (our usual experience).
At atmospheric pressure and low temperatures below 0 °C water is a solid. This is consistent with our experience of snow and ice on a cold winter's day. Or more trivially, the reason why we use a freezer to make ice. If we warm the ice, then at 0 °C it undergoes a phase change when it meets the melting curve. The melting curve defines when a substance transforms from a solid into a liquid (or vice versa) for any given combination of pressure and temperature. This is also an experience we are all familiar with when we see ice melting in a drink or on the roads. As we heat it up further, the water undergoes another phase change at 100 °C as it turns into gas when it reaches the vaporization curve. The vaporization curve defines when a substance transforms from a liquid into a gas (or vice versa) for any given combination of pressure and temperature. At this point, water boils. If we continue to heat it to very high temperatures (several thousand K), it would turn into a plasma as electrons are driven off the nuclei (this phase is not shown on the diagram).
There are two features of the diagram to point out. At high temperatures and pressures, there is a point called the critical point, at which gas and liquid become indistinguishable. Matter in this part of the phase diagram is called a supercritical fluid. This state of matter is not used in biological systems, although it has general importance for understanding the behavior of matter. Some exoplanets with high surface pressures and temperatures have been suggested to have atmospheres and surfaces with supercritical water (Chapter 20).
You will also notice that at pressures lower than atmospheric pressure, water boils at lower temperatures than 100 °C. This is consistent with the experience of mountaineers. The higher they go, the lower the temperature at which water boils (making it more difficult to cook vegetables). At the summit of Mount Everest (a height of 8848 m), the boiling point of water is 71 °C. If we continue to reduce the pressure, we hit a point on the graph called the triple point. The triple point is the point at which all three phases of matter can co-exist. You will see that at this point and at lower pressures, if we heat ice it turns directly into gas – it undergoes sublimation. The triple point of pure water is at 0.01 °C (273.16 K) and 611.2 Pa. There is no liquid phase
