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Building from the previous chapter, we could start with a very simple question: what is the minimum set of elements that life needs to assemble molecules? The matter from which life is constructed is unsurprisingly restricted to what is in the Periodic Table, and therefore its options are not unlimited.

Furthermore, we can assume that the Periodic Table is universal. This seems a reasonable working assumption, as no astrophysicist has yet observed a place in the Universe that seems to have an entirely alien chemistry; in fact quite the contrary – as we shall see later, many elements of importance to life, such as carbon, seem to be surprisingly abundant throughout the Universe. Therefore, the observations we make about the chemistry of terrestrial life are likely to reveal important insights into the potential chemistry of life anywhere in the Universe.

If we survey the elements from which life is constructed, we find six that are found in all living things; they have often been described as the “building block” elements of life (Figure 4.1). They are C (carbon), H (hydrogen), N (nitrogen), O (oxygen), P (phosphorus), and S (sulfur). They are sometimes remembered as the odd word CHNOPS, or the variation SPONCH – take your pick.

Figure 4.1 The six ubiquitous elements of life, CHNOPS. The van der Waals radius in picometers (pm) is the effective radius of the atom when modeled as a hard sphere. Also shown are the atomic mass and density for certain forms of the elements. The spheres show the relative sizes of the atoms. The colors are only for illustration purposes.

Carbon is used as the backbone of most complex molecules in life as we shall shortly see. Carbon comes in both inorganic and organic carbon forms. Inorganic carbon is carbon that lacks CH bonds and includes minerals such as calcium carbonate (CaCO₃). The form we are interested in, at least in terms of the assembly of life, is organic carbon, or carbon combined with elements such as H, N, and O. The name organic carbon, is rather unfortunate. It was originally derived from the view that such carbon could be found only in living things, although we now know that organic carbon compounds can be formed by non-biological processes.

What about the other five elements? Hydrogen is bound to many atoms, for example in –CH, –NH₂, –SH groups, and it is common in all organic compounds found in life (as well as being in life's solvent, water, in addition to oxygen). It usually ends carbon chains. You will remember from Chapter 3 that the hydrogen atom requires one electron to fill its electron shell. It tends to bind where there is an electron going spare, giving it a ubiquitous presence in many molecules in life.

Nitrogen is found in many carbon-based ring structures and amino acids and confers a greater degree of complexity on the range of organic molecules possible.

Oxygen is an essential atom in many organic compounds. It is found in ring structures, in –OH groups (for example in sugars and alcohols) and other bonds, again expanding the range of organic complexity.

Phosphorus is found in many proteins and in molecules such as adenosine triphosphate (ATP) involved in energy transfer in the cell. It is also found in the nucleic acids, such as DNA.

Sulfur is found in amino acids, such as cysteine (as we saw in Chapter 3, these take part in forming disulfide bridges that contribute to the three-dimensional structure of proteins), and it is found in iron–sulfur clusters involved in electron transfer in the cell.

These particular elements are found everywhere in life on account of a number of characteristics. They are neither too reactive nor inert, making stable covalent bonds with carbon. Apart from hydrogen, these elements can make multiple covalent bonds, making them versatile in linking between atoms. They generally have a sufficient natural abundance to make them widespread and available (this is not always the case, since ecosystems can be limited in one or more of these basic elements).

Needless to say, the CHNOPS elements must be in a biologically accessible state. For example, nitrogen gas (N₂) is energetically difficult to break up. The strength of the nitrogen–nitrogen triple bond (941 kJ mol⁻¹) is such that only certain bacteria with nitrogen fixation pathways can accomplish this task. Other organisms require nitrogen in more readily accessible states, such as ammonium (NH₄⁺) or nitrogen oxides, such as nitrate (NO₃⁻). Thus, the mere presence of nitrogen gas on a planet theoretically provides a source of N, but acquisition of this element requires evolutionary innovations which on Earth are restricted to certain specific organisms.

Another excellent example is the ubiquitous compound: water (H₂O). It clearly presents a potential source of hydrogen and oxygen atoms. Yet, in general, it is not easily broken up by biology (the H–OH bond energy has a relatively high value of 498.7 kJ mol⁻¹). Oxygenic photosynthetic organisms (Chapter 6) can accomplish the splitting of water as a source of electrons for photosynthetic energy acquisition. They have a specific water-splitting metal cluster in their photosystem that allows for this process. There is no known organism that has such a system to use water as a source of hydrogen and oxygen atoms for building biological molecules. This is likely because these atoms can be acquired less energetically from other sources. Non-biological processes can break down water, making its constituent atoms more available. For example, radiolysis, the breakdown of water by ionizing radiation, can release hydrogen, which is accessible to life.

To say that habitability is defined by the presence of biologically available CHNOPS elements may seem facile, but it is important to keep in mind the suggested source of these elements in any environment being studied for its habitability and how accessible they are.