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2.3  Chemicals that Make Cells Work

cell that the virus infects. So, in other words, they do not fulfil the criterion of independent

self-​replication and cannot thus be considered a basic unit of life, by this semiarbitrary defin­

ition. However, as we have discussed in light of the selfish gene hypothesis, this is still very

much an area of debate.

2.3  CHEMICALS THAT MAKE CELLS WORK

Several different types of molecules characterize living matter. The most important of these

is undeniably water, but, beyond this, carbon compounds are essential. In this section, we

discuss what these chemicals are.

2.3.1  IMPORTANCE OF CARBON

Several different atomic elements have important physical and chemical characteristics of

biological molecules, but the most ubiquitous is carbon (C). Carbon atoms, belonging to

Group IV of the periodic table, have a normal typical maximum valency of 4 (Figure 2.2a)

but have the lowest atomic number of any Group IV element, which imparts not only a

relative stability to carbon–​carbon covalent bonds (i.e., bonds that involve the formation of

dedicated molecular bonding electron orbitals) compared to other elements in that group

such as silicon, which contain a greater number of protons in their nuclei with electrons

occupying outer molecular orbitals more distant from the nucleus, but also an ability to

form relatively long chained molecules, or to catenate (Figure 2.2b). This property confers a

unique versatility in being able to form ultimately an enormous range of different molecular

structures, which is therefore correlated to potential biological functions, since the structural

properties of these carbon-​based molecules affect their ability to stably interact, or not, with

other carbon-​based molecules, which ultimately is the primary basis of all biological com­

plexity and determinism (i.e., whether or not some specific event, or set of events, is triggered

in a living cell).

KEY POINT 2.6

Carbon chemistry permits complex catenated biological molecules to be made, which

have intermediate chemical stability, that is, they are stable enough to perform bio­

logical functions but can also be relatively easily chemically modified to change their

functional roles.

FIGURE 2.2  Carbon chemistry. (a) Rod and space-​filling tetrahedral models for carbon atom

bound to four hydrogen atoms in methane. (b) Chain of carbon atoms, here as palmitic acid, an

essential fatty acid.