"The carbon atom, the primary constituent of organic molecules, is, in several respects, uniquely fit for the assembly of the complex macromolecules found in the cell.
First, due to the stability of carbon-carbon bonding, only carbon
can form long polymers of itself, forming long chains or rings, while also bonding to other kinds of atoms. Though silicon can also form long chains by bonding with itself, these bonds are significantly less stable than carbon-carbon bonds.
Plaxco and Gross note that “while silicon-silicon, silicon-hydrogen, and silicon-nitrogen bonds are similar in energy, the silicon-oxygen bond is far more stable than any of the other three types. As a consequence, silicon readily oxidizes to silicon dioxide, limiting the chemistry available to this atom whenever oxygen is present. And oxygen is the third most common atom in the Universe.”
As Primo Levi explains, carbon “is the only element that can bind itself in long stable chains without a great expense of energy, and for life on earth (the only one we know so far) precisely long chains are required. Therefore carbon is the key element of living substance.”
Second, carbon is tetravalent — that is, each atom can form four covalent bonds with other atoms.
Second, carbon is tetravalent — that is, each atom can form four covalent bonds with other atoms.
Third, carbon possesses a relatively small atomic nucleus, entailing short bond distances, thereby allowing it to form stable bonds with itself as well as other atoms. This property is also possessed by the other small, non-metal atoms in period two. Carbon is able to form single, double, and triple bonds with other atoms. Nitrogen can also form single, double, or triple bonds and oxygen can form single and double bonds. Contrast this with the nonmetal atoms directly beneath them in the periodic table — silicon, phosphorus, and sulfur — which possess larger atomic radii and therefore form such bonds less easily due to multiple bonds having reduced stability.
Another property of organic bonds is that their strength sits.jpg)
within a Goldilocks zone, being neither too strong nor too weak for biochemical manipulations in the cell. If the strength of those bonds were to be altered by a single order of magnitude, it would render impossible numerous biochemical reactions that take place in the cell.
Another property of organic bonds is that their strength sits
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--If it were too strong, the activation energy needed to break bonds could not be sufficiently reduced by enzymatic activity (enzymes strain chemical bonds by engaging in specific conformational movements while bound to a substrate).
--Conversely, if organic bonds were much weaker, bonds would be frequently disrupted by molecular collisions, rendering controlled chemistry impossible.
Another special characteristic of carbon is that there is not much variation in energy levels of carbon bonds from one atom to the next. Robert E. D. Clark explains that carbon “is a friend of all. Its bond.jpg)
energies with hydrogen, chlorine, nitrogen, oxygen, or even another carbon differ little. No other atom is like it.”
Another special characteristic of carbon is that there is not much variation in energy levels of carbon bonds from one atom to the next. Robert E. D. Clark explains that carbon “is a friend of all. Its bond
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Kevin W. Plaxco and Michael Gross further comment, “Carbon presents a fairly level playing field in which nature can shuffle around carbon-carbon, carbon-nitrogen, and carbon-oxygen single and double bonds without playing too great a cost to convert any one of these into another… Given all this, it’s no wonder that on the order of ten million unique carbon compounds have been described by chemists, which is as many as all of the described non-carbon-containing compounds put together.”
Jonathan McLatchie
Jonathan McLatchie
