Carbon is the backbone of all organic compounds. It forms the main structure of the molecule with Hydrogen, Nitrogen, Oxygen and occasionally other elements replacing Carbon or bonding to it at key points in the molecular structure. A carbon skeleton is the basis of all amino acids and nucleotides, the building blocks of proteins and of DNA/RNA respectively. The "R" in the image here represents a different side chain specific to each amino acid. There are 22 "standard" amino acids used in human proteins, of which eight are "essential" and can not be synthesized in the body so must be taken in as a food.
Carbon forms strong bonds with itself so it can form long chain compounds. Carbon forms more compounds than any other element, with almost ten million organic compounds described to date, plus an almost unlimited number of compounds that are theoretically possible.
Carbon also has some unique properties at the elemental level. It is the fourth most abundant element in the Universe, and is comparatively unreactive under normal conditions. It forms a number of allotropes most notably diamond, one of the hardest substances known and highly abrasive, and graphite, one of the softest substances and a very good lubricant. It can also form nanotubes, an area of intense research currently with significant possibilities in many areas. Carbon typically forms four bonds, filling up its 2p orbital.
Hydrogen is the filler of the open bonds with Carbon, any bond not linked to Oxygen, Nitrogen or other element in the compound will generally be linked to a Hydrogen atom. Thus a molecule made up of a Carbon chain with only Hydrogen atoms for any open bonds is called a Hydrocarbon. Examples of these would be many oils and fats as well as the main components of gasoline and natural gas. Gasoline is a mixture of mostly long chain Hydrocarbons. The Octane rating of a gasoline is partially based on the amount of Octane (a long chain Hydrocarbon containing eight Carbon atoms) found in the gasoline. Hydrogen also forms relatively weak bonds with other atoms in other molecules or even within the same molecule, helping to give specific shapes to long chain molecules like proteins.
Oxygen generally forms the acidic group in the organic compound. It typically is part of an -OH or -COOH group that can release a Hydrogen ion (H+). As a C=O group (double bond) found in the amino acid, it can also function as one side of the weak Hydrogen bonds, bonding with a Hydrogen in another molecule or another part of the molecular chain.
Nitrogen makes up the amino group of the amino acid (-NH2) that is a weak base (as in Ammonia NH3). This gives the Amino Acids both basic and acidic properties that allow for the many possible bonds and reactions seen in different proteins.
Besides being the chief components of organic compounds, these four elements are found all around us as inorganic compounds. Examples of some simple molecules in everyday life that contain these elements are: Nitrogen as N2, Oxygen as O2, Carbon Dioxide - CO2, Water - H2O, Methane - CH4 (natural gas) and Ammonia - NH3.
"God saw all that He had made, and it was very good. And there was evening, and there was morning--the sixth day. Thus the heavens and the earth were completed in all their vast array. By the seventh day God had finished the work He had been doing; so on the seventh day He rested from all his work. And God blessed the seventh day and made it holy, because on it He rested from all the work of creating that He had done. Gen 1:31-2:3 (NIV)".
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