There are roughly 92 naturally occurring elements on earth, but only four make up about 96% of all living organisms: oxygen, carbon, hydrogen and nitrogen. These elements combine to form life-sustaining biomolecules, which can be divided into four major groups of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, proteins, and lipids are used by cells as the building blocks for cells and for energy, while nucleic acids are the basis of genetic material (DNA and RNA).  A shared characteristic among all these macromolecules is their reliance on carbon to form the core backbone and scaffold of their molecular structures.  The role of carbon is so signficant that an entire branch of chemistry was dedicated to its study.  Organic Chemsitry, which many of you will take in a future semester is defined as “the chemistry of carbon compounds.”

It is not an overstatement to say that all life on earth is carbon based.  But, why carbon? If you could travel to distant universe and observe life on a habitable planet would it also be carbon based?   Several features of carbon make it incredible useful as the central building block of life.  Perhaps most important is the ability of carbon to consistently make 4 covalant bonds.  The feature allows for a vast variety of possible carbon frameworks.  Becuase of its intermediate electronegativity we see both polar and non-polar covanlent bonding. Its two simplest compounds carbon dioxide (CO2) and methane (CH4) are abundant and lightweight.  At most temperatures they exist as gases and are therefore easily distributed throughout the bioshpere ready to be used in living organisms. 


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