Carbon and the Molecular Diversity of Life
Overview: Carbon – The Backbone of Biological Molecules
* Although cells are 70–95% water, the rest consists mostly of carbon-based compounds. * Carbon is unparalleled in its ability to form large, complex, and diverse molecules. * Carbon accounts for the diversity of biological molecules and has made possible the great diversity of living things. * Proteins, DNA, carbohydrates, and other molecules that distinguish living matter from inorganic material are all composed of carbon atoms bonded to each other and to atoms of other elements. * These other elements commonly include hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P). (CHONPS)
Concept 4.1 Organic chemistry is the study of carbon compounds
* The study of carbon compounds, organic chemistry, deals with any compound with carbon (organic compounds). * Organic compounds can range from simple molecules, such as CO2 or CH4, to complex molecules such as proteins, which may weigh more than 100,000 daltons. * The overall percentages of the major elements of life (C, H, O, N, S, and P) are quite uniform from one organism to another. * However, because of carbon’s versatility, these few elements can be combined to build an inexhaustible variety of organic molecules. * Variations in organic molecules can distinguish even between individuals of a single species. * The science of organic chemistry began in attempts to purify and improve the yield of products obtained from other organisms. * Initially, chemists learned to synthesize simple compounds in the laboratory, but had no success with more complex compounds. * The Swedish chemist Jons Jacob Berzelius was the first to make a distinction between organic compounds that seemed to arise only in living organisms and inorganic compounds that were found in the nonliving world. * This led early organic chemists to propose vitalism, the belief that physical and chemical laws did not apply to living things. * Support for vitalism began to wane as organic chemists learned to synthesize complex organic compounds in the laboratory. * In the early 1800s, the German chemist Friedrich Wöhler and his students were able to synthesize urea from totally inorganic materials. * In 1953, Stanley Miller at the University of Chicago set up a laboratory simulation of chemical conditions on the primitive Earth and demonstrated the spontaneous synthesis of organic compounds. * Such spontaneous synthesis of organic compounds may have been an early stage in the origin of life. * Organic chemists finally rejected vitalism and embraced mechanism, accepting that the same physical and chemical laws govern all natural phenomena including the processes of life. * Organic chemistry was redefined as the study of carbon compounds regardless of their origin. * Organisms do produce the majority of organic compounds. * The laws of chemistry apply to inorganic and organic compounds alike.
Concept 4.2 Carbon atoms can form diverse molecules by bonding to four other atoms
* With a total of 6 electrons, a carbon atom has 2 in the first electron shell and 4 in the second shell. * Carbon has little tendency to form ionic bonds by losing or gaining 4 electrons to complete its valence shell. * Instead, carbon usually completes its valence shell by sharing electrons with other atoms in four covalent bonds. * This tetravalence by carbon makes large, complex molecules possible. * When carbon forms covalent bonds with four other atoms, they are arranged at the corners of an imaginary tetrahedron with bond angles of 109.5°. * In molecules with multiple carbons, every carbon bonded to four other atoms has a tetrahedral shape. * However, when two carbon atoms are joined by a double bond, all bonds around those carbons are in the same plane and have a flat,...