The late Dr. Carl Sagan loved to tell us our bodies are made of star dust. James D. Watson told us that life was just a matter of chemistry, a sequence of naturally occurring chemical reactions building more and more complex molecules. But what is this star dust, what are the raw materials, the elemental components of the human body.
Approximately 50 to 60 percent of the body weight of an average full grown human is water. Water, of course, is compose of the elements hydrogen and oxygen. But water is not the only compound in our bodies containing hydrogen and oxygen. The fundamental building blocks of the human body, and all life for that matter, are twenty different amino acids. They are the essential components of all proteins, DNA and RNA, which after the water has been removed make up the next largest amount of our body mass.
There are 20 amino acids found in human bodies, consisting of complex molecules containing an amino group, a carboxyl group and an “R” group. The amino group is essentially a molecule of ammonia, which consists of one atom of nitrogen covalently bonded to 3 atoms of hydrogen. An ammonia molecule will react with a molecule of methane, comprised of one atom of carbon bonded to 3 hydrogen atoms, to form a molecule called methylamine. Methylamine is the essential base molecule from which 19 of the twenty amino acids are formed. In the case of the twentieth amino acid, proline, the methane molecule is replaced by a propane one. When a molecule of methyl or propyl amine is formed the amino group essentially replaces one of the hydrogen atoms attached to the methyl or propyl component. The complete amino acid results when a carboxyl group (COOH) and R group are added. The R group can be any one of twenty organic compounds resulting in the differences between the basic amino acids found in human bodies. For instance, when the R group is a simple methane molecule, the simplest of the amino acids alanine, is the result. So basically, the major building blocks of the human body contain a hole lot of the elements hydrogen, carbon, oxygen and nitrogen.
Most of the structural tissues in our bodies are made up of proteins. Proteins themselves, are nothing but long single stranded chains of amino acids bonded together. The bond which links amino acids in proteins is called a peptide bond, and exists between the carboxyl group of one amino acid and the amino group of another. In addition to structural building material, proteins are also responsible for biosynthesis and metabolic regulation. All of the hormones and enzymes in our bodies would fall into this group of proteins. DNA and RNA provide the template to make every protein our body needs. Like proteins, DNA and RNA are long chains of amino acid based polymers called nucleotides; bonded with peptide bonds to form long chain molecules.
But these macromolecules are substantially more complex than simple proteins. First of all, they are double stranded, each strand bonded to the other with hydrogen bonds. Next, they include a kind of interposer to facilitate the vertical nucleotide peptide bond arrangement. These interposers consist of 5-carbon sugars (ribose in the case of RNA and Deoxy-ribose in the case of DNA), and a molecule of mono-phosphate (PO4). These sugar phosphate molecules essentially form what molecular biologists refer to as the back-bone of DNA. All the phosphorus contained in DNA and RNA represents about 1% of the human bodies total mass.
In addition to proteins, the human body uses the elements calcium as a structural building material for teeth and bone, representing about 1.5 percent of the bodies total mass. There are also trace amount of other elements, which generally facilitate metabolic processes.
When you add it all up in terms of elemental content, Hydrogen is the most abundant element in the human body, representing about 63 percent of the total atoms. When measured by atomic, or “molar”content, Oxygen comes in second at 26% followed by carbon at 9% and nitrogen at 1.25 percent. This represents 99.25 percent of the atoms in the human body. If you look at the bodies elemental content by weight, it adds up a little differently, as quantified in the following list. .
In the final analysis, it would appear the doctors, Watson and Sagan, summed up the elemental content of the human body pretty well, as a lot of chemistry and a little bit of star dust.
http://chemistry.about.com/cs/howthingswork/f/blbodyelements.htm (This 2nd source was used for corroborative purposes. analysis by weight is assumed but not stated in the article.)