¶ … abundant elements that occur in living organisms are as follows: carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous. These and all elements are made up of atoms, which are the primary units of matter. Molecules are groups of atoms that are covalently bonded. Covalent bonds occur when two atoms share one or more pairs of electrons. This is different from ionic bonds, which arise when an electrical attraction between two oppositely charged ions takes place. Elements are substances that cannot be chemically dissected into simpler kinds of matter. Compounds, however, are atoms of two or more elements that are united by chemical bonds.
The parts of an atom include protons, neutrons, and electrons. These are all called subatomic particles. Protons, which are part of the nucleus of an atom, have positive electrical charges. Neutrons, which are also part of the nucleus of an atom, and as the name indicates, are neutral -- they do not have any electrical charge. Rapidly rotating around the nucleus, electrons carry negative electrical charges.
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, with a typical ratio of 1:2:1, respectively; they tend to be polar; they are broken down during cellular respiration; and they provide most of the body's energy. Proteins, also organic compounds, consist of hydrogen, oxygen, carbon, and nitrogen atoms; they include chains of amino acids, which in turn form strings of polypeptides; they are usually soluble in water, salt solutions, or in alcohol; they have a coiled structure; and they are the major structural and functional material of body cells. Lipids are fatty compounds made of carbon, hydrogen, and oxygen, with an abundance of carbon-hydrogen bonds; they are not water soluble; and their purpose is to store energy. Nucleic acids are complex organic compounds that store important information in cells; they contain phosphorous, carbon, oxygen, hydrogen, and nitrogen; and they are mildly acidic. DNA and RNA are the most important types of nucleic acids.
