Chemistry for Physiology
There are 90 naturally occurring simplest substances called chemical “elements”. An additional 28 have been produced artificially. Thirty-four of the 118 elements are radioactive, while there also exist some radioactive forms of the stable elements, such as 14C, 40K and 3H. The smallest particle of an element is called an atom of that element. The names of these elements are shortened to one- or two-letter symbols that are displayed on the “periodic table” of elements. Metal elements (e.g. Li, Na, Ca, K) appear on the left, while non-metal elements (e.g. Cl, O, N) appear on the right-hand side of this table. A metal element may react with a non-metal element to form a new substance which will be a type of “ionic” compound. A non-metal element may react with another non-metal element to form a new substance which will be a type of “covalent” compound. Ionic compounds in solid form are continuous lattice structures, which when they dissolve, allow the particles to move about separately as positive ions if they have lost an electron(s) or negative ions if they gained electron(s). Covalent compounds exist as groups of atoms (known as molecules), with a fixed ratio of different atoms. The atoms in these molecules stay together. Examples are H2O (water), C6H2O6 (glucose), CO2 (carbon dioxide) and CH3COOH (acetic acid). Ions and small molecules, such as these, and amino acids and lipid molecules are able to move into and out of cells through pores in the plasma membrane during normal cell functioning.