Structural Biochemistry/Alkali Metals
The alkali metals are a series of chemical elements forming Group 1 (IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). (Hydrogen, although nominally also a member of Group 1, very rarely exhibits behavior comparable to the alkali metals). The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group.
Alkali Metals generally exist as cations with a 1+ charge, and exists as lustrous metals that conducts both heat and electricity well. They are water soluble, basic and very reactive. Alkali metals are the least electronegative group, and tend to react best with nonmetals and water.
Potassium and Sodium are vital in biochemical systems. Valinomycin complexes selectively with potassium for transporting molecules across the cell membrane. Crown ethers and cryptands are structures that complex with alkali metal ions; supramolecular chemists are studying these interactions. 
The sodium and potassium pump are important in maintaining equilibrium amounts both inside and outside the cell. Sodium is concentrated outside, whereas potassium is concentrated inside. Both of these metals also help to communicate electrical signals in nerves and in the heart. Humans obtain sodium by eating foods with table salt and baking soda. To obtain potassium, we can eat bananas, oranges, and avocados.
Lithium is the lightest of the alkali metals. Traces of lithium ion occur in animal tissues, but it has no known physiological role. Lithium salts were introduced into psychiatry in 1949 for the treatment of mania. It is believed that the lithium ion at concentrations of 1 to 10 mEq per liter inhibits the depolarization-provoked and calcium-dependent release of norepinephrine and dopamine, but not 5-HT, from nerve terminals. Lithium ion is small ( its radius is around 0.6 Angstrom) and mobile ( fast exchange ligands ). However, it weakly binds to ligands and is easily hydrated by water. In medicine, Lithum is used to control bipolar affective disorder such as manic depression.
Sodium reacts spontaneously with water and rapidly with oxygen. It is a silvery, soft solid in its pure form. There is an abundant amount of the sodium element, most commonly recognized as sodium chloride, or table salt. Sodium chloride is an important nutrition for animals.
Potassium reacts very spontaneously with water, igniting the hydrogen, causing it to burst into lavender-colored flames. It is the sixth most abundant element on earth and is used in fertilizer and drain cleaners.
Rubidium has similar characteristics as potassium but is much more reactive.
Cesium is a silver metal that is one of the five elements that are liquid at room temperature. It also has similar characteristics to potassium but is extremely reactive.
Francium is the second rarest natural and most unstable element. All the francium isotopes are radioactive and have short half-lives.
Applications of Alkali Metals
Most of the akali metals displayed on the periodic table have many applications aside from being used chemical reactions. Two of the most well known alkali pure metals used are rubidium and caesium. These two metals are used in atomic clocks, which is a device that uses electronic transition frequencies that most accurately depict time and frequency standards for known substances. Compounds such as sodium are also used in daily applications such as in the sodium-vapour lamp, which is simply a lamp that can emit very efficient light.
1. Goodman, Louis S, and Alfred Goodman Gilman. The Pharmacological Basis of Therapeutics. 7th ed. New York, N.Y.: Macmillan, 1985. 2.Bertini,Ivano. Biological Inorganic Chemistry : Structure and Reactivity.