Editing Period 3 element (section)

== Biological roles ==

Sodium is an [[Dietary mineral|essential element]] for all animals and some plants. In animals, sodium ions are used against [[potassium]] ions to [[Na+/K+-ATPase|build up charges on cell membranes]], allowing transmission of nerve impulses when the charge is dissipated; it is therefore classified as a dietary inorganic macromineral.

Magnesium is the eleventh most abundant element by mass in the [[human body]]; its ions are essential to all living [[Cell (biology)|cells]], where they play a major role in manipulating important biological [[polyphosphate]] compounds like [[Adenosine triphosphate|ATP]], [[DNA]], and [[RNA]]. Hundreds of [[enzyme]]s thus require magnesium ions to function. Magnesium is also the metallic ion at the center of [[chlorophyll]], and is thus a common additive to [[fertilizer]]s.<ref>{{cite web|url=http://www.mg12.info|title=Magnesium in health}}</ref> Magnesium compounds are used medicinally as common [[laxative]]s, antacids (e.g., [[milk of magnesia]]), and in a number of situations where stabilization of abnormal [[nerve]] excitation and blood vessel spasm is required (e.g., to treat [[eclampsia]]).

Despite its prevalence in the environment, [[aluminium salt]]s are not known to be used by any form of life. In keeping with its pervasiveness, it is well tolerated by plants and animals.<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a01_527.pub2 |chapter=Aluminum Compounds, Inorganic|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2007|last1=Helmboldt|first1=Otto|last2=Keith Hudson|first2=L.|last3=Misra|first3=Chanakya|last4=Wefers|first4=Karl|last5=Heck|first5=Wolfgang|last6=Stark|first6=Hans|last7=Danner|first7=Max|last8=Rösch|first8=Norbert|isbn=978-3527306732}}</ref> Because of their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.

Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals,<ref name="Niels">{{cite journal|doi=10.1146/annurev.nu.04.070184.000321|pages =21–41|journal=Annual Review of Nutrition|volume=4|year=1984|title=Ultratrace Elements in Nutrition|first=Forrest H.|last=Nielsen|pmid=6087860}}</ref> though various [[sea sponges]] need silicon in order to have structure. It is much more important to the metabolism of plants, particularly many grasses, and [[silicic acid]] (a type of silica) forms the basis of the striking array of protective shells of the microscopic [[diatom]]s.

Phosphorus is essential for life. As phosphate, it is a component of [[DNA]], [[RNA]], [[Adenosine triphosphate|ATP]], and also the [[phospholipid]]s that form all cell membranes. Demonstrating the link between phosphorus and life, elemental phosphorus was historically first isolated from human urine, and bone ash was an important early phosphate source. Phosphate minerals are fossils. Low phosphate levels are an important limit to growth in some aquatic systems. Today, the most important commercial use of phosphorus-based chemicals is the production of [[fertilizer]]s, to replace the phosphorus that plants remove from the soil.

Sulfur is an essential element for all life, and is widely used in biochemical processes. In metabolic reactions, sulfur compounds serve as both fuels and respiratory (oxygen-replacing) materials for simple organisms. Sulfur in organic form is present in the vitamins [[biotin]] and [[thiamine]], the latter being named for the Greek word for sulfur. Sulfur is an important part of many enzymes and in antioxidant molecules like [[glutathione]] and [[thioredoxin]]. Organically bonded sulfur is a component of all proteins, as the [[amino acid]]s [[cysteine]] and [[methionine]]. [[Disulfide]] bonds are largely responsible for the mechanical strength and insolubility of the protein [[keratin]], found in outer skin, hair, and feathers, and the element contributes to their pungent odor when burned.

Elemental chlorine is extremely dangerous and poisonous for all lifeforms, and is used as a [[pulmonary agent]] in [[chemical warfare]]; however, chlorine is necessary to most forms of life, including humans, in the form of [[chloride]] ions.

Argon has no biological role. Like any gas besides oxygen, argon is an [[asphyxiant]].