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Biological roles of the elements

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Biological roles of the elements

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A large fraction of the chemical elements that occur naturally on the earth's surface are essential to the structure and metabolism of living things. Four of these elements (hydrogen, carbon, nitrogen, and oxygen) are essential to every living thing and collectively make up 99% of the mass of protoplasm. Phosphorus and sulfur are also common essential elements, essential to the structure of nucleic acids and amino acids, respectively. Chlorine, potassium, magnesium, calcium and phosphorus have important roles due to their ready ionization and utility in regulating membrane activity and osmotic potential. The remaining elements found in living things are primarily metals that play a role in determining protein structure. Examples include iron, essential to hemoglobin; and magnesium, essential to chlorophyll. Some elements are essential only to certain taxonomic groups of organisms, particularly the prokaryotes. For instance, the lanthanide series rare earths are essential for methanogens. As shown in the following table, there is strong evidence that 19 of the elements are essential to all living things, and another 17 are essential to some taxonomic groups. Of these 17, most have not been extensively studied, and their biological importance may be greater than currently supposed.

The remaining elements are not known to be essential. There appear to be several causes of this.

Apart from the known essential elements, most elements have only received direct biological study in connection with their significance to human health; this has incidentally included study of some laboratory animals such as chickens and rats, and plants of agricultural importance. There is evidence that certain elements are essential to groups other than humans, but there has been little effort to systematically study any group other than humans or laboratory animals to determine the effects of deficiency of uncommon elements, and for these groups knowledge is largely limited to information that has been gathered incidentally to study of other aspects of each organism.
The noble gases helium, neon, argon, krypton, xenon are nonreactive and have no known direct biological role — albeit xenon nevertheless very surprisingly exhibits both anesthetic and neuroprotective side-effects despite usually being considered "chemically inert," and can activate at least one human transcription factor. (Radon is radioactive, discussed below.)
Some elements are very rare on the earth's surface and any lifeform to which these were essential would have limited habitat and possibly a limited term of existence as geological change altered the availability of these elements. Examples are rhodium and tantalum.
Some elements readily substitute for other, more common elements in molecular structures; e.g. bromine often substitutes for chlorine, or tungsten for molybdenum. Sometimes this substitution has no biological effect; sometimes it has an adverse effect.
Many elements are benign, meaning that they generally neither help nor harm organisms, but may be bioaccumulated. However, since the literature on these "benign" elements is almost entirely focused on their role in humans and laboratory animals, some of them may eventually be found to have an essential role in other organisms. In the following table are 56 benign elements.
A few elements have been found to have a pharmacologic function in humans (and possibly in other living things as well; the phenomenon has not been widely studied). In these, a normally nonessential element can treat a disease (often a micronutrient deficiency). An example is fluorine, which reduces the effects of iron deficiency in rats.
Some of the benign elements are radioactive. As such they alter life due to their potential to cause mutations. This effect could be interpreted as either adverse or beneficial, but since mutation would proceed even in the absence of ionizing radiation, these mutagenic elements are not essential to living things.
All elements with atomic number 95 or higher are synthetic and radioactive with a very short half-life. These elements have never existed on the surface of the earth except in minute quantities for very brief time periods. None have any biological significance.

Aluminum warrants special mention because it is the most abundant metal and the third most abundant element in the Earth's crust; despite this, it is not essential for life. With this sole exception, the eight most highly abundant elements in the earth's crust, making up over 90% of the crustal mass, are also essential for life.

The following list identifies in rank order the possible biological roles of the chemical elements, ranging from a score of 5 for elements essential to all living things, to a score of 1 for elements that have no known effects on living things. There are also letter scores for special functions of the elements. These rank scores are used to characterize each element in the following table.

Rank	Biological Importance
5	Essential for all (or most) living things.
4	Essential for some living things.
3	Not essential, but has a pharmacologic role; helps to treat disease in some organisms.
2	Benign: present in some organisms, sometimes bioaccumulating, but generally having no apparent effects (except possible harmful effects, notes “a” or “b”).
1	Extremely rare on the earth's surface (less than 1×10⁻⁷%, i.e. less than 1/10 as common as the least common essential element, selenium), thus has low potential for any kind of biological role.
a	Toxic in some molecular forms.
b	Radioactive.
c	Has uses in medicine as a drug or implant.

The following table identifies the 94 chemical elements that occur naturally on the earth's surface, their atomic numbers, their biological rank as defined above, and their general beneficial and harmful roles in living things.

Element	Z	Rank	Beneficial role	Harmful role
actinium	89	1b	Has no known biological role.	Radioactive.
aluminum	13	2a	Has no known biological role.	The metal, or various compounds, can be toxic to humans. In plants, aluminum can be the primary limitation on growth in acidic soils.
antimony	51	2c	Has no known biological role, but has a variety of uses in medicine, e.g. antibacterial.	Some compounds are highly toxic to humans.
argon	18	2	None known.	None known.
arsenic	33	4a	Essential to some species, including humans, for whom it is necessary for the functioning of the nervous system. Some marine algae and shrimp contain arsenic compounds.	Toxic to humans in some forms.
astatine	85	1b	None known.	Radioactive.
barium	56	2ac	Has no known biological role, but a variety of plants concentrate it from the soil, and it has a variety of uses in medicine.	Some compounds are toxic. In humans, barium ion affects the nervous system.
beryllium	4	2c	Has no known biological role, but has medical use in certain dental alloys	Toxic to humans, esp. via inhalation. Can substitute for magnesium in certain key enzymes, causing malfunction.
bismuth	83	2ac	Has no known biological role, but has a variety of uses in medicine, e.g. in antiulcer, antibacterial, anti‐HIV and radiotherapeutic uses.	Slightly toxic, perhaps the least toxic heavy metal, though poisonings have been reported.
boron	5	4	In plants, it has important roles in nucleic acid metabolism, carbohydrate and protein metabolism, cell wall synthesis, cell wall structure, membrane integrity and function, and phenol metabolism. Probably essential to animals, for reasons not well understood.	Toxic to both animals and plants.
bromine	35	5	Essential to membrane architecture and tissue development in animals. May have antibiotic effects in some compounds when it substitutes for chlorine. Bromine compounds are very common in and presumably essential to a variety of marine organisms, including bacteria, fungi, seaweeds, and diatoms. Most marine organobromine compounds are made by the action of a unique algal enzyme, vanadium bromoperoxidase	Toxic in excessive concentrations, causing the human disease bromism.
cadmium	48	4	A carbonic anhydrase using cadmium has been found in some marine diatoms that inhabit environments with very low zinc availability; the cadmium evidently provides a similar function. Many plants bioaccumulate cadmium, which deters herbivory. Cadmium deprivation in goats and rats leads to depressed growth, but has not been shown to be essential.	Cadmium poisoning is widely recognized in humans, but has not been described in other organisms. In general, cadmium acts by substituting for calcium, zinc, or iron, and can disrupt biochemical pathways dependent upon those metals.
calcium	20	5a	Ubiquitous, essential	Appears in various toxic organochemicals; contributes to diseases e.g. kidney stones.
carbon	6	5c	Ubiquitous, essential.	Its oxide is a pollutant.
cerium	58	4a	The methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV requires a lanthanide cofactor, lanthanum, cerium, praseodymium, or neodymium (or possibly other lanthanides) but it appears that any of these lanthanides can perform this function, so cerium is only essential if no other suitable lanthanides are available. Has medical uses, e.g. in burn treatment.	Can substitute for calcium with possible adverse effects, and in metallic form, is mildly toxic.
caesium	55	2a	Has no known biological role.	Can substitute for potassium (a biologically essential element) with possible adverse effects, particularly if the substitution is of radioactive cesium, which was the primary biologically active isotope released in the 1986 Chernobyl nuclear disaster.
chlorine	17	4a	Chlorine salts are critical for many species, including humans. Its ion is used as an electrolyte, as well as making the hydrochloric acid the stomach uses for digestion.	Elemental Cl₂ is toxic.
chromium	24	4	Appears to be essential in humans. Affects insulin metabolism. Also influences metabolism, replication and transcription of nucleic acids, and decreases the content of corticosteroids in plasma.	Toxic in some forms.
cobalt	27	5	Essential to the metabolism of all animals, as a key constituent of cobalamin, also known as vitamin B12.	Toxic in some forms, probably carcinogenic.
copper	29	5a	Essential in many ways; an important component of many enzymes, especially cytochrome c oxidase, which is present in nearly all living things.	Some compounds are toxic; the metal is highly toxic to viruses.
dysprosium	66	2	Has no known biological role.	Some salts have low toxicity.
erbium	68	2a	Has no known function in humans, and is not taken up by plants.	Soluble salts are mildly toxic.
europium	63	2a	Has no known function in humans, and is not taken up by plants.	Possible low toxicity in some forms.
fluorine	9	3a	Affects bone density in humans; creates fluoroapatite, which makes tooth enamel hard and relatively impervious to chemical action, compared to bone. Improves growth in rats; has pharmacologic effects – helps to treat other deficiencies, e.g. of iron. Absence of fluorine has no clear adverse consequences in animals.	Excess fluorine in humans results in fluoride toxicity, and can substitute for iodine, causing goitre.
francium	87	1b	Due to its very short half-life, there is almost no potential for a living thing to be exposed to it. Even synthesis cannot produce more than minute quantities before it decays, so there is no medical use.	Radioactive.
gadolinium	64	2ac	Has no known function in humans, and is not taken up by plants. There has been limited use in experimental medicine.	Soluble salts are mildly toxic. See medical discussion in Gadolinium: Safety.
gallium	31	2ac	Although nonessential, plays a complex role in humans, including concentrating in bone, binding to plasma proteins, and concentrating in malignancies. It is selectively taken up by plants, so there are a variety of possible roles in plant metabolism. There is limited medical use.	Inhibits iron uptake and metabolism in a variety of plants and bacteria.
germanium	32	2a	Some plants will take it up, but it has no known metabolic role.	Some salts are deadly to some bacteria.
gold	79	2a	Although some plants bioaccumulate gold, no living organism is known to require it. There are medical uses, including treatment of rheumatoid arthritis and fabrication of dental implants.	Some gold salts used in medicine have adverse side effects.
hafnium	72	2	Has no known biological role.	Salts have low toxicity.
helium	2	2	As with other noble gases, has no known biological role.	Has no known harmful role.
holmium	67	2a	This lanthanide has no known biological roles, and is not taken up by plants. There are medical uses; for example, holmium-containing nanoparticles are biocompatible and facilitate NMR imaging.	Some salts are known to be toxic to humans.
hydrogen	1	5	Ubiquitous, essential.	None known.
indium	49	2a	Has no known biological role.	Highly toxic to humans in fairly small doses; mildly toxic to plants, comparable to aluminum; may inhibit growth of some bacteria.
iodine	53	5ac	Iodine has a role in biochemical pathways of organisms from all biological kingdoms, indicating it is uniformly essential to life Widely used in medicine, mainly for treatment of goitre and for its antibacterial properties.	Highly toxic to humans in its elemental form.
iridium	77	1a	Due to its extreme rarity, iridium has no biological role.	The chloride is moderately toxic to humans.
iron	26	5	Essential to almost all living things, usually as a ligand in a protein; it is most familiar as an essential element in the protein hemoglobin.	Toxic in some forms.
krypton	36	1	As with other noble gases, has no known biological role. It is also the rarest non-radioactive element in the earth's crust.	None known.
lanthanum	57	4ac	The methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV requires a lanthanide cofactor, lanthanum, cerium, praseodymium, or neodymium (or possibly other lanthanides) but it appears that any of these lanthanides can perform this function, so lanthanum is only essential if no other suitable lanthanides are available. Among plants, Carya accumulates lanthanum and other lanthanides, perhaps as an adaptation to certain site-limiting environmental stresses.	The chloride is mildly toxic to humans.
lead	82	3a	Pb deprivation leads to suboptimal growth of rats, along with anemia, and reduced function of a variety of enzymes; but results have been inconclusive, and the effects may be pharmacologic.	Toxic in some forms, teratogenic, and carcinogenic; historically, lead poisoning has frequently been widespread in human societies. It seems to have been rarely documented in other organisms.
lithium	3	4a	There is some evidence that lithium deprivation adversely affects multiple functions, especially fertility and adrenal gland function, in rats and goats, and some plants accumulate lithium. However, it is not known to be essential for any organism. There are medical uses, especially in treatment of manic-depressive symptoms.	Toxic in some forms.
lutetium	71	2a	This lanthanide has no known biological roles, and is not taken up by plants.	Mildly toxic to humans in some forms.
magnesium	12	5a	Essential for almost all living things; needed for chlorophyll, and is a co-factor for many other enzymes; has multiple medical uses.	Large doses can have toxic effects.
manganese	25	5a	Essential for almost all living things, although in very small amounts; it is a cofactor for many classes of enzymes. At least one of these, mitochondrial superoxide dismutase (MnSOD), is present in all aerobic Bacteria and in the mitochondria of all eukaryotes.	Large doses can have toxic effects.
mercury	80	2ac	Although nearly ubiquitous in the environment, mercury has no known biological role. Traditionally used in medicine and dental fillings, it is now avoided due to toxic side effects.	Can inactivate certain enzymes, as a result, both the metal and some compounds (especially methylmercury) are harmful to most life forms; there is a long and complex history of mercury poisoning in humans.
molybdenum	42	5	Found in many enzymes; essential to all eukaryotes, and to some bacteria. Molybdenum in proteins is bound by molybdopterin or to other chemical moieties to give one of the molybdenum cofactors.	Metallic molybdenum is toxic if ingested.
neodymium	60	4	The methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV requires a lanthanide cofactor, lanthanum, cerium, praseodymium, or neodymium (or possibly other lanthanides) but it appears that any of these lanthanides can perform this function, so neodymium is only essential if no other suitable lanthanides are available.	Toxic in some forms. Anticoagulant.
neon	10	2	As with other noble gases, has no known biological role.	None known.
neptunium	93	1b	Has no known biological role.	Radioactive.
nickel	28	4	As a component of urease, and many other enzymes as well, nickel is needed by most living things in all domains. Nickel hyperaccumulator plants use it to deter herbivory.	Toxic in some forms.
niobium	41	2	Has no known biological role, although it does bioaccumulate in human bone. Is hypoallergenic and, both alone and in a niobium-titanium alloy, is used in some medical implants including prosthetics, orthopedic implants, and dental implants.	Toxic in some forms.
nitrogen	7	5	Ubiquitous, essential for all forms of life; all proteins and nucleic acids contain substantial amounts of nitrogen.	Toxic in some forms.
osmium	76	1a	None known. Osmium is very rare, substantially more so than any element essential to life.	The oxide is toxic to humans.
oxygen	8	5	Ubiquitous, essential for all forms of life; essentially all biological molecules (not to mention water) contain substantial amounts of oxygen.	In high concentrations, oxygen toxicity can occur.
palladium	46	2a	Has no known biological role. Medically, it is used in some dental amalgams to decrease corrosion and increase the metallic lustre of the final restoration.	Toxic in some forms.
phosphorus	15	5	Ubiquitous, essential for all forms of life; all nucleic acids contain substantial amounts of phosphorus; it is also essential to adenosine triphosphate (ATP), the basis for all cellular energy transfer; and it performs many other essential roles in different organisms.	Toxic in some forms; pure phosphorus is poisonous to humans.
platinum	78	2c	Has no known biological role, but it is a component of the drug cisplatin, which is highly effective in treating some forms of cancer.	Toxic in some forms. Contact can promote an allergic reaction (platinosis) in humans.
plutonium	94	1bc	Has no known biological role, and is extremely rare in the earth's crust. The isotope plutonium-238 is used as an energy source in some heart pacemakers.	Both toxic and radioactive.
polonium	84	1b	Has no known biological role, and due to its short half-life, is nearly nonexistent outside of research facilities.	Both highly toxic and radioactive.
potassium	19	5a	Essential for almost all living things, except perhaps some prokaryotes; performs numerous functions, most of which are related to the transport of potassium ions.	Potassium ion in excess causes paralysis and depresses central nervous system activity in humans.
praseodymium	59	4	The methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV requires a lanthanide cofactor, lanthanum, cerium, praseodymium, or neodymium (or possibly other lanthanides) but it appears that any of these lanthanides can perform this function, so praseodymium is only essential if no other suitable lanthanides are available.	Some forms are mildly toxic to humans.
promethium	61	1b	Has no known biological role; as it is radioactive with a short half-life, it is very rare and is seldom present for long.	Radioactive.
protactinium	91	1b	Has no known biological role; as it is radioactive with a short half-life, it is very rare and is seldom present for long.	Both toxic and highly radioactive.
radium	88	1bc	Has no known biological role; as it is radioactive it is very rare. There have been various medical uses in the past.	Radioactive; historically, there have been many cases of radium poisoning, most notably in the case of the Radium Girls.
radon	86	1bc	Has no known biological role. Historically, there have been various medical uses.	Radioactive, with a variety of documented harmful effects on human health.
rhenium	75	1	Has no known biological role, and is extremely rare in the earth's crust.	None known.
rhodium	45	1	Has no known biological role, and is extremely rare in the earth's crust.	Toxic in some forms.
rubidium	37	2c	Has no known biological role, although it seems to substitute for potassium, and bioaccumulates in plants. It has seen limited medical use.	None known.
ruthenium	44	1a	Has no known biological role; it bioaccumulates, but does not appear to have any function. It is extremely rare.	There is a highly toxic oxide, RuO₄, but it is not naturally occurring.
samarium	62	2ac	Has no known biological role, although it can bioaccumulate in some plants. One radioisotope is approved for medical use.	Toxic in some forms.
scandium	21	2a	Has no known biological role, but can bioaccumulate in some plants, perhaps because it can substitute for aluminum in some compounds.	Some compounds may be carcinogenic; some forms are mildly toxic to humans.
selenium	34	4	Selenium, which is an essential element for animals and prokaryotes and is a beneficial element for many plants, is the least-common of all the elements essential to life. Selenium acts as the catalytic center of several antioxidant enzymes, such as glutathione peroxidase, and plays a wide variety of other biological roles.	Toxic in some forms.
silicon	14	4c	Essential for connective tissue and bone in birds and mammals.Silica appears in many organisms; e.g. as frustules (shells) of diatoms, spicules of sponges, and phytoliths of plants. Also has medical uses, e.g. cosmetic implants.	Silicosis is a lung disease caused by inhalation of silica dust.
silver	47	2c	Has no known biological role, apart from medical use (antibiotic, mainly; also dental fillings).	Can produce a variety of toxic effects in humans and other animals; also toxic to various microorganisms.
sodium	11	5	Essential to animals and plants in many ways, such as osmoregulation and transmission of nerve impulses. Essential to energy metabolism of some bacteria, particularly extremophiles.	Toxic in some forms, and since it is essential to living things, either a lack or an excess can have harmful results.
strontium	38	4c	Essential to Acantharean radiolarians, which have skeletons of strontium sulfate. Also essential to some stony corals. Limited medical use in drugs such as strontium ranelate.	Non-toxic; in humans, it often substitutes for calcium.
sulfur	16	5	Sulfur is essential and ubiquitous, partly because it is part of the amino acids cysteine and methionine. Many metals that appear as enzyme cofactors are bound by cysteine, and methionine is essential for protein synthesis.	Toxic in some forms.
tantalum	73	1c	Has no known biological role, but is biocompatible, used in medical implants, e.g. skull plates.	Has not been found to be toxic, though some patients with tantalum implants have shown a mildly allergic reaction.
technetium	43	1b	Nonexistent (radioactive).	Nonexistent (radioactive).
tellurium	52	1a	Is not known to be essential to any organism, but is metabolized by humans, typically through methylation.	Toxic in some forms; the sodium salt is fatal to humans in small doses, and the oxide causes severe bad breath.
terbium	65	2a	Has no known biological role, but is probably similar to other lanthanides such as cerium and lanthanum, i.e., not known to be essential. Terbium is also one of the rarer lanthanides.	Toxic in some forms.
thallium	81	2a	Has no known biological role. Medically, it was used for many years to induce hair loss, but this has ended due to its numerous other toxic effects on human health. Its role, if any, in living things other than humans has been very little explored.	It is very toxic and there is evidence that the vapor is both teratogenic and carcinogenic. It can displace potassium in humans affecting the central nervous system. Thallium poisoning has a long history in humans, especially as it has sometimes been a preferred poison.
thorium	90	1b	Has no known biological role.	Radioactive.
thulium	69	2a	No known function in humans, and is not taken up by plants.	Toxic in some forms.
tin	50	4a	In mammals, deprivation causes impaired reproduction and other abnormal growth, suggesting that it is an essential element. Tin may have a role in tertiary structure of proteins. Some plants are tin hyperaccumulators, possibly to deter herbivory.	Toxic in some forms, especially the organotin compounds, which include many potent biocides.
titanium	22	2c	Present in most animals, possibly beneficial to plant growth, but not known to be essential; some plants are hyperaccumulators. Common in medical implants.	The common compounds are nontoxic.
tungsten	74	4a	Is a (presumably essential) component of a few bacterial enzymes, and is the heaviest biologically essential element. Appears to be essential in ATP metabolism of some thermophilic archaea. Can substitute for molybdenum in some proteins. Some plants hyperaccumulate it, though its function is unknown.	Toxic, at least to animals, in some forms.
uranium	92	4b	Some bacteria reduce uranium and use it as a terminal electron acceptor for respiration with acetate as electron donor. Some bacteria hyperaccumulate uranium.	Radioactive, and most compounds are also chemically toxic to humans.
vanadium	23	4a	Can mimic and potentiate the effect of various growth factors such as insulin and epidermal growth factor. Can also affect processes regulated by cAMP. Also used by some bacteria. Dinitrogenases, essential for nitrogen metabolism, normally use molybdenum but in its absence vanadium (or iron) will substitute. Vanadium is also an essential for a variety of peroxidases found in many taxonomic groups, including bromoperoxidases, haloperoxidases, and chloroperoxidases.	Some compounds are toxic, and are implicated in several human diseases of including diabetes, cancer, chlorosis, anemia, and tuberculosis.
xenon	54	1	Has no known biological role.	None known.
ytterbium	70	2a	No known function in humans, where it concentrates in bones. Not taken up by plants.	Toxic in some forms.
yttrium	39	2a	Not well understood. It occurs in most organisms and at widely varying concentrations, suggesting it does have a role, but not known whether essential.	Toxic in some forms, and it may be carcinogenic.
zinc	30	5a	Essential, involved in numerous aspects of cellular metabolism (more than 200 different proteins). Some plants are hyperaccumulators. There are also medical uses, e.g. in dentistry.	Some compounds are toxic.
zirconium	40	2a	Some plants have high uptake, but it doesn't appear to be essential or even to have a role; benign.	Compounds generally have low toxicity.

Biological roles of the elements

See also