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Carbon-based life
 
 
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Carbon
Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
 forms the backbone of biology
Biology

Biology is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment ....
 for all life
Life

Life is a characteristic of organisms that exhibit certain biological processes such as chemical reactions or other events that results in a transformation....
 on Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. Complex molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
s are made up of carbon bonded
Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
 with other elements
Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
, especially oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, hydrogen
Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
 and nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
. It is these elements that living organisms need, among others, and carbon is able to bond with all of these because of its four valence electrons. Since no life has been observed that is not carbon-based, it is sometimes assumed in astrobiology
Astrobiology

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and Planetary habitability outside our Solar System, the search for evidence of Abiogenesis, life on Mars and other bodies in our Solar Syst...
 that life elsewhere in the universe
Universe

The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them....
 will also be carbon-based.






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Carbon
Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
 forms the backbone of biology
Biology

Biology is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment ....
 for all life
Life

Life is a characteristic of organisms that exhibit certain biological processes such as chemical reactions or other events that results in a transformation....
 on Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. Complex molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
s are made up of carbon bonded
Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
 with other elements
Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
, especially oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, hydrogen
Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
 and nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
. It is these elements that living organisms need, among others, and carbon is able to bond with all of these because of its four valence electrons. Since no life has been observed that is not carbon-based, it is sometimes assumed in astrobiology
Astrobiology

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and Planetary habitability outside our Solar System, the search for evidence of Abiogenesis, life on Mars and other bodies in our Solar Syst...
 that life elsewhere in the universe
Universe

The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them....
 will also be carbon-based. This assumption is referred to by critics as carbon chauvinism
Carbon chauvinism

Carbon chauvinism is a relatively neologism meant to disparage the assumption that extraterrestrial life will resemble life on Earth. In particular, it would be applied to those who assume that the molecules responsible for the chemical processes of life must be constructed primarily from carbon....
, as it may be possible for life to form that is not based on carbon, even though it has never been observed.

In cinematic and literary science fiction
Science fiction

Science fiction is a broad genre of fiction that often involves speculations based on current or future science or technology. Science fiction is found in books, art, television, films, games, theatre, and other media....
, a moment when man-made machines cross from nonliving to living is often posited, this new form being the first example of non-carbon-based life. Since the advent of the microprocessor
Microprocessor

A microprocessor incorporates most or all of the functions of a central processing unit on a single integrated circuit . The first microprocessors emerged in the early 1970s and were used for electronic calculators, using Binary-coded decimal arithmetic on 4-bit Word ....
 in the late 1960s, these machines are often classed as computer
Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
s (or computer-guided robot
Robot

A robot is a virtual or mechanical artificial agent. In practice, it is usually an Electromechanics which, by its appearance or movements, conveys a sense that it has Intention or Agency of its own....
s) and filed under "silicon
Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".

Characteristics of carbon as a basis for life


The two most important characteristics of carbon as a basis for the chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 of life are that it has four valence bonds and that the energy required to make or break a bond is just at an appropriate level for building molecules which are not only stable but also reactive.

The fact that carbon atoms bond readily to other carbon atoms allows for the building of arbitrarily long and complex molecules
Organic compound

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered Inorganic compound....
.

There are not many other elements which appear to be even promising candidates for supporting life-like behavior, but the most frequent alternative suggested is silicon. This is in the same group in the Periodic Table
Periodic table

The periodic table of the chemical elements is a table method of displaying the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869....
 of elements
Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
 and therefore also has four valence bonds
Valence bond theory

In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that developed to use the methods of quantum mechanics to explain chemical bond....
. It also bonds to itself, but generally in the form of crystal lattices rather than long chains. However, its compounds are generally highly stable and do not support the ability to readily re-combine in different permutations in a manner that would plausibly support life-like processes.

This speculation of a life based on the chemistry of silicon is clearly distinct from "silicon-based life" in the above sense of artificial intelligence
Artificial intelligence

Artificial intelligence is the intelligence of machines and the branch of computer science which aims to create it. Major AI textbooks define the field as "the study and design of intelligent agents,"...
 based on electronic processes utilizing silicon integrated circuit
Integrated circuit

In electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin Wafer of semiconductor material....
s.

Key carbon-based molecules in the life processes


The most notable groups of chemicals used in the processes of living organisms include:

  • Proteins, which are the building blocks from which the structures of living organisms are constructed
  • Nucleic acids, which carry genetic
    Genetics

    Genetics , a discipline of biology, is the science of heredity and Genetic variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding....
     information
  • Enzymes, which catalyse organic chemical reactions
  • Carbohydrates, which store energy in a form that can be used by living cells
    Cell (biology)

    The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
  • Fats
    Lipid

    Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
    , which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.


See also

  • Alternative biochemistry
    Alternative biochemistry

    Alternative biochemistry is the speculative biochemistry of alien life forms that differ radically from those known on Earth. It includes biochemistries that use elements other than carbon to construct primary cellular structures and/or use solvents besides water....
  • CHON
    CHON

    CHON is an mnemonic acronym for the four most common chemical elements in living organisms: carbon, hydrogen, oxygen, and nitrogen. These four elements are also notable for being the least massive in their periodic table group in the periodic table....