Carbon-based life

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth.^[16][17] It is the second most abundant element in the human body by mass (about 18.5%) after oxygen.^[18]

The most important characteristics of carbon as a basis for the chemistry of cellular life are that each carbon atom is capable of forming up to four valence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive.^[19] Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long macromolecules and polymers in a process known as catenation.^[20][21][22] "What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", per Stephen Hawking in a 2008 lecture, "carbon [...] has the richest chemistry."^[23]

Norman Horowitz was the head of the Jet Propulsion Laboratory's bioscience section for the first U.S. mission, Viking Lander of 1976, to successfully land an unmanned probe on the surface of Mars. He considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. However, the results of this mission indicated that Mars was presently extremely hostile to carbon-based life. He also considered that, in general, there was only a remote possibility that non-carbon life forms would be able to evolve with genetic information systems capable of self-replication and adaptation.^[24]

Key molecules

The most notable classes of biological macromolecules used in the fundamental processes of living organisms include:^[25]

• catalyse organic chemical reactions).^[2]
• Amino acid, make up proteins, included the use in genetic code of life.^[2]
• Nucleic acids, which carry genetic information.^[2]
• Ribonucleic acid (RNA), production of proteins.^[26]
• Deoxyribonucleic acid (DNA), nucleic acid in genetic form.^[27]
• Peptide, building block of proteins.^[2]
• Lipids, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.^[2]
• Phospholipid used in cell membrane.^[2]
• Carbohydrates, which store energy in a form that can be used by living cells.^[2]
• Lectin, for binding proteins.^[28]
• Monosaccharide, simple sugars, including glucose and fructose.^[2]
• Disaccharides, sugar soluble in water, including lactose, maltose, and sucrose.^[2]
• Starch, made of amylose and amylopectin, plants energy storage.^[2]
• Glycogen, energy in animals.^[2]
• Cellulose, a biopolymer, found in the cell walls of plants.^[2]
• Fatty acid, two types, saturated fat and unsaturated fat (oil), are stored energy.^[2]
• Essential fatty acid, needed but not synthesized by the human body.^[2]
• Steroid, hormone, and used in cell membrane.^[2]
• signaling molecules.^[29]
• Cholesterol, used in the brain and spinal cord of animals.^[2]
• Wax, found in beeswax and lanolin. Plant wax used for protection.^[2]

Water

A few other elements have been proposed as candidates for supporting biological systems and processes as fundamentally as carbon does, for example, processes such as

Speculations about the chemical structure and properties of hypothetical non-carbon-based life have been a recurring theme in science fiction. Silicon is often used as a substitute for carbon in fictional lifeforms because of its chemical similarities. In cinematic and literary science fiction, when man-made machines cross from non-living to living, this new form is often presented as an example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, such machines are often classed as "silicon-based life". Other examples of fictional "silicon-based life" can be seen in the 1967 episode "The Devil in the Dark" from Star Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon.^[45] In the 1994 The X-Files episode "Firewalker", in which a silicon-based organism is discovered in a volcano.^[46][47]

In the

In JoJolion, the eighth part of the larger JoJo's Bizarre Adventure series, a mysterious race of silicon-based lifeforms "Rock Humans" serve as the primary antagonists.^[49]

Gallery

• 
  Correlation between the carbon cycle and formation of organic compounds.
• 
  Plant cell wall (cellulose) and chloroplasts conduct photosynthesis in plant cells and other eukaryotic organisms.
• 
  The arrangement of cellulose and other polysaccharides in a plant cell wall
• 
  Cell types: eukaryotic cell (left) and prokaryotic cell (right)
• 
  Fast carbon cycle showing the movement of carbon between land, atmosphere
• 
  Carbon stored in ecosystems
• 
  Where carbon goes when water flows
• 
  Oceanic carbon cycle
• 
  Simplified diagram of the global carbon cycle
• 
  Carbon cycle diagram
• 
  Triple bond of Carbon in Benzene
• 
  Model of diisobutylaluminium hydride, showing aluminium as pink, bonded to carbon in black, and hydrogen as white in Organoaluminium chemistry
• 
  Fatty acids made of long chains of carbon

See also

• Carbon source (biology)
• Cell biology
• CHONPS, a mnemonic
  acronym for the order of the most common elements in living organisms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur
• Habitable zone for complex life

References