• As for basic chemical elements (leaving aside more complex molecules, etc., formed from elements), life on Earth is primarily based on six elements (sometimes referred to by the acronym, CHNOPS), as summarized as follows.
  • Carbon  – element 6 on the periodic table, meaning that Carbon has an atomic number of 6, or 6 protons in its nucleus.  Carbon can be viewed as the “backbone” element for all life on Earth.  It bonds with many elements, including itself, to form a myriad of molecules and structures that are essential to all known life.
  • Hydrogen (H) – element 1.  Hydrogen is the lightest of all elements and the most abundant element in the universe.  It readily bonds with many other elements.  On Earth, hydrogen is rare in its elemental form (H₂), but exists plentifully bound up in molecules, including many that essential for known life.  It is, of course, also part of water (H₂O), the critical solvent for known life.
  • Nitrogen (N) – element 7.  Nitrogen makes up the bulk of Earth’s atmosphere, although it is uncommon in the Earth’s crust.  Nitrogen is essential to amino acids (and thus to proteins) and to nucleic acids (and thus to DNA and genetic material generally), and is thus essential to known life.  In the atmosphere, Nitrogen is found in the atmosphere in its elemental form (N₂) which is relatively inert (unreactive).  However, certain bacteria can “fix” N₂ (which process involces chemical reduction, or electron addition) to form ammonium NH₄⁺, which is much more reactive and can be further used biologically.
  • Oxygen (O) – element 8.  To understand oxygen’s overall role in life, it is first critical to appreciate the distinction between oxygen (O), an element that may be a constituent of a compound (such as H₂O) as distinct from “elemental oxygen”, which term generally refers to molecular oxygen in the form of O₂ (it could also refer to O₃, or ozone, but O₂ is of much more direct biological significance).  Oxygen as an element (O) is present in a multitude of compounds that are critical to known life, such as glucose (C₆H₁₂O₆), and, of course, water.  Elemental Oxygen (O₂) is also critical to many forms of life, but not all.   More specifically, all aerobic organisms use cellular respiration, which requires elemental oxygen (O₂).  However, anaerobic organisms use anaerobic respiration and do not require elemental oxygen (O₂) and, in fact, it is even toxic to some anaerobic organisms.
  • Phosphorus (P) – element 15.  Phosphorus plays a major role in life’s genetic material, cellular membranes and energy systems.
  • Sulfur (S) – element 16.  Sulfur occurs on earth in its elemental form as well as in compounds.  It is a key element in certain amino acids and in enzymes that are essential for known life.  Sulfur also plays a key role in the biochemistry and key biochemical reactions of certain microorganisms.  In particular, such microorganisms include archaea, methanogens (methane-producing) and sulfur-reducing bacteria (essentially, sulfur-“breathing” bacteria) that use chemical sources of energy.  This is of particular interest to astrobiologists, given the role of such microorganisms in very early life on Earth.  It is also of particular interest given the survival of (and key role of) such microorganisms around hydrothermal vents on Earth; hydrothermal vents on other bodies in the solar system could hypothetically provide a habitat (and energy sources) for life there.
  • It is notable that the first four elements represented in the CHNOPS acronym make up over 96% of the biomass on Earth.  They are also the most common chemically reactive elements in the universe.   These facts, as well as the great structural variety of carbon-based molecules, may suggest that life beyond Earth is likely to be based on a similar overall chemistry.
  • Resources include: http://en.wikipedia.org/wiki/Planetary_habitability#A_stable_habitable_zone

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