ELEMENTS IN LIVING THINGS There are about 92 elements occurring naturally in nature. From these 92 elements, only about 25 elements are needed to build living organisms. Not all these elements found in all living cells.

Main elements (CHON) are the most frequently found elements in cells, forming about 96% of the human body mass. Trace-elements are the elements found in small quantity in cells, but are important in biological processes.

This graph illustrates why the CHON elements are associated with living things.

The Primary elements of Earth Life are Carbon, Hydrogen, Oxygen and Nitrogen (“CHON’) Carbon chemistry is the basis of life (“Organic Chemistry”) Water is the universal solvent of life (H2O) Nitrogen is a key component of amino acids and DNA Also needs other elements, principally: P- Phosphorus (DNA, RNA, and ATP/ADP) S- Sulfur (amino acids cysteine and methionine)

Water is the universal solvent of life (H2O) The search for extraterrestrial life centers on the search for water The CHON elements and compounds made from them are important to life, but water is necessary for life as we know it Water is incredibly abundant in our universe

Best Chances For Life? Some nearby places have some good chemistry for life. Europa Mars Enceladus

Europa Europa has a huge liquid water ocean under the ice. Movie Europa Report is based on this.

Mars Mars had liquid Water in the past and has lots of ice. Underground life may be possible

Enceladus Enceladus has geysers that spray liquid water off the surface

Carbon nucleus had 6 protons (and 6 neutrons) Carbon has unique chemical properties that make it extremely versatile. Carbon nucleus had 6 protons (and 6 neutrons) Surrounded by an electron cloud containing 2 shells of 6 electrons total: 2 inner shell electrons 4 valence (outside) electrons Allow it to hook up and bond with many different elements. Can form single, double and triple bonds.

Carbon chemistry is very rich, able to form a vast variety of organic compounds. Long chains (polymers) Chains with complex branching Closed rings or networks of rings Carbon compounds dissolve readily in liquids, especially water. Millions of different complex organic compounds are possible…

Are there any other possibilities besides carbon? Silcon based “SHON”

Silicon chemistry is analogous to carbon chemistry in many ways. In the same family on periodic table Also has 4 outer electrons to share. Can also form Si chains and rings In principle, Silicon chemistry could be as rich and complex as carbon chemistry No evidence for “SHON” but an interesting alternative

Silicon-based life would we quite different than carbon-based life. Could be crystalline in structure and tolerate high temperatures. Silicon-based life is favorite theme of science fiction writers (e.g the Horta from the original Star Trek episode “Devil in the Dark”)

Can there be life without Chemistry? This is unknown Life on neutron stars? Collapsed cores of evolved stars Extreme gravity (trillion g’s) High temperatures (Million K) Nuclear reactions instead of chemical reactions Belongs to the realm of science fiction

Contemplating possible extraterrestrial life helps frame questions relevant to know how life works on Earth. Asking how life might emerge on other worlds focuses on what processes were most important in how life emerged here. Asks questions that get at the heart of the inner working of life and biology. Helps us to better understand what kinds of questions to ask about life’s history on Earth.