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ELEMENTS IN LIVING THINGS
• There are about 92 elements occurring
naturally in the universe.
• 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)
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…
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
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.