Encyclopedia of the Solar System 2nd ed

852 Encyclopedia of the Solar System

TABLE 2 Examples of Metabolic Pathways

Heterotrophy

1. Fermentation C 6 H 12 O 6 →2CO 2 +2C 2 H 5 OH


2. Anaerobic Respiration C 6 H 12 O 6 +12NO− 3 →6CO 2 +6H 2 O+12NO− 2


3. Aerobic Respiration C 6 H 12 O 6 +6O 2 →6CO 2 +6H 2 O
   Photoautotrophy


4. Anoxic photosynthesis 12CO 2 +12H 2 S+hν→2C 6 H 12 O 6 +9S+3SO 4


5. Oxygenic photosynthesis 6CO 2 +6H 2 O+hν→C 6 H 12 O 6 +3O 2
   Chemoautotrophy
   Anaerobic


6. Methanogens CO 2 +4H 2 →CH 4 +2H 2 O
   CO+3H 2 →CH 4 +H 2 O
   4CO+2H 2 O→CH 4 +3CO 2


7. Acetogens 2CO 2 +4H 2 →CH 3 COOH+2H 2 O


8. Sulphate Reducers H 2 SO 4 +4H 2 →H 2 S+4H 2 O


9. Sulfur Reducers S+H 2 →H 2 S


10. Thionic Denitrifiers H 2 S+2NO− 3 →SO^24 −+H 2 O+N 2 O
    3S+4NO− 3 +H 2 →3SO^24 −+2N 2 +2H+


11. Iron Reducers 2Fe^3 ++H 2 →2Fe^2 ++2H+
    Aerobic


12. Sulfide Oxidizers 2H 2 S+3O 2 →2SO 4 S+2H 2 O


13. Iron Oxidizers 4FeO+O 2 →2Fe 2 O 3

to biochemistry because it acts as the solvent in which bio-
chemical reactions take place and, furthermore, it interacts
with many biochemicals in ways that influence their proper-
ties. For example, water forms hydrogen bonds with some
parts of a large molecule, the hydrophilic groups, and repels
other parts, the hydrophobic groups, thereby forcing these
molecules to curl up with their hydrophobic groups in the
interior and the hydrophilic groups on the exterior in con-
tact with the water. Certain organisms, notably lichen and
somealgae, are able to use water in the vapor phase if the
relative humidity is high enough. Many organisms can con-
tinue to metabolize at temperatures well below the freez-
ing point of pure water because their intracellular material

contains salts and other solutes that lower the freezing point

of the solution. No microorganism currently known is able

to obtain water directly from ice. Many organisms, such

as the snow algaeChlamydomonas nivalis thrive in liquid

water associated with ice but in these circumstances the or-

ganisms are the beneficiaries of external processes that melt

the ice. There is no known occurrence of an organism using

metabolic methods to overcome the latent heat of fusion of

ice thereby liquefying it.

Because liquid water is universally required for known

life and because it appears to be rare in the Solar System,

the search for life beyond the Earth begins first with the

search for liquid water.

TABLE 3 Elemental Abundances by Mass

1 Cosmic Earth’s Crust Humans Bacteria

1 H 70.7% O 46.6% O 64% O 68%

2 He 27.4 Si 29.7 C 19 C 15

3 O 0.958 Al 8.13 H 9 H 10.2

4 C 0.304 Fe 5.00 N 5 N 4.2

5 Ne 0.174 Ca 3.63 C 1.5 P 0.83

6 Fe 0.126 Na 2.83 P 0.8 K 0.45

7 N 0.110 K 2.59 S 0.6 Na 0.40

8 Si 0.0706 Mg 2.09 K 0.3 S 0.30

9 Mg 0.0656 Ti 0.44 Na 0.15 Ca 0.25

10 S 0.0414 H 0.14 Cl 0.15 Cl 0.12