From the mightiest blue whale to the most miniscule paramecium, life as we know it takes dramatically different forms. Nonetheless, all organisms are built from the same six essential elemental ingredients: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (CHNOPS).

Why those elements? To find out, Life's Little Mysteries consulted Matthew Pasek, a biogeochemist at the University of South Florida.

"First off, carbon enters easily into bonds with other carbon atoms. This means it forms vast chains that act as a nice skeleton for other atoms to bond to," Pasek said. In other words, carbon atoms are the perfect building blocks for large organic molecules. "This lends itself to complexity."

But what explains the other five chemical ingredients of life? "One thing that makes nitrogen, hydrogen and oxygen good is that they're abundant," Pasek said. "They also exhibit acid-base effects, which allows them to bond with carbon to make amino acids, fats, lipids and the nucleobases from which DNA and RNA are built."

"Sulfur provides electron shuffle," Pasek continued. "Basically, with their surplus of electrons, sulfides and sulfates help catalyze reactions. Some organisms use selenium in place of sulfur in their enzymes, but not many."

Last but not least, phosphorus, usually found in the molecule phosphate, is vital to metabolism, because polyphosphate molecules such as ATP (adenosine triphosphate) are able to store a huge amount of energy in their chemical bonds. Breaking the bond releases its energy; do this enough times in, say, a group of muscle cells, and you can move your arm.

Late last year, NASA scientists discovered the only known exception to the phosphorus requirement in an arsenic-rich California lake. They found a strain of microbes able to substitute arsenic atoms for phosphorus in their molecules when supplies of phosphorus are low. Arsenic is chemically similar to phosphorus, making it poisonous to most life forms because it disrupts metabolic pathways.

In summary, "With a few exceptions, what you need for life is CHNOPS, plus a dash of salt and a few metals," Pasek said. "Of course, those ingredients do have to be in the correct bonding structure, but this seems to occur naturally. Amino acids occur spontaneously, as do sugars and lipids and nucleobases."

That's true, at least, on Earth. For the necessary molecular structures to form, a planet must be just the right distance from its sun it can't be too hot or too cold for liquid water to exist. Having an abundant supply of water also helps, because it makes it easier for the ingredients to move around and bump into each other to form interesting compounds. Gravity must be just right, too. Finally, a dash of lightning can provide the much-needed energy to catalyze a reaction that will ultimately lead to the production of the complex moleculesamino acids, proteins, fats, carbohydrates, RNA and DNAthat lend themselves to producing life. At least as we know it.

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