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You can't see it or feel it, but your life depends on tiny chemical `coin'

WASHINGTON—The most common currency in the world isn't the dollar or the euro, but a tiny chemical "coin" that scientists call ATP.

Your body contains roughly 60 billion trillion—that's 60 followed by 21 zeros—molecules of ATP. All together, they weigh only about a tenth of a pound, but you couldn't exist without them. Only the DNA molecule is more important to life.

ATP is "the major energy currency of the cell," John Kimball, a retired biology professor at Harvard University, said in an e-mail. "What I find most remarkable about this molecule is that, directly or indirectly, it is involved in virtually every activity of cells and thus of organisms."

Each ATP molecule stores a unit of energy and pays it out whenever you need it, like a miniature bank. What you withdraw is immediately replaced by new energy supplied by the food you eat or drink, keeping the account in balance.

Counting all the deposits and withdrawals, an adult turns over his or her weight in ATP every day, according to Arthur Kornberg, a biochemistry professor at Stanford University in Palo Alto, Calif., who won a Nobel Prize in 1959 for his work on this molecule.

"A staggering amount of ATP is turned over each day to convert the energy in food into the forms we need to live and work," he wrote in his 1989 autobiography, "For the Love of Enzymes."

ATP supplies the energy that moves muscles, triggers nerve action, keeps cells in proper shape, assembles proteins and synthesizes DNA, the instruction book for life. It also lights up fireflies, powers sperm cells and signals when your bladder is full.

Living organisms need a common energy currency because they get their supply from many different sources. Plants take their energy from the sun. Animals receive theirs by feeding on plants or on other animals.

"Imagine the confusion" if there weren't a common currency for energy, Kornberg wrote. "Each of the diverse foodstuffs would generate different energy currencies, and each of the great variety of cellular functions would have to trade in its unique currency."

Paul May, a chemist at Bristol University in England, called ATP "nature's universal energy store." He likened it to "a chemical battery, storing energy when it is not needed, but able to release it instantly when the organism requires it."

Here's how ATP works: The three letters stand for adenosine triphosphate (add-a-NO-seen try-FOSS-fate), a tangle of several dozen atoms of carbon, hydrogen, nitrogen and oxygen, plus three atoms of phosphorus. Each phosphorus atom combines with oxygen to make phosphate, a familiar ingredient of fertilizer.

When it's needed, an ATP molecule kicks out a phosphate group, releasing about as much energy as is contained in a peanut, according to Carl Nave, a physicist at Georgia State University, in Atlanta. In the process, ATP becomes a slightly smaller molecule known as ADP—adenosine diphosphate—which has only two phosphorus atoms.

Meanwhile, new energy is created by the combustion of glucose, a simple sugar that's supplied by the food you eat or created by photosynthesis in a plant. This energy reattaches a phosphate group to the ADP molecule, converting it back into ATP.

"The combustion of sugar captures the energy of sugar and stores it in ATP for later use," Kornberg wrote. This cycle—ATP to ADP to ATP—takes place about three times per minute, he said.

ATP works about the same in all complex organisms. "At this fundamental level, there are few distinctions between a human, a mouse and a yeast cell," Kornberg said.

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For more information go to http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/ATP.html

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(c) 2007, McClatchy-Tribune Information Services.

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