NASA recently held a press conference announcing the first demonstration that organisms could use arsenic in place of phosphorus in their cells. Not surprisingly, science fiction got there first.
Kirk told [Bones] about the tabekh sauce. Bones nodded at that and said, "Yes, I've heard of it. I don't think you'd want to try it, though."
"One of the other ingredients is arsenic."
"Apparently they like the bitter taste," Bones said. "Also, the arsenides are pretty important in their diet. Klingons can get into horrible arsenide deficiencies if they're not careful, especially in stressful situations –"
~ Doctor's OrdersOne creature's poison is another one's meat by Diane Duane (1990)
While Klingons need a bit of arsenic in their diet, the element is a deadly poison to humans and most other Earthly life forms. A big part of what makes arsenic toxic is that it has very similar - but not identical - chemical properties as phosphorus, one of the fundamental building blocks of life.
Phosphorus is a component of the phospholipid molecules that form cellular membranes, the backbone of DNA and, importantly adenosine triphosphate (ATP). ATP is a molecule our cells use to store and transport energy. When arsenic is present in cells it interferes with the formation of ATP, which eventually leads to cell death. For for a visual demonstration of how arsenic can substitute for phosphorus in biomolecules, see the video at the bottom of this page
Because of the similarities between the properties of arsenic and phosphorus it's natural to speculate that organisms could evolve so that they could use arsenic in place of phosphorus. Dr. Felisa Wolfe-Simon, a geobiologist with the US Geological Survey had been testing that idea by studying bacteria living in California's Mono Lake, which has naturally high arsenic levels.