  Radical-ion pairs are ubiquitous in a wide range of biochemical reactions,
ranging from photosynthesis to magnetic sensitive chemical reactions underlying
avian magnetic navigation. We here show that the charge recombination of a
radical-ion-pair is a continuous quantum measurement process that interrogates
the spin state of the pair. This naturally leads to the appearance of the
quantum Zeno effect, explaining a large amount of data on unusually long-lived
radical-ion-pairs.
