Digital SAT Reading and Writing Practice Question #611

Scientists hypothesized that cryptochrome-4 (CRY4) in the retinas of European robins mediates magnetic compass navigation via a quantum radical pair mechanism. In a recent experiment, robins were exposed to specific radio frequencies definitively known to disrupt spin-correlated radical pairs without affecting any other biological processes. Under green light, normal robins exposed to these frequencies completely lost their ability to orient magnetically. Under red light, however, normal robins could not orient magnetically at all, regardless of the presence or absence of the radio frequencies, because red light photons lack the energy required to trigger CRY4 excitation. Furthermore, mutant robins lacking functional CRY4 were entirely unable to orient magnetically under any lighting conditions.