Remarkably, a single photon absorbed by a resting rod cell produces a measurable response in the form of a small hyperpolarization in the membrane potential of about 1 mV, which in amphibians lasts a second or two. Humans are able to detect a flash of as few as five photons with their hundreds to thousands of rods; such responses to single-photon absorption are highly relevant for night vision. The light-detecting system is very sensitive because the signal is greatly amplified by the signal transduction pathway. During the time it is active, each rhodopsin molecule in the disk membrane of a rod cell can activate ~500 Gαt molecules, two of which in turn activate a single phosphodiesterase molecule. Each PDE molecule hydrolyzes hundreds of cGMP molecules during the fraction of a second it remains active. Thus absorption of a single photon—yielding a single activated rhodopsin molecule—can trigger the closing of thousands of cation channels (or about 5 percent of the open channels) in the plasma membrane and cause a measurable change in the membrane potential of the cell.