In rod cells, phototransduction consists of 4 steps:

1. Activation of rhodopsin

2. Activation of a G-protein (transducin), resulting in conversion of GTP to GDP

3. Activation of phosphodiesterase, resulting in the conversion of cGMP to GMP

4. Closure of a Na+ channel, resulting in hyperpolarization of the rod.

During each step in this process, there is amplification of the signal, as 1 photon can result in the activation of 100,000 cGMP molecules/sec.  This amplification contributes greatly to the sensitivity of the phototransduction process.