From https://en.wikipedia.org/wiki/Rod_cell https://en.wikipedia.org/wiki/Rod_cell "Activation of a single unit of rhodopsin, the photosensitive pigment in rod

From https://en.wikipedia.org/wiki/Rod_cell https://en.wikipedia.org/wiki/Rod_cell "Activation of a single unit of rhodopsin, the photosensitive pigment in rods, can lead to a large reaction in the cell because the signal is amplified. Once activated, rhodopsin can activate hundreds of transducin molecules, each of which in turn activates a phosphodiesterase molecule, which can break down over a thousand cGMP molecules per second (Kandel et al. 2000). Thus, rods can have a large response to a small amount of light."