Source: Hardie, R. C. and Raghu, P. (2001) Visual transduction in Drosophila. Nature 413: 186-193. The diagram shows the major elements that link the capture of a photon of light to the opening of membrane channels. Rhodopsin is the visual pigment molecule. Each molecule can capture one photon of light, which activates it. G is the G-protein activated by rhodopsin. The G-protein activates phospholipase C (PLC), which opens the channels through a series of intermediate steps. TRP is the transient receptor protein channel. When open, it allows Ca++ and other small cations to enter, depolarizing the photoreceptor cell.
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