Rhodopsin-mediated photosensing in green flagellated algae

Green flagellated algae possess a primitive visual system that regulates th e activity of their motor apparatus. Photoexcitation of a rhodopsin-type ph otoreceptor protein gives rise to the photoreceptor current, which, above a certain threshold of stimulus intensity, induces the flagellar current. It is probable that the photoinduced alteration in flagellar beating is gover ned by changes in intracellular Ca2+ concentration. This rhodopsin-mediated sensory system serves to align the swimming path with the direction of the light stimulus, whereas processes of energy metabolism determine whether t he oriented movement is directed towards or away from the light source.