AbstractIn darknessChlamydomonascells swim forward with a helical motion of low amplitude. When cells are exposed to light conditions they are not adapted to, they perform direction changes or stop responses depending on how far the stimulant irradiance is shifted from the former adaptation level. Here we present the utility of a commercially available motion analysis system for the analysis of theChlamydomonasstop response.Chlamydomonascells stop in darkness only occasionally with a random temporal distribution but with a highly increased frequency after a flash or a step‐up light stimulation. The delay time, tD, defined as the minimal time difference between flash and a light‐induced stop, was below 50 ms. The reaction time, tR, defined as the time difference between flash and the maximal probability for a cell to stop was found to be 140 ms. During a stop the cells swim revers for some 300 ms with 20 of the forward swimming speed.To a given stimulation program cells adapt with a first‐order kinetic. In the case of a single step‐up or step‐down stimulation this adaptation consists of a single stop response followed by direction changes which decrease in frequency to a certain steady‐state level. To repetitive light pulses the cells respond with a gradual disappearance of step‐up stop responses and a concurrent appearance of step‐down responses.External calcium influences the stop response in a multifunctional way. For stop responses to occur 300 nM calcium are required. At increasing calcium concentrations the duration of a stop response is extended. Besides light, calcium regulates the time course of light‐adaption and the absolute adaptation level.A kinetic model for the description of adapta
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