Cercadian Change of Photosensiuvity in Paramecium Bursaria

Circadian changes in behaviour and electrical properties of the membrane were investigated in Paramecium bursaria. Both photoaccumulation and swimming velocity were at a higher level during the day than at night. During the day, swimming velocity decreased as light intensity was increased and the cells almost stopped at the intensity at which they showed the maximum degree of photoaccumulation. When the intensity was increased further, the avoiding reaction was frequently observed. Step increases in the light intensity depolarized the membrane potential. The depolarization was larger in the night cells than in the day cells. The resting potential measured in the dark was about 4 mV more positive in the night cells than in the day cells. Membrane conductance was larger in the day than at night, which can explain the changes in the resting potential and also the photosensitive behaviour.