Hypothesis of gravikinesis in Paramecium. The cell membrane of Paramecium contains K- and Ca-mechanoreceptor channels. Their number follow two gradients: At the posterior cell pole mainly K-mechanorec. are found. At the anterior end mainly Ca-mechanorec. are found. The cytoplasm, whose density exceeds the density of the surrounding medium by 4%, causes a pressure on the lower cell membrane, stimulating the receptor channels. Depending on cell orientation the causes either a depolarization by stimulation of Ca-mechanorec. of a hyperpolarization by stimulation of K-receptorc. Because of the electromotor coupling a downwards swimming cell decreases actively its swimming rate, whereas an upwards swimming cell actively increases its swimming rate. This gravikinesis compensates part of the sedimentation rate.