The action of sinusoidally modulated light on the membrane potential of Acetabularia

Cells of the alga Acetabularia crenulata were illuminated withlight of sinusoidally modulated intensity of different frequenciesand changes in membrane potential were measured with microelectrodes.The frequency response, investigated in the range of frequenciesof 8 to 1/40 cycles/min, displayed the following features: 1.The slope of the frequency response is, according to our results,6 db/octave or steeper. Experiments should be extended to higherfrequencies to gain more information concerning the slope ofthe frequency response and, thus, of the number of reactionsin the series. 2. The time course of changes in membrane potentialcaused by light reveals non-linearities in time-scale and amplitude,indicating that not only first-order reactions are involved.3. Time constants are slightly influenced by the intensity oflight. 4. Further evidence is given that the characteristicof the action of light is logarithmic. 5. A phenomenon similarto resonance is observed, which corresponds to the overshootsgenerated by square-wave illumination. Models show that thisis not necessarily explained by feedback systems, but also bycounteracting parallel pathways (Received October 8, 1970; )