Measurement of the path-length difference dependence from the gradient of electric field was carried out using phase microscope. The dependence was shown to be linear. The constant of electrooptical effect of membrane in multilayer structure was 5.5 x 10(-9) m/V. Similar values are obtained in experiments with electrooptical effect on the mitochondrial membrane - numerical values of this constant varied from 2.4 x 10(-9) to 1.2 x 10(-8) m/V. Bases on similarity of these values, we conclude that the phase-shift on mitochondria and black membrane has the same physical nature. After the weak-bounded water removing, the constant of electrooptical effect decreases in several times. Based on this observation we conclude that phase-length changes of the electric field are due to the existence of the weak-bounded water layer. The obtained results shade light on new aspects of the influence of the membrane electric potential on functioning of biological systems.