Rheological properties of living cytoplasm: Endoplasm of Physarum plasmodium

Abstract

Magnetic sphere viscoelastometry, video microscopy, and the Kamiya double chamber method have been combined in an optical and rheological investigation of the living endoplasm of Physarum polycephalum. The rheological properties examined were yield stress, viscocity (as a function of shear), and elasticity. These parameters were evaluated in directions perpendicular; (X) and parallel (Y) to the plasmodial vein. Known magnetic forces were used for measurements in the X direction, while the falling ball technique was used in the Y direction. Approximate yield stresses were calculated in the X and Y directions of 0.58 and 1.05 dyn/cm2, respectively. Apparent viscosities measured in the two directions (η(x) and η(y)) were found to fluctuate with time. The fluctuations in η(x) and η(y) were shown, statistically, to occur independently of each other. Frequency correlation with dynamoplasmograms indicated that these fluctuations probably occur independently of the streaming cycle. Viscosity was found to be a complex function of shear, indicating that the endoplasm is non-Newtonian. Plots of shear stress vs. rate of shear both parallel and perpendicular to the vein, showed that endoplasm is not a shear thinning material. These experiments have shown that living endoplasm of Physarum is an anisotropic viscoelastic fluid with a yield stress. The endoplasm appears not to be a homogeneous material, but to be composed of heterogeneous domains.