Velocity Distributions of Characean Cytoplasmic Streaming are Consistent with Random Cross-bridge Contractions

Abstract

We have obtained velocity distributions for cytoplasmic streaming in Chara and Nitella by three separate techniques: laser light scattering, where a Doppler shift is measured; clocking of organelles over a standard distance using low-power microscopy; and frame-by-frame analysis of cine film taken at highpower. The laser-derived distribution is a symmetrical Lorentzian centred at the average velocity, with half-bandwidth 12% of the average. The distribution as seen by low-power microscopy is different: it is skewed, showing a sharp drop off at the high-velocity end. The cine distribution is more symmetrical and indicates by its spread that streaming is not a smooth process when viewed on a small time and distance scale. We propose that organelles advance in a jerky fashion caused by brief cross-bridge contractions which are randomly timed. Streaming would nevertheless appear smooth under low-power as each organelle would undergo a larger number of contractions to cover the longer distances involved. Velocity spread in the laser distribution, where measurements are made over distances the order of a wavelength, cannot reasonably be attributed to Brownian motion.