Dielectric polarization, electrical conduction, information processing and quantum computation in microtubules. Are they plausible?

The multitude and diversity of functions performed by the cytoskeleton of eukaryotic cells poses a major scientific puzzle. Microtubules which are the main components of the cytoskeleton are engaged in such important activities as: material transport, cell motility, cell division, signal transduction and possibly information processing within axons of nerve cells. The latter aspect has been recently brought to the forefront of consciousness studies by R. Penrose and S. Hameroff. In this paper we discuss the potential of microtubules as information processing units from a physical standpoint. In particular, since electric dipoles are a characteristic property of protein molecules, and as such may undergo various ordering phase transitions, it can be expected that microtubules support the existence of various ferroelectrically–ordered states. It is also argued that the piezoelectric effect may link electric and elastic properties of a dielectric polymer system and hence explain a number of experimental observations both in vitro and in vivo. Furthermore, preliminary results are shown of recent quantum mechanical calculations for the electrical conduction properties of microtubule protofilaments which support the assertion that the latter may function under certain conditions very much like semiconducting devices. Indirect evidence links the cytoskeleton with information processing and cognitive function. We discuss some plausible ways in which axonal microtubules can be involved in the functioning of the brain. Finally, we present a hierarchy of energy values for a number of physical and chemical interactions of interest and discuss their relative importance.