Moving Life

Previous chapters have dealt with the phenomenon centered on the structure and function of biological membranes. The lipid membrane is an essential structure that guarantees the steady state of the life in the ever-changing environment. Non-stop exchange of matters and information with the environment is a salient feature of the activity of living thing. The previous chapters describe various facts of these aspects. Voluntary movement is another remarkable feature of living things. Thus in Chap. 7, various types of biological motions are described. First, muscle contraction is briefly described, because it is one of the best known and studied example of the motor-based biological motions. Another well-studied example of the motor-based motion is the organelle transport; microscopic observations and analysis of the motion are described as examples. Motors related to the transport motility are briefly described and actin and microtubule tracks are described in some detail. The dynamism of the cell is largely depends on the dynamism of the system of actin filaments, microtubules and intermediate filaments. These filament systems are assembled or disassembled according to the needs of the cell, which is adopted in generating cellular movements. In the case of actin filaments and microtubules the movements such as lamellipodial protrusion, segregation of chromosomes and organelle transport, are driven. For these phenomena polymerization and depolymerization of the subunit of these polymers play essential roles. Thus, the polymerization/depolymerization-driven motions in the cell are one of the main theme of the last half of this chapter.First, structure of actin monomer and tubulin, actin filament and microtubule are described, and kinetics of polymerization of actin is then explained, because it is necessary to understand the polymerization/depolymerization-driven motions in the cell. Following these sections, overview of the cell motility is given, which is followed by explanation on the actin-related structures in the cell. An important question on the actin-based motility is that if it can really develop the force that leads to the mechanical work such as deformation of the cell membranes in lamellipodial protrusion. Three examples are presented: (1) the measurements done on two types of cells keratocyte, a highly motile cell, and a fibroblast, rather quiescent cell; (2) the reconstituted systems, liposomes encapsulating actin or monomeric unit of microtubule (tubulin); (3) actin-based movement of bacteria. Then, theories that were developed on the basis of thermodynamics or on the basis of thermal fluctuation, and the experimental evaluation of the theories are described. In the last section microtubule-based, depolymerization-driven motility is briefly explained.