4 research outputs found
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
A circle swimmer at low Reynolds number
Swimming in circles occurs in a variety of situations at low Reynolds number.
Here we propose a simple model for a swimmer that undergoes circular motion,
generalising the model of a linear swimmer proposed by Najafi and Golestanian
(Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres
arranged in a triangular configuration, joined by two links of time-dependent
length. For small strokes, we discuss the motion of the swimmer as a function
of the separation angle between its links. We find that swimmers describe
either clockwise or anticlockwise circular motion depending on the tilting
angle in a non-trivial manner. The symmetry of the swimmer leads to a
quadrupolar decay of the far flow field. We discuss the potential extensions
and experimental realisation of our model.Comment: 9 pages, 9 Figure