37 research outputs found
Pattern formation of reaction-diffusion system having self-determined flow in the amoeboid organism of Physarum plasmodium
The amoeboid organism, the plasmodium of Physarum polycephalum, behaves on
the basis of spatio-temporal pattern formation by local
contraction-oscillators. This biological system can be regarded as a
reaction-diffusion system which has spatial interaction by active flow of
protoplasmic sol in the cell. Paying attention to the physiological evidence
that the flow is determined by contraction pattern in the plasmodium, a
reaction-diffusion system having self-determined flow arises. Such a coupling
of reaction-diffusion-advection is a characteristic of the biological system,
and is expected to relate with control mechanism of amoeboid behaviours. Hence,
we have studied effects of the self-determined flow on pattern formation of
simple reaction-diffusion systems. By weakly nonlinear analysis near a trivial
solution, the envelope dynamics follows the complex Ginzburg-Landau type
equation just after bifurcation occurs at finite wave number. The flow term
affects the nonlinear term of the equation through the critical wave number
squared. Contrary to this, wave number isn't explicitly effective with lack of
flow or constant flow. Thus, spatial size of pattern is especially important
for regulating pattern formation in the plasmodium. On the other hand, the flow
term is negligible in the vicinity of bifurcation at infinitely small wave
number, and therefore the pattern formation by simple reaction-diffusion will
also hold. A physiological role of pattern formation as above is discussed.Comment: REVTeX, one column, 7 pages, no figur
Intracellular microrheology of motile Amoeba proteus
The motility of motile Amoeba proteus was examined using the technique of
passive particle tracking microrheology, with the aid of newly-developed
particle tracking software, a fast digital camera and an optical microscope. We
tracked large numbers of endogeneous particles in the amoebae, which displayed
subdiffusive motion at short time scales, corresponding to thermal motion in a
viscoelastic medium, and superdiffusive motion at long time scales due to the
convection of the cytoplasm. Subdiffusive motion was characterised by a
rheological scaling exponent of 3/4 in the cortex, indicative of the
semiflexible dynamics of the actin fibres. We observed shear-thinning in the
flowing endoplasm, where exponents increased with increasing flow rate; i.e.
the endoplasm became more fluid-like. The rheology of the cortex is found to be
isotropic, reflecting an isotropic actin gel. A clear difference was seen
between cortical and endoplasmic layers in terms of both viscoelasticity and
flow velocity, where the profile of the latter is close to a Poiseuille flow
for a Newtonian fluid