4,409 research outputs found
Bistability: Requirements on Cell-Volume, Protein Diffusion, and Thermodynamics
Bistability is considered wide-spread among bacteria and eukaryotic cells,
useful e.g. for enzyme induction, bet hedging, and epigenetic switching.
However, this phenomenon has mostly been described with deterministic dynamic
or well-mixed stochastic models. Here, we map known biological bistable systems
onto the well-characterized biochemical Schloegl model, using analytical
calculations and stochastic spatio-temporal simulations. In addition to network
architecture and strong thermodynamic driving away from equilibrium, we show
that bistability requires fine-tuning towards small cell volumes (or
compartments) and fast protein diffusion (well mixing). Bistability is thus
fragile and hence may be restricted to small bacteria and eukaryotic nuclei,
with switching triggered by volume changes during the cell cycle. For large
volumes, single cells generally loose their ability for bistable switching and
instead undergo a first-order phase transition.Comment: 23 pages, 8 figure
Entropy production selects nonequilibrium states in multistable systems
Far-from-equilibrium thermodynamics underpins the emergence of life, but how
has been a long-outstanding puzzle. Best candidate theories based on the
maximum entropy production principle could not be unequivocally proven, in part
due to complicated physics, unintuitive stochastic thermodynamics, and the
existence of alternative theories such as the minimum entropy production
principle. Here, we use a simple, analytically solvable, one-dimensional
bistable chemical system to demonstrate the validity of the maximum entropy
production principle. To generalize to multistable stochastic system, we use
the stochastic least-action principle to derive the entropy production and its
role in the stability of nonequilibrium steady states. This shows that in a
multistable system, all else being equal, the steady state with the highest
entropy production is favored, with a number of implications for the evolution
of biological, physical, and geological systems.Comment: 15 pages, 4 figure
Noise characteristics of the Escherichia coli rotary motor
The chemotaxis pathway in the bacterium Escherichia coli allows cells to
detect changes in external ligand concentration (e.g. nutrients). The pathway
regulates the flagellated rotary motors and hence the cells' swimming
behaviour, steering them towards more favourable environments. While the
molecular components are well characterised, the motor behaviour measured by
tethered cell experiments has been difficult to interpret. Here, we study the
effects of sensing and signalling noise on the motor behaviour. Specifically,
we consider fluctuations stemming from ligand concentration, receptor switching
between their signalling states, adaptation, modification of proteins by
phosphorylation, and motor switching between its two rotational states. We
develop a model which includes all signalling steps in the pathway, and discuss
a simplified version, which captures the essential features of the full model.
We find that the noise characteristics of the motor contain signatures from all
these processes, albeit with varying magnitudes. This allows us to address how
cell-to-cell variation affects motor behaviour and the question of optimal
pathway design. A similar comprehensive analysis can be applied to other
two-component signalling pathways.Comment: 22 pages, 7 figures, 3 tutorials, supplementary information;
submitted manuscrip
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