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Abstract
Artificial protocellular compartments and lipid vesicles have been used as model systems to understand the origins and requirements for early cells, as well as to design encapsulated reactors for biotechnology. One prominent feature of vesicles is the semi-permeable nature of their membranes, able to support passive diffusion of individual solute species into/out of the compartment, in addition to an osmotic water flow in the opposite direction to the net solute concentration gradient. Crucially, this water flow affects the internal aqueous volume of the vesicle in response to osmotic imbalances, in particular those created by ongoing reactions within the system. In this theoretical study, we pay attention to this often overlooked aspect and show, via the use of a simple semi-spatial vesicle reactor model, that a changing solvent volume introduces interesting non-linearities into an encapsulated chemistry. Focusing on bistability, we demonstrate how a changing volume compartment can degenerate existing bistable reactions, but also promote emergent bistability from very simple reactions, which are not bistable in bulk conditions. One particularly remarkable effect is that two or more chemically-independent reactions, with mutually exclusive reaction kinetics, are able to couple their dynamics through the variation of solvent volume inside the vesicle. Our results suggest that other chemical innovations should be expected when more realistic and active properties of protocellular compartments are taken into account.
Highlights
The rise of cellular life on the early Earth provided a unique opportunity for escaping from the vagaries of chemical interactions happening in a compartment-free context [1]
In trying to solve fixed points of the variable volume vesicle reactor model ODE Equations (7), it can be observed that the dilution terms Equation (8) can be usefully disregarded, since the vesicle volume is not changing at steady state
We perform three case studies that assess the behavior of basic chemical reaction sets inside the variable volume vesicle reactor model
Summary
The rise of cellular life on the early Earth provided a unique opportunity for escaping from the vagaries of chemical interactions happening in a compartment-free context [1]. Some authors [4] claim that metabolisms should no longer be conceived just as complex networks of cyclic, strongly-regulated and coupled reactions, but in addition, ought to be understood as intrinsically “vectorial” [5], i.e., involving membrane processes and dynamics from their very core In this theoretical study, we take as a starting point a prebiotic scenario in which primitive protocells (lipid vesicles) would spontaneously form and behave as dynamic supramolecular structures that can host and get coupled to various chemical reactions within their inner aqueous medium. Martín et al [17] have used the dilution term to model primitive cells, where metabolic complexity is strongly reduced In this protocell scenario, the complete description of the reaction system encapsulated within the membrane is known, and this permits the variation of volume to be formulated, via osmotic considerations, as explicitly depending on the total concentration of internal species. The Supplementary Material (on line) contains essential supporting derivations and data
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