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Emergent chemical behavior in variable-volume protocells.

Shirt-Ediss B, Solé RV, Ruiz-Mirazo K - Life (Basel) (2015)

Bottom Line: 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.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.Our results suggest that other chemical innovations should be expected when more realistic and active properties of protocellular compartments are taken into account.

View Article: PubMed Central - PubMed

Affiliation: ICREA-Complex Systems Lab, Institut de Biologia Evolutiva, CSIC-UPF, 08003 Barcelona, Spain. ben@shirt-ediss.me.

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.

No MeSH data available.


Related in: MedlinePlus

Steady State 1 (SS1) and SS2 internal solute concentrations. Solute concentrations inside the vesicle reactor, at spherical steady states SS1 and SS2, for each reaction scheme reported in Figure 2. Comparing SS1 and SS2 for each scheme, it can be observed that there are quantifiable differences in solute concentrations between steady states, but these differences are fairly small. Owing to the osmotic water balance, the total concentration of solutes inside the vesicle (height of the stacked bars) is always constrained to be equal to the total external solute concentration of the environment, Cε. Thus, in the vesicle reactor model, the main feature distinguishing steady states is vesicle size (see Section 4.2 for details). Symbol b∗denotes the steady-state concentration of the BTbuffer molecules trapped inside the vesicle. At SS2, b∗→ 0, due to large vesicle sizes, and the diffusing solutes constitute the majority of total internal concentration. The Supplementary Material supplies data supporting the figure.
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life-05-00181-f003: Steady State 1 (SS1) and SS2 internal solute concentrations. Solute concentrations inside the vesicle reactor, at spherical steady states SS1 and SS2, for each reaction scheme reported in Figure 2. Comparing SS1 and SS2 for each scheme, it can be observed that there are quantifiable differences in solute concentrations between steady states, but these differences are fairly small. Owing to the osmotic water balance, the total concentration of solutes inside the vesicle (height of the stacked bars) is always constrained to be equal to the total external solute concentration of the environment, Cε. Thus, in the vesicle reactor model, the main feature distinguishing steady states is vesicle size (see Section 4.2 for details). Symbol b∗denotes the steady-state concentration of the BTbuffer molecules trapped inside the vesicle. At SS2, b∗→ 0, due to large vesicle sizes, and the diffusing solutes constitute the majority of total internal concentration. The Supplementary Material supplies data supporting the figure.

Mentions: In contrast, the following two case studies provide counterexamples that support the opposite scenario, in which interesting emergent chemistry may develop as a consequence of the volume-changing aqueous interior of a vesicle. Figure 2 displays the main results for Case Studies 1–3. It demonstrates that under the correct parameter regimes, each of the two reaction schemes discussed in each case study can show bistability within the full vesicle model. Vesicle morphology space and bifurcation curves are drawn for each reaction scheme. Even the Schlögl and Wilhelm reaction systems of the present case study can exhibit bistability for some parameters (Figure 2a(i) and 2a(ii), respectively). Figure 3 accompanies Figure 2, to make clear the concentration values of all internal solutes at stable steady states SS1 and SS2 for each of the six schemes.


Emergent chemical behavior in variable-volume protocells.

Shirt-Ediss B, Solé RV, Ruiz-Mirazo K - Life (Basel) (2015)

Steady State 1 (SS1) and SS2 internal solute concentrations. Solute concentrations inside the vesicle reactor, at spherical steady states SS1 and SS2, for each reaction scheme reported in Figure 2. Comparing SS1 and SS2 for each scheme, it can be observed that there are quantifiable differences in solute concentrations between steady states, but these differences are fairly small. Owing to the osmotic water balance, the total concentration of solutes inside the vesicle (height of the stacked bars) is always constrained to be equal to the total external solute concentration of the environment, Cε. Thus, in the vesicle reactor model, the main feature distinguishing steady states is vesicle size (see Section 4.2 for details). Symbol b∗denotes the steady-state concentration of the BTbuffer molecules trapped inside the vesicle. At SS2, b∗→ 0, due to large vesicle sizes, and the diffusing solutes constitute the majority of total internal concentration. The Supplementary Material supplies data supporting the figure.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4390847&req=5

life-05-00181-f003: Steady State 1 (SS1) and SS2 internal solute concentrations. Solute concentrations inside the vesicle reactor, at spherical steady states SS1 and SS2, for each reaction scheme reported in Figure 2. Comparing SS1 and SS2 for each scheme, it can be observed that there are quantifiable differences in solute concentrations between steady states, but these differences are fairly small. Owing to the osmotic water balance, the total concentration of solutes inside the vesicle (height of the stacked bars) is always constrained to be equal to the total external solute concentration of the environment, Cε. Thus, in the vesicle reactor model, the main feature distinguishing steady states is vesicle size (see Section 4.2 for details). Symbol b∗denotes the steady-state concentration of the BTbuffer molecules trapped inside the vesicle. At SS2, b∗→ 0, due to large vesicle sizes, and the diffusing solutes constitute the majority of total internal concentration. The Supplementary Material supplies data supporting the figure.
Mentions: In contrast, the following two case studies provide counterexamples that support the opposite scenario, in which interesting emergent chemistry may develop as a consequence of the volume-changing aqueous interior of a vesicle. Figure 2 displays the main results for Case Studies 1–3. It demonstrates that under the correct parameter regimes, each of the two reaction schemes discussed in each case study can show bistability within the full vesicle model. Vesicle morphology space and bifurcation curves are drawn for each reaction scheme. Even the Schlögl and Wilhelm reaction systems of the present case study can exhibit bistability for some parameters (Figure 2a(i) and 2a(ii), respectively). Figure 3 accompanies Figure 2, to make clear the concentration values of all internal solutes at stable steady states SS1 and SS2 for each of the six schemes.

Bottom Line: 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.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.Our results suggest that other chemical innovations should be expected when more realistic and active properties of protocellular compartments are taken into account.

View Article: PubMed Central - PubMed

Affiliation: ICREA-Complex Systems Lab, Institut de Biologia Evolutiva, CSIC-UPF, 08003 Barcelona, Spain. ben@shirt-ediss.me.

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.

No MeSH data available.


Related in: MedlinePlus