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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

Graphical intuition into emergent bistability through osmotic coupling. (a)Emergent bistability in the vesicle reactor model: (i) two chemically-independent unimolecular reactions can be understood by taking a “reactions-eye view” from the perspective of each reaction; (ii) from the perspective of the X→Y reaction (Reaction 1), all molecules associated with the P→Q reaction simply act as extra inert buffer (B2) in addition to the trapped impermeable buffer molecules BT inside the compartment; (iii) conversely, from the perspective of the P→Q reaction (Reaction 2), all molecules associated with the P→Q reaction act as extra inert buffer (B1). (b) Graph showing how the total steady-state particle number of each reaction responds to the extra number of buffer molecules that the other reaction is providing, where Reaction 1 has the y-axis as independent and Reaction 2 has the x-axis as independent. The three cross points represent fulfillment of the cyclic condition referred to in the text. The dotted line shows the relation B1 = B2. Two chemically-independent reaction sets with identical stoichiometry and identical kinetic constants would give curves that are reflections in this line. (c) When the chemical transformation between P and Q is removed, the latter solutes simply diffuse across the membrane until their respective concentration gradients are equalized. A unimolecular reaction sharing the vesicle compartment with such inert diffusing species cannot be bistable (see the text).
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life-05-00181-f005: Graphical intuition into emergent bistability through osmotic coupling. (a)Emergent bistability in the vesicle reactor model: (i) two chemically-independent unimolecular reactions can be understood by taking a “reactions-eye view” from the perspective of each reaction; (ii) from the perspective of the X→Y reaction (Reaction 1), all molecules associated with the P→Q reaction simply act as extra inert buffer (B2) in addition to the trapped impermeable buffer molecules BT inside the compartment; (iii) conversely, from the perspective of the P→Q reaction (Reaction 2), all molecules associated with the P→Q reaction act as extra inert buffer (B1). (b) Graph showing how the total steady-state particle number of each reaction responds to the extra number of buffer molecules that the other reaction is providing, where Reaction 1 has the y-axis as independent and Reaction 2 has the x-axis as independent. The three cross points represent fulfillment of the cyclic condition referred to in the text. The dotted line shows the relation B1 = B2. Two chemically-independent reaction sets with identical stoichiometry and identical kinetic constants would give curves that are reflections in this line. (c) When the chemical transformation between P and Q is removed, the latter solutes simply diffuse across the membrane until their respective concentration gradients are equalized. A unimolecular reaction sharing the vesicle compartment with such inert diffusing species cannot be bistable (see the text).

Mentions: In Figure 5, graphical intuition is developed as to why the two chemically-independent unimolecular reactions, Equation (15), can display emergent bistability in the variable volume vesicle model. It is not at all trivial that two such reactions should display bistability, since both are kinetically simple mechanisms with rates linearly dependent on the reactant concentration. Furthermore, stoichiometrically speaking, there is no net production of molecules in the system (one consumed molecule per produced one, in both reactions), so the asymmetry triggering volume changes can only come from the different diffusion (or permeability) properties of the four chemical species involved.


Emergent chemical behavior in variable-volume protocells.

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

Graphical intuition into emergent bistability through osmotic coupling. (a)Emergent bistability in the vesicle reactor model: (i) two chemically-independent unimolecular reactions can be understood by taking a “reactions-eye view” from the perspective of each reaction; (ii) from the perspective of the X→Y reaction (Reaction 1), all molecules associated with the P→Q reaction simply act as extra inert buffer (B2) in addition to the trapped impermeable buffer molecules BT inside the compartment; (iii) conversely, from the perspective of the P→Q reaction (Reaction 2), all molecules associated with the P→Q reaction act as extra inert buffer (B1). (b) Graph showing how the total steady-state particle number of each reaction responds to the extra number of buffer molecules that the other reaction is providing, where Reaction 1 has the y-axis as independent and Reaction 2 has the x-axis as independent. The three cross points represent fulfillment of the cyclic condition referred to in the text. The dotted line shows the relation B1 = B2. Two chemically-independent reaction sets with identical stoichiometry and identical kinetic constants would give curves that are reflections in this line. (c) When the chemical transformation between P and Q is removed, the latter solutes simply diffuse across the membrane until their respective concentration gradients are equalized. A unimolecular reaction sharing the vesicle compartment with such inert diffusing species cannot be bistable (see the text).
© Copyright Policy
Related In: Results  -  Collection

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

life-05-00181-f005: Graphical intuition into emergent bistability through osmotic coupling. (a)Emergent bistability in the vesicle reactor model: (i) two chemically-independent unimolecular reactions can be understood by taking a “reactions-eye view” from the perspective of each reaction; (ii) from the perspective of the X→Y reaction (Reaction 1), all molecules associated with the P→Q reaction simply act as extra inert buffer (B2) in addition to the trapped impermeable buffer molecules BT inside the compartment; (iii) conversely, from the perspective of the P→Q reaction (Reaction 2), all molecules associated with the P→Q reaction act as extra inert buffer (B1). (b) Graph showing how the total steady-state particle number of each reaction responds to the extra number of buffer molecules that the other reaction is providing, where Reaction 1 has the y-axis as independent and Reaction 2 has the x-axis as independent. The three cross points represent fulfillment of the cyclic condition referred to in the text. The dotted line shows the relation B1 = B2. Two chemically-independent reaction sets with identical stoichiometry and identical kinetic constants would give curves that are reflections in this line. (c) When the chemical transformation between P and Q is removed, the latter solutes simply diffuse across the membrane until their respective concentration gradients are equalized. A unimolecular reaction sharing the vesicle compartment with such inert diffusing species cannot be bistable (see the text).
Mentions: In Figure 5, graphical intuition is developed as to why the two chemically-independent unimolecular reactions, Equation (15), can display emergent bistability in the variable volume vesicle model. It is not at all trivial that two such reactions should display bistability, since both are kinetically simple mechanisms with rates linearly dependent on the reactant concentration. Furthermore, stoichiometrically speaking, there is no net production of molecules in the system (one consumed molecule per produced one, in both reactions), so the asymmetry triggering volume changes can only come from the different diffusion (or permeability) properties of the four chemical species involved.

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