Abstract
Unilamellar lipid vesicles can serve as model for protocells. We present a vesicle fission mechanism in a thermal gradient under flow in a convection chamber, where vesicles cycle cold and hot regions periodically. Crucial to obtain fission of the vesicles in this scenario is a temperature-induced membrane phase transition that vesicles experience multiple times. We model the temperature gradient of the chamber with a capillary to study single vesicles on their way through the temperature gradient in an external field of shear forces. Starting in the gel-like phase the spherical vesicles are heated above their main melting temperature resulting in a dumbbell-deformation. Further downstream a temperature drop below the transition temperature induces splitting of the vesicles without further physical or chemical intervention. This mechanism also holds for less cooperative systems, as shown here for a lipid alloy with a broad transition temperature width of 8 K. We find a critical tether length that can be understood from the transition width and the locally applied temperature gradient. This combination of a temperature-induced membrane phase transition and realistic flow scenarios as given e.g. in a white smoker enable a fission mechanism that can contribute to the understanding of more advanced protocell cycles.
Highlights
Before life on earth emerged, only small molecules existed
While modern cell division is a complicated mechanism mediated by many complex proteins, an early earth protocell must have had a simple fission mechanism
The concept of a primitive cell cycle with division of cell-sized lipid vesicles is modelled in several studies[16]: in all cases the concept introduces an imbalance of vesicle surface and volume[15,17,18,19], as shown above, or shear forces and thermodynamic instability as shown by Szostak et al In combination with a spontaneous growth mechanism, this “[...] could lead to a primitive cell cycle controlled entirely by the biophysical properties of the membrane and environmental forces”[17,20]
Summary
Before life on earth emerged, only small molecules existed. The development of complex structures or already simple reactions were unlikely and energetically unfavorable due to the assumed very dilute settings of early earth[1,2]. They do not result in two or more sized vesicles resembling modern cell division Such a temperature setup does not resemble scenarios possible on early earth, as closed fluid compartments with rapid temperature change over some seconds without flow do barely exist in nature. The concept of a primitive cell cycle with division of cell-sized lipid vesicles is modelled in several studies[16]: in all cases the concept introduces an imbalance of vesicle surface and volume[15,17,18,19], as shown above, or shear forces and thermodynamic instability as shown by Szostak et al In combination with a spontaneous growth mechanism, this “[...] could lead to a primitive cell cycle controlled entirely by the biophysical properties of the membrane and environmental forces”[17,20]
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