Primitive Membrane Formation, Characteristics and Roles in the Emergent Properties of a Protocell

Abstract

All contemporary living cells are composed of a collection of self-assembled molecular elements that by themselves are non-living but through the creation of a network exhibit the emergent properties of self-maintenance, self-reproduction, and evolution. This short review deals with the on-going research that aims at either understanding how life emerged on the early Earth or creating artificial cells assembled from a collection of small chemicals. In particular, this article focuses on the work carried out to investigate how self-assembled compartments, such as amphiphile and lipid vesicles, contribute to the emergent properties as part of a greater system.

Current Ideas about Prebiological Compartmentalization

“…Recognizing the fact that lipid-based vesicles resemble the basic morphology of all cells, as far as the basic design is considered—an aqueous volume enclosed by a lipid-based boundary, they have been often proposed as the most cell-like models for prebiotic compartmentalization [ 6 , 20 , 32 , 72 , 76 , 77 ]. Vesicular compartments offer unique possibilities for mimicking the structural and dynamic properties of biological cells and their protocellular precursor structures: (i) vesicles form from potentially prebiotic lipidic amphiphiles [ 20 ]; (ii) the internal volume of vesicles can vary between about 50 nm and several 100 μm, encompassing the size range of prokaryotes; (iii) inorganic and organic molecules can be trapped inside vesicles; (iv) the permeability can be modified, or “fine-tuned”, by either choosing the membrane-forming lipids accordingly or by adding membrane-soluble compounds, or physically (e.g., by varying the temperature); (v) the vesicle interior or the vesicle surface can promote and regulate chemical reactions [ 50 , 78 ]; (vi) chemical reactions leading to the formation of lipidic vesicle membrane-forming amphiphiles can be accelerated by the presence of the vesicles, thereby leading to vesicle growth and reproduction [ 6 , 75 , 79 ]; (vii) vesicles can aggregate to form vesicle colonies [ 80 ]; and (viii) vesicles can entrap other vesicles to form so-called “multivesicular vesicles” [ 81 ], similar to the internal compartmentalization of eukaroytic cells.…”

An Origin-of-Life Reactor to Simulate Alkaline Hydrothermal Vents

“…The natural inorganic compartments in alkaline vents could facilitate not only the concentration of organics by thermophoresis, but also the beginnings of selection for metabolism (Branciamore et al 2009 ; Koonin and Martin 2005 ). The two processes combined could potentially drive the replication of simple organic vesicles composed of mixed amphiphiles enclosing primitive replicators within vent pores (Budin et al 2009 ; Mauer and Monndard 2011 ). Such vesicles are capable of growth and division, while retaining RNA (Hanczyc et al 2003 ; Mansy et al 2008 ) and are en route to the known end-point, modern cells with lipid membranes.…”

Growth and Division in a Dynamic Protocell Model

“…Finally, note that the presence of high protein concentrations in small vesicles are also experimentally reported, as for example in [ 65 ]; these differences between internal and external chemical concentrations could induce the presence of interesting osmotic processes [ 66 ].…”