Current Ideas about Prebiological Compartmentalization

Abstract

Contemporary biological cells are highly sophisticated dynamic compartment systems which separate an internal volume from the external medium through a boundary, which controls, in complex ways, the exchange of matter and energy between the cell's interior and the environment. Since such compartmentalization is a fundamental principle of all forms of life, scenarios have been elaborated about the emergence of prebiological compartments on early Earth, in particular about their likely structural characteristics and dynamic features. Chemical systems that consist of potentially prebiological compartments and chemical reaction networks have been designed to model pre-cellular systems. These systems are often referred to as "protocells". Past and current protocell model systems are presented and compared. Since the prebiotic formation of cell-like compartments is directly linked to the prebiotic availability of compartment building blocks, a few aspects on the likely chemical inventory on the early Earth are also summarized.

Figure 1

Inventory of some of the ideas about prebiological compartmentalization on Earth in a scenario that considers a terrestrial origin of life [43], see also the main text.

Figure 2

Design of protocells based on lipidic amphiphiles. (A) Interfacial bilayer systems, possibly as a boundary of a compartment system with an inorganic interior (bottom left), as a boundary of an organic vesicle with an aqueous interior (bottom middle), or adsorbed on a flat mineral surface (bottom right). The compounds that form catalytic networks can interact with the amphiphilic structure, either through a lipophilic association within the bilayers or by electrostatic interactions. In the former case, two categories of compounds exist, very hydrophobic molecules (hydrophobic chemicals or anchors are violet) that will be located within the hydrophobic core of the structures and molecules that are amphiphilic, i.e., compounds with a polar or charged moiety (hydrophilic compounds or moieties are in orange) and a hydrophobic moiety that can anchor them in the bilayers. (B) Encapsulation of water-soluble solutes within the aqueous lumen of a vesicular compartment (shaded in light blue). In (A), a direct access to substrates (in green) present in the external environment is possible. The B design requires the substrates (S, in green) to diffuse across the bilayers either through transient packing defects or via a partitioning into the bilayers. The reaction waste (W) is in red.