Figure 13.

Example features of the Metabolic GARD (M-GARD) model. (a) The reactions shown are assumed to be non-enzymatically catalysed by other lipids as part of a GARD mutually catalytic network, analogous to that of the standard GARD model. (i) Lipid headgroup is modified on the outer leaflet of the bilayer, via a covalent reaction with an extraneous hydrophilic compound (yellow); (ii) non-covalent lipid flipping to the inner leaflet; (iii) covalent clipping that releases the hydrophilic compound into the vesicle aqueous lumen. This progression is equivalent to catalysed transport that confers chemical specificity on lumen enrichment. In modified form, if a high-energy lipid (e.g. with oligophosphate headgroup) is involved as a reactant, this could constitute a primitive form of active transport. (b) The reactions in a could underlie a coveted scenario in which under modified GARD compositional reproduction dynamics homeostatic growth and fission could lead to the generation of progeny with faithful copying of the composition of both the bilayer shell and the enclosed aqueous lumen.