Vesicle size and membrane composition control monomer transfer pathways in multicomponent lipid vesicles.

Lipid exchange between populations of non-fusing protocells, mediated by monomer diffusion through the aqueous phase, is a key process underlying protocell growth and compositional equilibration. While most theoretical descriptions consider single-component membranes with constant desorption rates, the influence of vesicle size and lipid composition on lipid-transfer dynamics remains poorly understood. Here, we develop a kinetic model for lipid exchange in multicomponent protocell populations that incorporates lipid species with different desorption rates and composition-dependent membrane packing effects. We show that lipid composition heterogeneity introduces multiple equilibration timescales and can generate non-monotonic dynamics, including transient overshoots of intermediate lipid species. Vesicle-size asymmetry controls the direction and rate of lipid transfer, determining the donor or acceptor character of individual vesicle populations. Membrane compression further modifies lipid-transfer kinetics and vesicle-area evolution. These results demonstrate how vesicle size, lipid composition, and membrane packing jointly govern lipid-transfer dynamics in heterogeneous protocell populations.