Figure 2. Membrane diffusion imposes a minimum cell size threshold for stable polarisation.

(a-d) Polarity across parameter space defined by system size (L) and the pool (OT/GOR/WP) or ratio of pools (PAR) of available species. All exhibit a region of parameter space (grey) that permits maintenance of polarity, which is bounded by a CPSS (dashed lines). Insets show schematic representation of the steady-state (polarized or unpolarized). For the PAR system, whether A or P is the dominant membrane species in the unpolarized state is colour-coded. (e) CPSS varies linearly with $\sqrt{D}$ for all models. (f) Conceptual model for a cell-size-induced polarity switch in a stem cell-like lineage. A stem cell polarizes and divides asymmetrically to generate another stem cell and a differentiating cell. Absent cell growth, the stem cell becomes smaller at each division. If cell size limits polarisation, at some point the stem cell will fail to polarize leading to symmetric division.