14.5 Phase-Separated Compartments

Phase-Separated Compartments are synthetic cell compartments that achieve internal-external separation through liquid-liquid phase separation rather than through any discrete membrane or solid boundary, forming a dense, compartment-like liquid region dispersed within a surrounding dilute liquid phase. This class, exemplified by coacervates, relies on differential molecular affinity and solubility rather than a structural barrier to concentrate and retain contents.

Foundational Physical Process

Liquid-Liquid Phase Separation

Liquid-liquid phase separation is the physical process by which a homogeneous mixture of molecules spontaneously demixes into two distinct liquid phases, each enriched in different components, when conditions such as concentration or molecular interaction strength cross a critical threshold. This process is the underlying physical driver of every phase-separated compartment described in this branch.

Coacervate Phase Formation

Coacervate phase formation is the specific instance of liquid-liquid phase separation in which oppositely interacting macromolecules, typically driven by electrostatic or other favorable interactions, condense out of solution into a distinct liquid droplet phase. Coacervate formation is the primary route by which phase-separated compartments are realized in a synthetic cell context.

The Two-Phase Architecture

Coacervate Dense Phase

The coacervate dense phase is the macromolecule-rich liquid region that forms as a result of phase separation, functioning as the compartment interior analogous to the lumen of a membrane-bound vesicle, despite lacking any discrete boundary structure.

Coacervate Dilute Phase

The coacervate dilute phase is the macromolecule-poor liquid region that surrounds the dense phase after phase separation, functioning as the external environment relative to the compartment, analogous to the medium surrounding a membrane-bound vesicle.

Coacervate Interface

The coacervate interface is the boundary region separating the dense and dilute phases, distinguished not by a discrete physical membrane but by a sharp gradient in molecular composition and concentration between the two liquid phases.

Compositional Behavior of the Dense Phase

Molecular Partitioning

Coacervate molecular partitioning describes the tendency of specific molecules to distribute unevenly between the dense and dilute phases, with certain molecules favoring the dense phase and others remaining predominantly in the dilute phase, based on their chemical compatibility with the concentrated macromolecular environment.

Macromolecule Enrichment

Coacervate macromolecule enrichment describes the elevated concentration of macromolecules that accumulate within the dense phase relative to the surrounding dilute phase, a direct consequence of the phase separation process that defines the dense phase in the first place.

Reaction Concentration

Coacervate reaction concentration describes how the enrichment of reactant molecules within the dense phase can elevate local reaction rates or shift reaction equilibria relative to what would occur in the more dilute surrounding phase, without requiring any structural barrier to achieve this concentrating effect.

Selective Recruitment

Coacervate selective recruitment describes the capacity of the dense phase to preferentially draw in specific molecular species from the surrounding dilute phase based on their chemical affinity for the dense phase environment, effectively organizing composition without a membrane-based selectivity mechanism.

Dynamic Behavior

Dynamic Exchange

Coacervate dynamic exchange describes the continuous, unhindered movement of molecules between the dense and dilute phases, occurring more freely than the comparatively restricted exchange characteristic of membrane-bound compartments, since no discrete barrier exists to physically block such movement.

Coacervate Fusion

Coacervate fusion occurs when two separate dense-phase droplets come into contact and merge into a single, larger droplet, a behavior enabled by the liquid, boundary-free nature of the dense phase.

Coacervate Growth

Coacervate growth describes an increase in the size of a dense-phase droplet over time, occurring either through fusion with other droplets or through continued recruitment of additional macromolecular material from the dilute phase.

Coacervate Dissolution

Coacervate dissolution describes the reversal of phase separation, in which the dense phase disperses back into the surrounding dilute phase as a single homogeneous mixture, typically triggered by a change in conditions that no longer favors phase separation.

Sensitivity and Structural Limitations

Environmental Sensitivity

Coacervate environmental sensitivity describes the strong dependence of phase separation behavior on surrounding conditions such as ionic strength, pH, and temperature, meaning that a coacervate compartment's existence and stability can be readily disrupted by changes in its environment.

Membraneless Compartment Boundary Limitation

Membraneless compartment boundary limitation refers to the inherent constraint that, lacking any discrete physical barrier, a phase-separated compartment cannot achieve the same degree of controlled, selective isolation from its surroundings that a membrane-bound compartment can, since separation relies entirely on differential solubility rather than a structural gatekeeping mechanism.

Overall Assessment

Phase-Separated Synthetic Cell Suitability

Phase-separated synthetic cell suitability describes how well the properties of coacervate compartments, including their capacity for reaction concentration and selective recruitment set against their environmental sensitivity and lack of a discrete boundary, match the requirements of a given synthetic cell application relative to membrane-bound alternatives such as lipid vesicles, polymersomes, or protein-based compartments.