Membrane composition and thermodynamic identity as boundaries of life for synthetic cell research.

Biological membranes are more than structural boundaries: they are the thermodynamic interfaces that enable living systems to maintain a state far from equilibrium, continuously exchanging matter and energy with their environment. In this review, we explore three interconnected themes. First, we examine the biochemical composition of cell membranes, with particular focus on Escherichia coli, including lipid diversity, membrane asymmetry, and homeoviscous adaptation. Second, we reframe the membrane as the physical representation of the Markov blanket, the statistical boundary separating internal from external states, and discuss how biological systems minimize free energy to maintain homeostasis. Third, we evaluate synthetic cell research strategies for reconstructing life-like systems, highlighting how out-of-equilibrium approaches, based on selective membrane transport coupled to metabolic networks, may recapitulate the thermodynamic identity of living cells.