The Influence of Environmental Conditions, Lipid Composition, and Phase Behavior on the Origin of Cell Membranes

Abstract

At some point in life's development, membranes formed, providing barriers between the environment and the interior of the 'cell.' This paper evaluates the research to date on the prebiotic origin of cell membranes and highlights possible areas of continuing study. A careful review of the literature uncovered unexpected factors that influence membrane evolution. The major stages in primitive membrane formation and the transition to contemporary cell membranes appear to require an exacting relationship between environmental conditions and amphiphile composition and phase behavior. Also, environmental and compositional requirements for individual stages are in some instances incompatible with one another, potentially stultifying the pathway to contemporary membranes. Previous studies in membrane evolution have noted the effects composition and environment have on membrane formation but the crucial dependence and interdependence on these two factors has not been emphasized. This review makes clear the need to focus future investigations away from proof-of-principle studies towards developing a better understanding of the roles that environmental factors and lipid composition and polymorphic phase behavior played in the origin and evolution of cell membranes.

Protein-mediated Selective Enclosure of Early Replicators Inside of Membranous Vesicles: First Step Towards Cell Membranes

“…It is known that phospholipids could have formed prebiotically from the same components, by heating the solution to dryness (Hargreaves and Deamer 1978;Rao et al 1982Rao et al , 1987, but this is unlikely due to the low occurrence on the starting materials on the early Earth Thomas and Rana 2007). However, simpler amphiphiles, such as alkyl phosphates, alkyl sulfates, fatty acids and other amphiphilic hydrocarbons could have formed in prebiotic planetary conditions.…”

“…They also depend on the concentration and composition of the amphiphile mixtures, so that vesicles are formed only when a critical amphiphile concentration is reached (called either critical bilayer concentration, CBC, or critical concentration for vesicle formation, CVC). The formed vesicles are then in equilibrium in the solution with single molecules and micelles (Thomas and Rana 2007;Namani and Deamer 2008). Octanoic acids-eight carbon chains-are the shortest monocarboxylic acids that can form vesicles, and this happens at the CBC of 130 mM .…”

“…Vesicle formation is also highly dependent on various environmental factors including temperature, pH, ionic concentration and presence of other amphiphiles and hydrophobic compounds Furuuchi et al 2005;Thomas and Rana 2007;Namani and Deamer 2008). Typically, the fatty acids can form vesicles at pH near to the pK value of the acid (ranging approximately from 7 to 10 for C 8 to C 18 carbon chains), and at temperatures near the melting point of the acid (ranging from 30 to 70°C for C 8 to C 18 saturated chains).…”

“…These supposedly were present in the early oceans at least in 2 mM concentrations, and if so, would have reacted with the anionic fatty acids forming insoluble soaps Deamer 2002, 2003). Also moderate NaCl concentrations disrupt pure fatty acid vesicles (Thomas and Rana 2007). However, mixtures of different hydrophobic molecules improve the formation and stability of the fatty acid membranes in different ionic and pH conditions.…”

“…Intensive experimental work has demonstrated that, indeed, such amphiphiles can assemble into vesicles, and that such primitive vesicles can enclose functional polymers, take up precursors, and even grow and divide. However, these processes can happen only in very specific environmental conditions, with strictly fixed concentrations of starting materials (Deamer 1998;Deamer et al 2002;Chen et al 2004; Thomas and Rana 2007;Namani and Deamer 2008;. Although simple membranes may have provided some kind of (transient) compartmentalisation already during the early stages of prebiotic processes, their role in molecular evolution involves some profound questions.…”