Solute transport at the plasmalemma and the early evolution of cells

The evolution of the plasmalemma and its porter systems is considered in relation to selective pressures on primitive cells. Initially the polar lipid bilayer acted to separate the genetic apparatus of the protocell from the rest of the world. The requirement for the supply of nutrients and removal of waste products resulted in the evolution of passive uniporters for a number of organic and inorganic solutes. There was also a requirement for primary active transport, whereby one or more solutes is transported across the membrane contrary to the direction predicted from passive driving forces, with an energy input from light, redox reactions, “high-energy phosphate” or some other metabolic process. Active transport is discussed in terms of cytoplasmic pH regulation, cytoplasmic volume regulation, Ca²⁺ exclusion/phosphate accumulation, and the accumulation of organic (heterotrophic) substrates.It is suggested that volume regulation in wall-less cells was initially achieved by Na⁺ exclusion with active Na⁺ extrusion as a later refinement; the same applies to the maintenance of the characteristically low free Ca²⁺ level in the cytoplasm. A requirement for active phosphate influx is also likely in view of the high concentrations of orthophosphate required for phosphorylation reactions relative to the likely external concentration of phosphate and the inside-negative potential difference. This p.d., which results inter alia from Na⁺ extrusion, makes the maintenance of intracellular pH via passive H⁺ fluxes very difficult in the face of continued intracellular production of H⁺ during fermentation. Hence an early role for primary active extrusion (uniport) of H⁺ is very likely. Such uniport is of universal occurrence in present-day cells. Besides its role in pH regulation and in energy-coupling, H⁺ transport energises secondary (H⁺-linked) transport of many other solutes. We suggest that transport of HCO₃^? might also have a pH-regulating role, but apparently HCO₃^? cannot substitute for H⁺ with respect to energy-coupling and secondary active transport.