The earliest problem faced by living cells was an unavoidable trend to swelling and lysis, as extracellular salts and water leaked through a semipermeable plasma membrane enclosing impermeant macromolecules. Two solutions countered this ever-present threat—a rigid cell wall to resist expansion and ion pumps to offset the passive influx with an active efflux. On the premise that ion pumps were the first solution, we have constructed an evolutionary sequence to account for the diversity of ion pumps found today.
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Maloney et al. (1985) studied this question.