CASE 1 The First Cell: Life’s Origins

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What are the consequences of the ability of phospholipids to form a bilayer when placed in water? The bilayer structure forms spontaneously, dependent solely on the properties of the phospholipid and without the action of an enzyme, as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell. The pH is important because it ensures that the head groups are in their ionized (charged) form and thus suitably hydrophilic. Thus, if phospholipids are added to a test tube of water at neutral pH, they spontaneously form spherical bilayer structures called liposomes that surround a central space (Fig. 5.3c). As the liposomes form, they may capture macromolecules present in solution.

Such a process may have been at work in the early evolution of life on Earth. Experiments show that liposomes can form, break, and re-form in environments like tidal flats that are repeatedly dried and flooded with water. The liposomes can even grow, incorporating more and more lipids from the environment, and capture nucleic acids and other molecules in their interiors. Depending on their chemical composition, early membranes might have been either leaky or almost impervious to the molecules of life. Over time, they evolved in such a way as to allow at least limited molecular traffic between the environment and cell interior. At some point, new lipids no longer had to be incorporated from the environment. Instead, proteins guided lipid synthesis within the cell, although how this switch to protein-mediated synthesis happened remains uncertain.

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All evidence suggests that membranes formed originally by straightforward physical processes, but that their composition and function evolved over time. François Jacob once said that evolution works more like a tinkerer than an engineer, modifying already existing materials rather than designing systems from scratch. It seems that the evolution of membranes is no exception to this pattern.