The formation of membranes is an indispensable stage in increasing complexity, because they are able to isolate and protect macromolecules with specific functions from external influences. Their formation enables cells to be constructed.
How can a membrane form? Certain molecules have the property of being am-biphilic, that is, they have one end that is hydrophilic (polar) and the other hydrophobic (non-polar). Once immersed in liquid water the ambiphilic molecules form on the surface, the hydrophilic heads in contact with the liquid water and the hydrophobic tail pointing outwards. They thus form a monomolecular layer (Fig. 9.12). Several types of ambiphilic molecules are able to form more complex structures, such as multiple layers or spherical structures, which were without any doubt at the origin of cellular membranes (Fig. 9.13).
Laboratory experiments (Sydney Fox, 1958) have shown that polymerization of amino acids may lead to the formation of 'proteinoids', which once dissolved in hot water and then cooled, cluster together in microspheres with a double membrane. In 1985, David Deamer observed that the amino acids in the Murchison meteorite, once dissolved in water, also formed micro-bubbles with membranes. These structures therefore have the property of being able to withstand an episode of dehydration, and then be re-activated in an aqueous medium.
Life as we know it uses DNA to store genetic information and RNA to transmit this information within the cell. In addition, specific proteins, the enzymes, are necessary to catalyze reactions, which the nucleic acids cannot do. In what order did these three types of molecules appear?
Fig. 9.12 Formation of a monomolecular layer of a Lipid (After Gilmour and Sephton, 2003)
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