The Biological Membrane: The Phospholipid Bilayer
The formation of the biological membrane and its selective permeability to various substances is the one most important factor that led to the origination of life. However, this formation, despite its importance, is quite simple. In fact, it's simultaneous -- provided that all the essential elements are available (which were -- please refer to the section The Conditon of the Early Earth).
When phospholipids are mixed with water, they self-assemble to form films due to insolubility. Agitation then breaks the films into sphere. Even these primitive membranes have some ability to control the passage of substances between the contents of a sphere and the aqueous environment outside. Phospholipids were probably among the organic molecules that predated life on the primitive Earth, and their spontaneous assembly to form membranes was a major step toward protocells that could maintain internal environments differing from the surroundings (Campbell).
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