Early Barrier Theories
Since the invention of the microscope in the seventeenth century it has been known that plant and animal tissue is composed of cells. The plant cell wall was easily visible even with these early microscopes but no similar barrier was visible on animal cells, though it stood to reason that one must exist. By the mid 19th century, this question was being actively investigated and Moritz Traube noted that this outer layer must be semipermeable to allow transport of ions. Traube had no direct evidence for the composition of this film, though, and incorrectly asserted that it was formed by an interfacial reaction of the cell protoplasm with the extracellular fluid.
The lipid nature of the cell membrane was first correctly intuited by Quincke, who noted that a cell generally forms a spherical shape in water and, when broken in half, forms two smaller spheres. The only other known material to exhibit this behavior was oil. He also noted that a thin film of oil behaves as a semipermeable membrane, precisely as predicted. Based on these observations, Quincke asserted that the cell membrane comprised a fluid layer of fat less than 100 nm thick. This theory was further extended by evidence from the study of anesthetics. Hans Meyer and Ernest Overton independently noticed that the chemicals which act as general anesthetics are also those soluble in both water and oil. They interpreted this as meaning that to pass the cell membrane a molecule must be at least sparingly soluble in oil, their “lipoid theory of narcosis.” Based on this evidence and further experiments, they concluded that the cell membrane might be made of lecithin (phosphatidylcholine) and cholesterol. Interestingly, one of the early criticisms of this theory was that it included no mechanism for energy-dependent selective transport. This “flaw” would remain unanswered for nearly half a century until the discovery that specialized molecules called integral membrane proteins can act as ion pumps.
Read more about this topic: History Of Cell Membrane Theory
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