Editing Cell theory (section)

=== Membrane and bulk phase theories ===

Two opposing concepts developed within the context of studies on [[osmosis]], permeability, and electrical properties of cells.<ref>{{cite book|last=Ling|first=Gilbert N.|title=In search of the physical basis of life|date=1984|publisher=Plenum Press|location=New York|isbn=0306414090}}</ref> The first held that these properties all belonged to the plasma membrane whereas the other predominant view was that the [[protoplasm]] was responsible for these properties. The [[History of cell membrane theory|membrane theory]] developed as a succession of ad-hoc additions and changes to the theory to overcome experimental hurdles. Overton (a distant cousin of [[Charles Darwin]]) first proposed the concept of a lipid (oil) plasma membrane in 1899. The major weakness of the [[lipid membrane]] was the lack of an explanation of the high permeability to water, so Nathansohn (1904) proposed the mosaic theory. In this view, the membrane is not a pure lipid layer, but a mosaic of areas with lipid and areas with semipermeable gel. Ruhland refined the mosaic theory to include pores to allow additional passage of small molecules. Since membranes are generally less permeable to [[anions]], [[Leonor Michaelis]] concluded that [[ions]] are [[adsorbed]] to the walls of the pores, changing the permeability of the pores to ions by [[electrostatic repulsion]]. Michaelis demonstrated the [[membrane potential]] (1926) and proposed that it was related to the distribution of ions across the membrane.<ref>{{Cite journal|last1=Michaelis|first1=L.|title=Contribution to the Theory of Permeability of Membranes for Electrolytes|journal=The Journal of General Physiology|volume=8|issue=2 |pages=33–59|year=1925|pmid=19872189|pmc=2140746|doi=10.1085/jgp.8.2.33}}</ref>

Harvey and Danielli (1939) proposed a [[lipid bilayer]] membrane covered on each side with a layer of protein to account for measurements of surface tension. In 1941 Boyle and Conway showed that the membrane of frog muscle was permeable to both {{chem|K|+}} and {{chem|Cl|-}}, but apparently not to {{chem|Na|+}}, so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to {{chem|K|+}}, {{chem|H|+}}, and {{chem|Cl|-}} as well as the impermeability to {{chem|Na|+}}, {{chem|Ca|+}}, and {{chem|Mg|2+}}.
Over the same time period, it was shown (Procter and Wilson, 1916) that gels, which do not have a semipermeable membrane, would swell in dilute solutions.{{citation needed|date=September 2023}}

[[Jacques Loeb]] (1920) also studied [[gelatin]] extensively, with and without a membrane, showing that more of the properties attributed to the plasma membrane could be duplicated in [[gels]] without a membrane. In particular, he found that an electrical potential difference between the gelatin and the outside medium could be developed, based on the {{chem|H|+}} concentration. Some criticisms of the membrane theory developed in the 1930s, based on observations such as the ability of some cells to swell and increase their surface area by a factor of 1000. A lipid layer cannot stretch to that extent without becoming a patchwork (thereby losing its barrier properties). Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties.{{citation needed|date=September 2023}}

In 1938, Fischer and Suer proposed that water in the protoplasm is not free but in a chemically combined form—the protoplasm represents a combination of protein, salt and water—and demonstrated the basic similarity between swelling in living tissues and the swelling of gelatin and [[fibrin]] gels. Dimitri Nasonov (1944) viewed proteins as the central components responsible for many properties of the cell, including electrical properties.
By the 1940s, the bulk phase theories were not as well developed as the membrane theories. In 1941, Brooks and Brooks published a monograph, "The Permeability of Living Cells", which rejects the bulk phase theories.{{citation needed|date=September 2023}}