<P> The cell was first discovered by Robert Hooke in 1665 using a microscope . The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s . In this theory the internal contents of cells were called protoplasm and described as a jelly - like substance, sometimes called living jelly . At about the same time, colloidal chemistry began its development, and the concepts of bound water emerged . A colloid being something between a solution and a suspension, where Brownian motion is sufficient to prevent sedimentation . The idea of a semipermeable membrane, a barrier that is permeable to solvent but impermeable to solute molecules was developed at about the same time . The term osmosis originated in 1827 and its importance to physiological phenomena realized, but it wasn't until 1877, when the botanist Pfeffer proposed the membrane theory of cell physiology . In this view, the cell was seen to be enclosed by a thin surface, the plasma membrane, and cell water and solutes such as a potassium ion existed in a physical state like that of a dilute solution . In 1889 Hamburger used hemolysis of erythrocytes to determine the permeability of various solutes . By measuring the time required for the cells to swell past their elastic limit, the rate at which solutes entered the cells could be estimated by the accompanying change in cell volume . He also found that there was an apparent nonsolvent volume of about 50% in red blood cells and later showed that this includes water of hydration in addition to the protein and other nonsolvent components of the cells . </P> <P> Two opposing concepts developed within the context of studies on osmosis, permeability, and electrical properties of cells . 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 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 . 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 & Conway showed that the membrane of frog muscle was permeable to both K and Cl, but apparently not to Na, so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to K, H, and Cl as well as the impermeability to Na, Ca, and Mg . Over the same time period, it was shown (Procter & Wilson, 1916) that gels, which do not have a semipermeable membrane, would swell in dilute solutions . 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 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 . 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 & Brooks published a monograph, "The Permeability of Living Cells", which rejects the bulk phase theories . </P> <P> With the development of radioactive tracers, it was shown that cells are not impermeable to Na . This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove Na as it permeates cells . This drove the concept that cells are in a state of dynamic equilibrium, constantly using energy to maintain ion gradients . In 1935, Karl Lohmann discovered ATP and its role as a source of energy for cells, so the concept of a metabolically - driven sodium pump was proposed . The tremendous success of Hodgkin, Huxley, and Katz in the development of the membrane theory of cellular membrane potentials, with differential equations that modeled the phenomena correctly, provided even more support for the membrane pump hypothesis . </P> <P> The modern view of the plasma membrane is of a fluid lipid bilayer that has protein components embedded within it . The structure of the membrane is now known in great detail, including 3D models of many of the hundreds of different proteins that are bound to the membrane . These major developments in cell physiology placed the membrane theory in a position of dominance and stimulated the imagination of most physiologists, who now apparently accept the theory as fact--there are, however, a few dissenters. (citation needed) </P>

Which is a component of the modern cell theory