Cell theory
To further support his theory, Matthias Schleiden and Theodor Schwann both also studied cells of both animal and plants.
The expanded use of lenses in eyeglasses in the 13th century probably led to wider spread use of simple microscopes (magnifying glasses) with limited magnification.
Compound microscopes, which combine an objective lens with an eyepiece to view a real image achieving much higher magnification, first appeared in Europe around 1620.
In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia.
Hooke also used a simpler microscope with a single lens for examining specimens with directly transmitted light, because this allowed for a clearer image.
This came from the Latin word Cella, meaning ‘a small room’ like monks lived in, and also Cellulae, which meant the six-sided cell of a honeycomb.
Though he did not have much formal education, he was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676.
Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker.
[12] Remak published observations in 1852 on cell division, claiming Schleiden and Schawnn were incorrect about generation schemes.
[citation needed] 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.
[citation needed] Two opposing concepts developed within the context of studies on osmosis, permeability, and electrical properties of cells.
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.
Ruhland refined the mosaic theory to include pores to allow additional passage of small molecules.
[18] 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.
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.
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.
Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties.
[citation needed] 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.
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 [de] discovered ATP and its role as a source of energy for cells, so the concept of a metabolically-driven sodium pump was proposed.
[citation needed] The modern view of the plasma membrane is of a fluid lipid bilayer that has protein components embedded within it.
[citation needed] In 1962, Gilbert Ling became the champion of the bulk phase theories and proposed his association-induction hypothesis of living cells.
