Editing Cell theory (section)

===Steady-state membrane pump concept ===

With the development of [[radioactive tracers]], it was shown that cells are not impermeable to {{chem|Na|+}}. This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove {{chem|Na|+}} as it permeates cells. This drove the concept that cells are in a state of [[dynamic equilibrium]], constantly using energy to maintain [[ion gradient]]s. In 1935, {{ill|Karl Lohmann (chemist)|de|Karl Lohmann (Biochemiker)|lt=Karl Lohmann}} discovered [[Adenosine triphosphate|ATP]] and its role as a source of energy for cells, so the concept of a metabolically-driven [[sodium pump]] was proposed.{{citation needed|date=September 2023}}
The success of [[Alan Lloyd Hodgkin|Hodgkin]], [[Andrew Huxley|Huxley]], and [[Bernard Katz|Katz]] in the development of the membrane theory of cellular membrane potentials, with differential equations that modeled the phenomena correctly, provided further support for the membrane pump hypothesis.{{citation needed|date=September 2023}}
 
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|date=March 2020}}