por What is the membrane folding theory?
The membrane folding theory proposes that the organelle membranes of eukaryotic cells originate from the expansion and invagination of the plasma membrane.
JD Robertson, a pioneer in electron microscopy, noted in 1962 that several intracellular bodies possessed a structure visually identical to the plasma membrane.
The idea of a structure that delimits cells arose immediately after the concept of «cell» arose, so several studies were carried out to understand the characteristics of such a structure.
plasma membrane
The plasmatic membrane is a structure formed by a double layer of phospholipids organized in such a way that the polar groups are oriented towards the cytosol and the extracellular environment, while the apolar groups are organized towards the interior of the membrane.
Its main function is to define cells, both eukaryotic and prokaryotic, since it physically separates the cytoplasm from the extracellular medium.
Despite its structural function, it is well known that the membrane is not static, but rather an elastic and dynamic barrier where a large number of essential processes for the cell occur.
Some processes that occur in the membrane are the anchoring of the cytoskeleton, the transport of molecules, or the signaling and connection with other cells to form tissues. In addition, a great variety of organelles also have a membrane where other important processes occur.
Background to the membrane folding theory
Electrophysiological studies
Long before Robertson came to propose the theory of membrane folding in 1962, studies were carried out to determine what this structure was like. In the absence of the electron microscope, electrophysiological studies predominated, among which the following stand out:
1895
Overton noticed that lipids crossed the cell membrane more easily than molecules of another nature, therefore, he inferred that the membrane must be made up mostly of lipids.
1902
J. Bernstein presented his hypothesis, which mentioned that cells consisted of a solution with free ions bounded by a thin layer impermeable to these charged molecules.
1923
Fricke measured the capacity of the erythrocyte membrane to store charges (capacitance), determining this value to be 0.81 µF/cm2.
Subsequently, it was determined that the membranes of other cell types had similar capacitance values, therefore the membrane should be a unitary structure.
1925
Gorter and Grendel measured the area of mammalian erythrocytes with the aid of a microscope. They then extracted the lipids from a known number of these cell types and measured the area they occupied.
They obtained as a result a 1:2 cell:membrane ratio. This meant that the cell membrane was a double structure, thus giving rise to the term “lipid bilayer”.
1935
Studies prior to 1935 suggested the presence of proteins in the membrane, this led Danielli and Davson to propose the sandwich model, or Protein-Lipid-Protein model.
According to this model, the plasma membrane consists of two layers of phospholipids that lie between two layers of proteins, both of which are associated with the membrane through electrostatic interactions.
Electron microscopy studies
In 1959, thanks to the appearance of electron microscopy, J. David Robertson collected sufficient evidence to confirm and complement the models of Gorter and Grendel (1925) and Danielli and Davson (1935), and to propose the «Unitary Membrane» model.
This model retains the characteristic of the lipid bilayer model proposed by Danielli and Davson, with the variation of the protein layer which, in this case, is asymmetric and discontinuous.
What is the membrane folding theory about?
The advent of electron microscopy allowed us to have a fairly clear idea of how the plasma membrane was formed.
However, this fact was accompanied by the visualization of multiple intracytoplasmic membranes that formed intracellular compartments, which led Robertson in 1962 to propose the theory of membrane folding.
The theory of membrane folding is that the plasma membrane increased its surface area and invaginated to give rise to intracytoplasmic membranes, these membranes surrounded molecules that were in the cytosol and gave rise to organelles.
According to this theory, the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vacuoles could have originated in this way.
The continuity that exists between the plasmatic membrane with the first three organelles mentioned above has been confirmed by electron microscopy studies in different cell types.
However, Robertson also proposed in his theory that vesicular organelles, like lysosomes and vacuoles, also originated by invaginations that subsequently separated from the membrane.
Due to the characteristics of the membrane folding theory, it is considered an extension of the unitary membrane model that he himself proposed in 1959.
Micrographs taken by Robertson show that all of these membranes are the same and therefore should be fairly similar in composition.
However, the specialization of the organelles considerably modifies the composition of the membranes, which reduces the properties that they have in common at the biochemical and molecular level.
In the same way, the fact that membranes have the main function of serving as a stable barrier for aqueous media is maintained.
Importance of this theory
Thanks to all the tests carried out between 1895 and 1965, especially the microscopy studies carried out by JD Robertson, the importance of cell membranes was emphasized.
From his unitary model, the essential role played by the membrane in the structure and function of cells began to be highlighted, to such an extent that the study of this structure is considered a fundamental topic in current biology.
However, currently the theory as such has no acceptance.
However, at the time it gave rise to more experts in the area trying to elucidate the origin not only of cell membranes, but also the origin of the eukaryotic cell itself, as Lynn Margulis did in 1967 when proposing the endosymbiotic theory.
References
Lodish H, Berk A, Kaiser C, et al. (2008). Molecular Cell Biology.
Heuser JE (1995). In Memory of J. David Robertson. Taken from heuserlab.wustl.edu.
Lee A. (2001). Membrane structure. curr. Biol.