A "classical" membrane is a planar layer of a solid or liquid substance insoluble in two adjacent phases (in the case of molecular thickness this "solid or liquid" cannot be determined), a layer of a substance, separating these two phases from each other. In the case of a protein membrane this does not apply to the vital-primary membrane, which -- already because it is intramolecular -- is soluble in both boundary phases. But as a result of adsorptional insertion [or deposition], often connected with a coagulation or full-denaturation, this primary membrane transforms into the insoluble insertion membrane. Basically, the layer may consist of a single phase, in the case of the living condition even consisting of just a part of a molecule.
One may recognize the spontaneous formation of plasmatic skinlets [membranes] when one squeezes suitable plasm objects beneath the microscopic cover glass in a dye solution. In this, dye may be enclosed into vacuoles, but the true inside never becomes dyed. Not taking into account organ-like skin formations, as also the strengthening, stiffening, and sectioning by plasmatic skinlets, and "improvements" by preferred permeability-regulating adsoption of certain substances from the cell content, -- no special membrane construction materials, being present in the protoplasm, are needed, the denaturation membrane forms as automatically as does the surface-adsorption-layer in a liquid. Skin formation as a result of surface adsorption is in physiology known under the name of haptogene-membrane. About the further functional use of this spontaneous membrane nothing is said in all this.