What holds a cell membrane together from falling apart ?
I can't definitively answer your question; I'm not much of a biochemist. But I can say that whilst a phospholipid micelle is a useful conceptual analogy, a plasma membrane is a lot more complex than a simple phospholipid micelle. If you take into consideration the following, perhaps you can appreciate that the PM will not behave like a simple micelle.
Taking animal cells as an example, there are five major phospholipids that comprise the PM – phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. Distribution of the different lipids varies between the outer and inner leaflet.
In addition to the phospholipids, the PM contains glycolipids and cholesterol. Glycolipids are a minor component whereas cholesterol is a major membrane constituent of animal cells, being present in about the same molar amounts as the phospholipids.
Of course, there are many proteins in a PM, the so-called ‘peripheral’ and ‘integral’ membrane proteins. Lipids are the fundamental structural elements whereas proteins are responsible for carrying out specific membrane functions. Most plasma membranes consist of approximately 50% lipid and 50% protein by weight, with the carbohydrate portions of glycolipids and glycoproteins constituting 5 to 10% of the membrane mass.
Many of the peripheral membrane proteins are components of the cortical cytoskeleton, which underlies the plasma membrane and determines cell shape and contributes to cell function, eg. spectrin, actin, ankyrin. Ankyrin serves as the principal link between the plasma membrane and the cytoskeleton. There are a number of linkages between cytoskeleton and PM.
In contrast to transmembrane proteins, a variety of proteins (many of which behave as integral membrane proteins) are anchored in the plasma membrane by covalently attached lipids or glycolipids, eg. glycosylphosphatidylinositol (GPI) anchors. GPI anchors are added to certain proteins that have been transferred into the endoplasmic reticulum and are anchored in the membrane by a C-terminal transmembrane region.
Other proteins are anchored in the inner leaflet of the plasma membrane by covalently attached lipids. Rather than being processed through the secretory pathway, these proteins are synthesized on free cytosolic ribosomes and then modified by the addition of lipids.
Info taken from various sections of:
The Cell: A Molecular Approach (2nd ed)
Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.