Fluid membranes can be described using physical laws. The membrane has some tension which resists the formation of invaginations of finite area. The area of each invagination must be removed from the rest of the cell membrane under tension in order to form a bud. The membrane has a rigidity which parameterizes the stiffness of the membrane. Patches (rafts) on the membrane which have a distinct chemical composition also experience a line tension that acts at the interface of the patch with the rest of the membrane. This line tension acts to minimize the length of the contact line and tends either to make the domain circular or to form a bud, with a much reduced contact region near the neck of the bud. Finally, membrane inclusions such as caveolin that are distributed asymmetrically in the membrane can always be expected to make the membrane curve. It is argued that the predominantly random coil of the N-terminal section of caveolin acts to bend the membrane away from it and hence favors the formation of endo, rather than exo, buds. These physical ingredients can be combined in simple theories to understand when, and if, caveolae-like invaginations should be stable and, if so, at what length scales. The characteristic scales that arise from comparing surface tension and line tension with rigidity both are in the 100 nm range, and thereby provide an early hint that a balance between the various physical effects listed above could be expected to give rise to invaginations similar to caveolae. We describe how this can be analyzed in more detail, leading to a theory for the stability of the buds under variation of surface tension. We find that buds of a single characteristic size stabilized by line tension can act as mechanical tension regulators and how these could very effectively buffer the cell's tension in the physiological range.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.