问题：

How do cells restrict the movement of their membra

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答案：

Cells have ways of confining particular proteins to localized areas within the bilayer, thereby creating functionally specialized regions, or membrane domains, on the surface of the cell or organelle.
Plasma membrane proteins can be tethered to structures outside the cell—for example, to molecules in the extracellular matrix or on an adjacent cell— or to relatively immobile structures inside the cell, especially to the cell cortex.
This asymmetric distribution of membrane proteins is maintained by a barrier formed along the line where the cell is sealed to adjacent epithelial cells by a so-called tight junction. At this site, specialized junctional proteins form a continuous belt around the cell where the cell contacts its neighbors, creating a seal between adjacent plasma membranes. Membrane proteins are unable to diffuse past the junction.
Additionally, cells can create barriers that restrict particular membrane components to one membrane domain. In epithelial cells that line the gut, for example, it is important that transport proteins involved in the uptake of nutrients from the gut be confined to the apical surface of the cells (which faces the gut contents) and that other transport proteins—including those involved in the export of solutes out of the epithelial cell into the tissues and bloodstream—be confined to the basal and lateral surfaces.