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The membrane that surrounds the cell is a remarkable structure. It is made up of lipids and proteins and is semipermeable, allowing some substances to pass through it and excluding others. However, it's permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. The nucleus and other organelles in the cell are bound by similar membranous structures.
The lipid bilayer is not miscible with either the extracellular fluid or the intracellular fluid. Therefore it constitutes a barrier against movement of water molecules and water- soluble substances between the extracellular and intracellular fluid compartments.
