The cytoplasm of cells is bounded by a membrane referred to as the cell membrane or plasma membrane. The fluid-mosaic model for the structure of the cell membrane holds that the membrane consists of a protein-embedded bilayer of phospholipids, where the lipids' hydrophobic tails are pointed toward the center of the membrane and their hydrophilic heads toward the surfaces of the membrane. The proteins embedded within the lipid bilayer can exist in several different positions: extending through both layers, projecting out one side or the other, or totally contained within the bilayer. Those proteins that extend entirely through the lipid layers often form pores, allowing substances to pass through the membrane; the amino acid structure of the proteins gives the pores selectivity in determining what substances can pass through.
In addition, the fluid-mosaic model holds that the structure of the membrane is not static. But, whereas lipid molecules, which are linked together only by weak bonds, can move freely within their layers, proteins have much less freedom of movement, and those whose functions are linked to their position on the cell membrane (such as enzymes that serve as receptor sites) cannot move at all.
Besides giving a cell shape and strength, the cell membrane is differentially permeable, allowing some materials to pass through while preventing others from doing so. With regard to molecules, the cell membrane is very permeable to water, certain lipid-soluble substances, amino acids, and sugars, but relatively impermeable
