The plasma membrane in Eukaryotes separates the cell's cytoplasm from the surrounding extracellular environment. The plasma membrane is a lipid bilayer, a double sheet of phospholipids.
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Phospholipid are remarkable for being hydrophilic (blue head) and hydrophobic (yellow tail). The hydrophobic tail is formed by long carbohydrate chains of lipid acids while the hydrophilic head bears a phosphate whose polarity attracts other polar molecules, most abundantly water. At the other end, the hydrophobic sites tend to congregate thereby forming stable structures impermeable for polar molecules.
A similar membrane insulates compartments within a cell, organelles, and there is a perpetual exchange between these membranes with endocytosis and exocytosis describing the route from and to the cell membrane.
