Plasma membranes in cells are basically phospholipid bilayers with proteins embedded in them. These phospholipid layers are liquid films, like the surface of soap bubbles, so plasma membranes are able to flow and re-form like a fluid. They function as effective barriers because water-soluble molecules have trouble passing through the fat-soluble interior between the two layers. (Fat-soluble molecules are generally stored in vesicles and not freely floating in the cell’s cytoplasm.)

Proteins embedded in the phospholipid bilayer enable the plasma membrane to regulate the flow of molecules through the membrane. These proteins are able to sit within the plasma membrane because they are assembled from a combination of both polar and nonpolar amino acids.

In addition to enclosing the cell and regulating the flow of molecules into and out of the cell, plasma membranes can also be used to enclose organelles and regulate the flow of molecules into and out of those organelles.

When a cell builds more phospholipid molecules, those molecules can be used to either grow the size of the cell, increasing its surface area and volume, or to build an organelle within the cell.