One universal feature of all cells is an outer limiting membrane called the plasma membrane.
In addition, all eukaryotic cells contain elaborate systems of internal membranes which set up various membrane-enclosed compartments within the cell.
Cell membranes are built from lipids and proteins.
The plasma membrane serves as the interface between the machinery in the interior of the cell and the extracellular fluid (ECF) that bathes all cells.
The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other. Substantial amounts of cholesterol are tucked within the hydrocarbon tails (not shown).
In all these cases, the portion within the lipid bilayer consists primarily of hydrophobic amino acids. These are usually arranged in an alpha helix so that the polar -C=O and -NH groups at the peptide bonds can interact with each other rather than with their hydrophobic surroundings.
Those portions of the polypeptide that project out from the bilayer tend to have a high percentage of hydrophilic amino acids. Furthermore, those that project into the aqueous surroundings of the cell are usually glycoproteins, with many hydrophilic sugar residues attached to the part of the polypeptide exposed at the surface of the cell.Some transmembrane proteins that span the bilayer several times form a hydrophilic channel through which certain ions and molecules can enter (or leave) the cell. [Example]