A phospholipid, or phosphatide, is a member of a large class of fat-like organic compounds that in their molecular structure resemble the triglycerides, except for the replacement of a fatty acid with a phosphate-containing polar group. The polar end of the molecule is soluble in water (hydrophilic) and water solutions (including cytoplasm); the other, fatty-acid end is soluble in fats (hydrophobic). In a watery environment phospholipids naturally combine to form a two-layer structure, lipid bilayer, with the fat-soluble ends sandwiched in the middle and the water-soluble ends sticking out. Such lipid bilayers are the structural basis of cell membranes. Phospholipids are the principal components of the myelin sheaths of neurons. Examples of phospholipids include lecithin, cephalins, phosphoinositides (in the brain), and cardiolipin (in the heart).