The hydrophobic portions stick together, as do the hydrophilic. A very stable form of this arrangement is the lipid bilayer. This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic. Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment. These are known as membrane vesicles and are used to study membrane properties experimentally.
Some proteins span the membrane while others are found on the outside or the inside.
Many of the membrane spanning proteins are involved in transport or energy generation. The cytoplasmic membrane is stabilized by hydrophobic interactions between neighboring lipids and by hydrogen bonds between neighboring lipids. Hydrogen bonds can also form between membrane proteins and lipids. Some proteins may move within the plane of the membrane while others are anchored to structures in or near the membrane. The result is that the membrane is actually fluid and has the consistency of a light grade oil.