Draw a portion of a plasma membrane of a cell and label the cytoplasmic and extracellular surface. Place the following four proteins in the membrane: a) peripheral membrane protein (cytoplasmic exposure), b) integral protein (transmembrane exposure), c) integral protein (extracellular exposure) and d) peripheral protein (extracellular exposure)

Integral proteins: Proteins that are permanently encrusted in the membrane. These molecules might be classified into three groups: transmembrane proteins, those encrusted in one of the monolayers, and those that only join a membrane protein.

Transmembrane proteins cross the membrane from side to side. They have three domains in their monoacidic sequences: an extracellular hydrophilic domain, an intracellular hydrophilic domain, and an interior hydrophobic domain. The aminoacidic chain of these molecules might cross the membrane just once or many times, depending on the protein. Transmembrane proteins accomplish many functions, such as substance adhesion, ions interchange, molecules transport, communication, among others.

Proteins encrusted in one of the monolayers do not cross the membrane. Their monoacidic sequences are integrated with only one of the monolayer lipids, hence they have an intramembrane domain and only one extramembrane domain, which might be either cytosolic or extracellular, depending on their position in the membrane.

Peripheral membrane protein: Proteins that are not encrusted in the membrane, but just united to the internal or external membrane surfaces, and they can easily separate from it. Opposite to integral proteins, peripheral ones are not in a permanent association with the membrane. Instead, they are just temporally united to the membrane. This makes them useful for cellular communication. They can be attached to integral proteins and little portions of the lipidic bilayer.