Proteins
Membranes also contain proteins. Typically 20-30% of membrane associated protein is soluble in water and loosely associated. The other 70-80% is tightly bound to the membrane, often spanning both sides. These proteins are also often amphipathic molecules (contain both hydrophobic and hydrophilic portions) with stretches of hydrophilic amino acids and stretches of hydrophobic amino acids. Most of them are placed in the membranes so that the hydrophobic amino acids associate with the lipids in the membrane and the hydrophilic amino acids are outside the membrane interacting with either the cytoplasm or the periplasm.
