Since the hydrophobic core is impermeable to many molecules, the presence of membrane-proteins helps to mediate the transport of specific molecules, ions and water across.

2. There are three forms in which a protein can link to a membrane:

  • Integral-membrane (Transmembrane) protein; is located in between the lipid bilayer (which is why it is called transmembrane). It has a region that faces inside of a cell called the cytoplasmic face or domain and a region facing outside of the cell called exoplasmic face or domain. The part of the protein that is in between the lipid membrane is called the transmembrane domain. Integral membrane proteins are also called Integrins for short. The transmembrane domain, unlike the other two domains, contain many hydrophobic amino acids whose side-chains interact with the hydrocarbon core of the phospholipid bilayer and form channels and cores that allow molecules to move into-and-out of the cell or organelle. A third title give to integrins because of this is Channel Proteins.

Three types of membrane proteins. Click on image for credit.

  • Peripheral membrane protein; partially penetrates the lipid bilayer (one leaflet) and sometimes even interacts with the integrin protein and can either be found on the cytoplasmic domain or the exoplasmic domain. It usually functions either as a receptor of signal molecules or as an enzyme.
  • Lipid anchored membranes; are covalently bound to one or more lipid molecules on the biomembrane surface. The hydrophobic carbon chain of the lipid, more specifically the leaflet, is responsible for the attachment and so it does not penetrate the phospholipid bilayer. It also functions similar to peripheral proteins.