There are four main types of membrane proteins:-

Transporters – these help the passage of ions, sugars, amino acids, nucleotides and many other metabolites which cross the lipid bilayer too slowly by simple diffusion.

Linkers – these strengthen and support the cell membrane as part of the membrane linked cytoskeleton, give each cell its distinctive shape and are important in intercellular junctions

Receptors – These detect chemical signals in the cells environment and relay them to the cell’s interior.

Enzymes – These catalyse specific reactions

Two main classes of membrane transport proteins can be distinguished

Carrier proteins –

These bind a solute on one side of the membrane and deliver it to the other through a change in the configuration (shape) of the carrier protein. An important example of this type of protein is the sodium-potassium pump

This pump exchanges three sodium ions from the inside of the cell for two potassium ions from the outside of the cell.

This process uses energy in the form of ATP, enzymes and involves the protein changing shape

Channel proteins

These form tiny hydrophilic pores in the membrane, through which solutes can pass by diffusion. Most channel proteins let through inorganic ions only and are often called ion channels.

Both channel proteins and carrier proteins allow movement of molecules in the same direction as the concentration gradient. This is called passive or facilitated diffusion. Only carrier proteins can carry out active transport where molecules are moved against the concentration gradient. Active transport requires energy

There are six main stages to the sodium-potassium pump – see diagram

There are a large number of enzyme proteins associated with the cell membrane. These all speed up reactions.

Kinases catalyse the addition of phosphate to molecules –

Two important groups of enzymes are the kinases and phosphatases both of which help in the sodium – potassium pump

Phosphatases catalyse the removal of phosphate from molecules

Note kinase is the the same as a phosphorylase but not a phosphatase

These are intrinsic transmembrane proteins which have a variety of functions

a) Strengthening and supporting cell membrane.

These determine the shape of the cell and the mechanical properties of the PM in particular which is covered in the inner (cytosolic) surface by a meshwork of fibrous proteins called the cell cortex

b) Joining cells together into strong layers or tissues. This is known as cell adhesion.

c) Restricting the movement of membrane proteins by forming intercellular junctions e.g. gut cells are linked together in a way that prevents molecules moving between the gaps

Glycoproteins (proteins with short chains of sugar attached to them) are only found on the non cytosolic side of the membrane where they act with other molecules to form a sugar coating. This coating has many uses e.g.

• It protects the cell surface from mechanical damage

• It prevents cells from sticking together or to surrounding tissues

• Cell-cell recognition

• ‘docking’ sites for extracellular signalling molecules such as hormones from the cells surroundings. - see next lesson