Structure and function of cell membrane proteins

Membrane proteins are of two kinds, and the distinction is how firmly they are attached.

Integral, or transmembrane, proteins sit in the phospholipid bilayer and are permanently attached. Six functional varieties are named: channels, carriers, pumps, receptors, enzymes and linkers.

Peripheral membrane proteins sit on the surface, mainly outside, are temporarily attached, and interact with the cytoskeletal elements to influence cell shape and movement.

The distinction between a carrier and a pump is the one to be sure of: a carrier translocates by facilitated diffusion, a pump transports actively and is engaged in active transport.

An integral protein, also called a transmembrane protein, is a protein in the phospholipid bilayer that is permanently attached to the membrane.

There are six functional varieties.

Channels form transmembrane pores, which allow ions to cross the membrane in either direction.

Carriers bind small molecules and translocate them across the membrane. They come in three arrangements:

• uniport, facilitated diffusion;
• symport, also called cotransport, carriers;
• antiport, exchange diffusion.

The names describe how many things move and in which direction: one substance alone, two together the same way, or two in opposite directions.

Pumps transport certain ions, such as Na⁺, actively across the membrane. They are engaged in active transport.

A pump is not a fast carrier. The difference is energy: a carrier lets a molecule go where it was already going, and a pump makes it go where it would not.

Receptors recognise and respond to chemical signals.

Enzymes help chemical reactions to occur.

Linkers attach the intracellular cytoskeleton with the extracellular matrix.

Peripheral membrane proteins are proteins on the surface, mainly outside.

• They are temporarily attached.
• They interact with the cytoskeletal elements to influence shape and cell movement.