Transport Proteins are involved in the transportation of particles ranging from electrons to macromolecules. Messengers Proteins are involved in the transmission of nervous impulses, by acting as receptors of small molecules which cross junctions separating nerve cells. Within an organism, biological processes must be coordinated between cells in tissues and indeed between different organs. Proteins also act as hormone receptors. Enzymes synthesize proteins by translating sequences of DNA, and this production can be promoted or repressed by other proteins, in complex feedback mechanisms.
Some proteins have a structural role, providing mechanical support.
On a larger scale, muscle contraction depends on the action of large protein assemblies. Other organic material such as hair and bone are also based on protein. Collagen is found in all multicellular animals, occurring in almost every tissue. The monomers are amino acids, and there are 20 types, which themselves have a range of chemical properties. There is therefore a great diversity of possible protein sequences.