Cell signalling molecules
Cell signalling molecules transmit biological information between cells of multicellular organisms. Although all these molecules act as ligands that bind to receptors expressed by their target cells, there is considerable variation in the structure and function of the different types of molecules that serve as signal transmitters.
Cell signalling molecules can carry signals over long distances, whereas others act locally to convey information between neighbouring cells. In addition, signalling molecules differ in their mode of action on their target cells.
The different pathways that cell siganlling molecules can travel are:
Signalling molecules can be hydrophobic and can cross the hydrophobic region of the phospholipid bilayer of the cell membrane and bind to intracellular receptors in the cytoplasm or nucleus. Steroid hormones and NO gas are examples of hydrophobic signalling molecules.
However, most signalling molecules are hydrophillic and therefore cannot pass through the cell membrane. Instead, they bind to specific receptors expressed on their respective target cell surface membrane.