Signaling agents could be physical agents like mechanical pressure, voltage, temperature, and light or chemical agents like peptides, steroids, terpenoids, etc. It may be food material or pathogen-associated patterns, or it may be oxygen or carbon dioxide levels or specially biosynthesised signaling molecules like hormones and ferromones (ektohormones). Signaling molecules vary greatly in their physio-chemical properties such as solubility (hydrophobic or hydrophillic). Some of the signaling molecules are gaseous, such as nitric oxide. Additionally, proteins on the surface of neighboring cells could also be signals.