Molecular signals received by cells that have receptors turned to specific signals. When signal molecule binds receptor cellular response: changes in gene expression protein synthesis, transport of molecules across pm, metabolic reactions, secretion, movements, cell division, apoptosis (programmed cell death) Direct channels of communication: gap junctions (animal cells) spread of electrical signals between cells in cardiac muscle, plasmodesmata (plant cells) Specific contact between cells: cell adhesion molecules integral membrane proteins that bind clls to one another or to extracellular matrix. Not every cell in organism is able to respond to every single signal that it could encounter. Different cells are able to respond to different signals, depend on what the receptors that are being produced. Examples of stimulation of glycogen breakdown in liver cells by epinephrine. Secretion of hormone epinephrine by adrenal gland leads to increase in blood glucose: hormone is first messenger, leads to formation of second messenger, leads to activation of enzyme for glycogen to glucose conversion.