Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In words, this receptor change intracellular molecules changes a response. There are two levels in this procedure: the signaling molecule activates a particular receptor protein on the cell membrane. A 2nd messenger transfers the signal into the cell and eliciting a physiological response. In either level the signal can be amplified. Whereas, one signalling molecule can cause several responses. Gene activations and metabolism alterations are instances of cellular responses to extracellular stimulation which requires signal transduction.