Lecture 22 signal transduction 2- g proteins/lipid derived signaling molecules/second messengers. G proteins are called that because they bind to guanine nucleotides, either to gtp or gdp. We will also talk about lipid signalling and second messengers. There are a lot more acronyms in this lecture so there"s a lot of memorization needed. What you see is the generalized signaling pathway. At the top is a signaling cell or an external signal that the cell finds itself in the presence of. This extracellular signaling molecule is often referred to as a first messenger or as the. Regardless of what it is, either hormone or small molecule or ion, this takes place at the cell surface. There are a couple of different types of receptors that exist. Keep in mind that a cell can only perceive a signal and it contains a receptor for that specific type of signal.