Cell signaling involves chemical signals binding to cell surface receptors and activating intracellular signal transduction pathways. There are two main ways signals work: by changing the activity of existing proteins or by changing the amount of proteins in the cell. Common types of signals include hormones, growth factors, and neurotransmitters. Signal transduction involves second messengers such as cAMP and IP3 that activate downstream effector proteins like protein kinases. G-protein coupled receptors are a major class of receptors that activate intracellular signaling pathways.