In this section learn that a signaling pathway begins with the arrival of a chemical signal - such as a hormone or growth factor - at the cell surface.

A signaling pathway begins with the arrival of a chemical signal – such as a hormone or growth factor – at the cell surface. The gray structures sticking out of the cell membrane are receptors for these incoming signals. The signal, in this case a platelet-derived growth factor (here in purple and blue), encounters and binds to its matching receptor. A second receptor protein joins in, making the growth factor fit like a key in a lock. The binding of the growth factor causes the receptors to change shape. This change in the protein will be conducted through the membrane and into the cell's interior – the cytoplasm. Molecules identified: Platelet derived growth factor (PDGF): PDGF stimulates growth and division in cells such as fibroblasts and smooth muscle cells. PDGF is important in tissue repair, activating cells of the immune system and synthesizing components of the extracellular matrix. Some cancer cells produce their own PDGF, which releases them from their dependency on growth signals from other cells. Platelet derived growth factor (PDGF) receptors: PDGF receptors are large membrane-spanning proteins with an extracellular and an intracellular component. Two PDGF receptor proteins "dimerize" to bind a single platelet-derived growth factor. The cytoplasmic portion has kinase activity – able to add phosphate molecules to other molecules to activate them. This receptor can contribute to cancer if rendered active for an extended period of time.

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