Final answer:
Growth factors are external signals that bind to receptor tyrosine kinases (RTKs) on the cell surface, initiating cell signaling pathways such as the MAP kinase pathway, which promote cell division.
Step-by-step explanation:
Growth factors are external signals received at a cell's plasma membrane that promote cell division. These molecules are crucial for the regulation of the cell cycle by stimulating cellular responses such as division and growth. Growth factors typically bind to receptors on the cell surface, specifically to receptor tyrosine kinases (RTKs), initiating a cascade of signaling pathways. A well-known pathway involves the activation of RTKs leading to the stimulation of a G-protein called RAS, which in turn activates the MAP kinase pathway. The MAP kinase then drives the expression of proteins necessary for the cell to progress through the cell division cycle.
External factors such as the availability of growth factors, hormones like human growth hormone (HGH), and cellular interactions with the extracellular matrix influence whether a cell will enter the cell cycle and divide. The absence of these signals can inhibit cell division, which can result in conditions like dwarfism, while an excess can lead to gigantism. Conversely, overcrowding of cells tends to inhibit cell division as a regulation mechanism. Therefore, the balance and regulation of these signals are paramount for normal growth and development.