G proteins are a group of proteins that act as molecular switches inside cells. They play a crucial role in transmitting signals from outside the cell to the inside, helping to regulate various cellular processes. When a signal molecule, such as a hormone, binds to a receptor on the cell surface, it activates the G protein, which then triggers a cascade of reactions within the cell. These proteins are composed of three subunits: alpha, beta, and gamma. The alpha subunit binds to GDP or GTP, which determines whether the G protein is active or inactive. Once activated, G proteins can influence other proteins and enzymes, leading to changes in cell behavior, such as growth, movement, or metabolism.