A G-protein is a protein that operates inside the cytoplasm of a cell and takes it's direction from a G-protein receptor, which is located in the cell membrane. Specific substances bind to the G-protein receptor, the receptor sends signals to the G-protein, and the G-protein activates other substances and organelles within the cell. The G-protein is similar to the ligand protein in that it stimulates specific responses for activities both within the cell and outside the cell, but differs in that it is specific to certain particles, where the ligand protein is broader based. The G-protein is very important in helping the cell maintain and carry out it's life functions and processes as it helps regulate the traffic across the cell membrane. The cell membrane is considered the gate-keeper of the cell, allowing some substances to enter, but denying others.