specifically bind to the “locks” that are membrane proteins, changing something about the cell to allow metabolic changes to occur within the cell. One interesting protein in the cell membrane is the membrane-associated G-proteincoupled protein, also referred to as G-protein-coupled receptors (GPCRs). These will attach to many things outside of the cell and, when attached to, they change the interior of the cell. It is a single globular protein imbedded into the cell membrane, having several segments. There is an alpha protein subunit that binds to GTP (guanine triphosphate) or GDP (guanine diphosphate), which are energy molecules like ATP. The active form of GPCRs binds to GTP and uses its energy to drive intracellular processes. There are about 1000 different GPCRs in animal cells that bind to things like drugs and other molecules, using the energy to allow signals in the cell to deeply enter the cell in order to affect a change within the cell. These are important receptors that drive many intracellular processes.
ORGANELLES The term, organelles, involves those structures within the cell that serve specific functions. Many are membrane-bound and act to help the cell perform its metabolic processes. Others house cell structures, creating an internal environment that allows for specific things being done in their interior. Organelles are not freely floating within the cell but are held in relative space by the cytoskeleton or “cell skeleton”. Figure 15 shows the internal structures of the cell:
66
