Which process is an example of cell signalling?
The active transport of hydrogen ions (H+H^+H+) out of companion cells into the cell wall of source tissues
The binding of glucagon to complementary glycoprotein receptors on the cell surface membrane of a liver cell
The facilitated diffusion of glucose molecules through GLUT4 transporter proteins into a skeletal muscle cell
The mass flow of water and inorganic ions through the dead, lignified vessels of the xylem
