During the fight-or-flight response the body needs to get energy to the muscles in the form of sugar molecules. One of the methods used to do that is called glycogenolysis. During this process, glucose subunits are removed from long branching glycogen molecules. In the movie, we show this occurring in liver cells, but it can also occur in the muscle cells themselves.
Epinephrine binds to an Alpha-1 Adrenergic Receptor* on liver cell initiating glycogenolysis.
*NOTE: The Alpha-1 adrenergic receptor also causes contraction of the arrector pili muscles raising hairs on the skin's surface (seen later in the movie) when epinephrine binds to it.
When it binds, epinephrine stimulates the receptor to change shape. This conformational change causes the G protein complex to become activated and uncoupled.
IP3 binds to receptors on the surface of the endoplasmic reticulum (ER). The ER is stimulated to release calcium ions.
When activated by the G protein, Phospholipase-C begins adding water molecules (called hydrolyis) onto PIP2 embedded in the cell membrane (not shown) forming the IP3 signaling molecule.
Interestingly, PIP2 is a phospholipid and is present in the cell's lipid bilayer. Cell membranes contain a variety of different classes of lipids and phospholipids. Often, they are used for other cellular functions besides just acting to form a barrier.
The calcium ions interact with phosphorylase kinase which is decorated with glycogen phosphorylase When the calcium activates the phosphorylase kinase (via calmodulim subunits, not shown).
Individual glucose subunits are broken off of a glycogen molecule by activated glycogen phosphorylase. The glucose is passively transported (GLUT2 transporter not shown) across the membrane and into the bloodstream.