Epinephrine causes the enzyme glycogen phosphorylase to break down glycogen within a cell. Epinephrine performs this task on intact cells, meaning cells that have a membrane (not just cell contents placed in a medium). What does this fact reveal

The fact that it reveals is that:

Epinephrine binds to the receptor tyrosine kinase on the cell membrane and does not act on glycogen phosphorylase.

Explanation:

In cellular respiration, epinephrine is a neurotransmitter and a plasma membrane hormone receptor.  It is not a lipid-derived hormone, but rather an amino acid-derived hormone. As a result, they are unable to pass through the plasma membrane of cells. They bind to receptors on the outer surface of the plasma membrane through plasma membrane hormone receptors (receptor's tyrosine kinase of the cell membrane) because they are lipid insoluble hormones.

Unlike steroid hormones, lipid insoluble hormones (epinephrine) do not directly influence glycogen phosphorylase or the target cell because they cannot enter the cell and operate directly on DNA. The activation of a signaling pathway occurs when these hormones attach to a cell surface receptor; this activates intracellular activity and performs the hormone's specific effects. Nothing crosses through the cell membrane in this fashion; the hormone that binds at the cell's surface stays at the cell's surface, while the intracellular component stays within the cell.

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