Adrenaline and the Second Messenger Model

Adrenaline is a hormone made from an amino acid. It is not lipid-soluble, so it cannot cross the cell-surface membrane. Instead it binds to specific receptors on the membrane of its target cells, and a molecule inside the cell carries the signal on. This is the second messenger model.In a liver cell:
1. Adrenaline, the first messenger, binds to a specific adrenaline receptor on the cell-surface membrane. The receptor has a shape complementary to adrenaline.
2. Binding activates a G protein in the membrane, which in turn activates the enzyme adenyl cyclase.
3. Activated adenyl cyclase converts ATP into cyclic AMP (cAMP).
4. cAMP is the second messenger. It activates the enzyme protein kinase A.
5. Protein kinase A starts a cascade of enzyme-controlled reactions that activates the enzymes breaking glycogen down into glucose (glycogenolysis).
6. Glucose diffuses out of the liver cell into the blood, raising blood glucose for respiration in the muscles.The cascade amplifies the signal: each activated enzyme activates many molecules of the next, so a few adrenaline molecules release a very large number of glucose molecules.Worked example: suppose one adrenaline molecule leads to 100 cAMP molecules, and each cAMP molecule, through the cascade, releases 10 000 glucose molecules.glucose released = 100 × 10 000 = 10⁶ moleculesBecause the signal is carried inside the cell by a second messenger, the same hormone can cause different responses in different cells, depending on which enzymes the second messenger activates.