The second messenger model of adrenaline and glucagon action, involving…

The idea

Adrenaline and glucagon are first messengers. Both are hydrophilic and cannot cross the phospholipid bilayer, so they act from outside the cell. The hormone binds to a specific receptor on the cell-surface membrane of a liver cell, changing the receptor's shape. The altered receptor activates the membrane-bound enzyme adenylate cyclase, which catalyses the conversion of ATP into cyclic AMP. Cyclic AMP is the second messenger: it moves through the cytoplasm and activates protein kinase, which phosphorylates and activates further enzymes. The cascade ends with glycogen phosphorylase, which catalyses the phosphorolysis of glycogen to glucose-1-phosphate; in liver cells this is converted to glucose, which leaves the cell. Because one hormone molecule causes many cAMP molecules to be made, and each kinase activates many enzyme molecules, the signal is amplified through the cascade.

What earns the marks

• Adrenaline or glucagon binds to a specific receptor on the cell-surface membrane of a liver cell.
• The activated receptor stimulates adenylate cyclase, a membrane-bound enzyme.
• Adenylate cyclase catalyses the conversion of ATP into cyclic AMP.
• Cyclic AMP acts as a second messenger that activates protein kinase.
• Protein kinase initiates a cascade that activates enzymes, ending with glycogen phosphorylase, which breaks down glycogen; glucose-1-phosphate is produced and converted to glucose in liver cells.
• Amplification explained: one hormone molecule leads to many cAMP molecules and many activated enzyme molecules, so a low hormone concentration produces a large response.

Common mistakes and how to put them right

• Saying the hormone enters the cell and acts on the enzyme directly.

  Put it right: The hormone is hydrophilic and binds to a cell-surface receptor; the second messenger carries the signal inside.

• Calling cyclic AMP the first messenger, or the hormone the second messenger.

  Put it right: The hormone is the first messenger; cAMP is the second messenger.

• Writing that adenylate cyclase converts ADP or glucose into cyclic AMP.

  Put it right: Adenylate cyclase catalyses the conversion of ATP into cyclic AMP.

• Omitting protein kinase, so the chain jumps straight from cyclic AMP to glycogen breakdown.

  Put it right: cAMP activates protein kinase, which phosphorylates further enzymes.

• Stating that glycogen phosphorylase hydrolyses glycogen directly to glucose.

  Put it right: It catalyses phosphorolysis to glucose-1-phosphate, which is converted to glucose in liver cells.

Exam tips

• Learn the chain in order: hormone, receptor, adenylate cyclase, ATP to cAMP, protein kinase, glycogen phosphorylase, glucose-1-phosphate to glucose.
• Say the words second messenger explicitly when you name cyclic AMP.
• If asked why this system is efficient, answer with amplification: one hormone molecule leads to many glucose molecules released.