Chemical signalling practice questions.

When blood glucose falls, which hormone stimulates the breakdown of glycogen to glucose in the liver?

• Insulin
• Glucagon
• Adrenaline only
• Thyroxine

Answer: Glucagon

Glucagon, secreted by pancreatic alpha cells, promotes glycogenolysis and gluconeogenesis in the liver, raising blood glucose. Insulin has the opposite effect, lowering blood glucose.

Distinguish how a hydrophilic peptide hormone (e.g. insulin) transmits its signal into a target cell.

• It diffuses through the plasma membrane and binds a receptor in the nucleus
• It binds a cell-surface receptor, triggering a signal transduction cascade
• It is transported into the cell by endocytosis and acts as a transcription factor
• It dissolves in the phospholipid bilayer and directly activates DNA

Answer: It binds a cell-surface receptor, triggering a signal transduction cascade

Because peptide hormones cannot cross the hydrophobic membrane, they bind surface receptors and act through second messengers and signal transduction.

Explain why a steroid hormone can bind receptors located inside the cell rather than on its surface.

• Steroid hormones are large and charged
• Steroid hormones are lipophilic and cross the membrane
• Steroid hormones are broken down at the membrane surface
• Steroid hormones require a transport protein to enter the cell

Answer: Steroid hormones are lipophilic and cross the membrane

Steroid hormones are lipid-soluble, so they pass through the phospholipid bilayer and bind intracellular receptors that often act on gene transcription.

State the general term for the internal signalling molecule (such as cyclic AMP) produced when a hormone binds a cell-surface receptor.

• First messenger
• Second messenger
• Ligand
• Neurotransmitter

Answer: Second messenger

The hormone is the first messenger; molecules like cyclic AMP generated inside the cell are second messengers that relay and amplify the signal.

Determine the advantage of a signal transduction cascade involving multiple enzyme-activation steps.

• It slows the response so it lasts longer
• It amplifies the signal many-fold
• It prevents the signal from reaching the nucleus
• It converts the hormone into a steroid

Answer: It amplifies the signal many-fold

Each step activates many molecules in the next, so a small number of hormone molecules trigger a greatly amplified response.

Distinguish paracrine signalling from endocrine signalling.

• Paracrine signals travel long distances in the blood; endocrine signals act locally
• Paracrine acts on nearby cells; endocrine travels in blood to distant targets
• Paracrine signalling only uses steroid hormones; endocrine only uses peptides
• Paracrine signalling occurs only in neurons; endocrine only in muscle

Answer: Paracrine acts on nearby cells; endocrine travels in blood to distant targets

Paracrine signals diffuse to neighbouring cells over short distances, whereas endocrine (hormonal) signals are transported through the bloodstream to distant target cells.