Cell Signalling Pathways Questions and Answers

Answer: C

A third of 900 is 300. The proportion illustrates how central this one receptor family is to pharmacology, and why so much effort goes into understanding it. Their structures were long difficult to determine, which slowed drug design.

Answer: D

Binding causes two receptors to pair and phosphorylate each other, creating docking sites for relay proteins. The insulin receptor and many growth factor receptors work this way. Cross-phosphorylation between the paired receptors starts the cascade.

Answer: A

One activated receptor can switch on many G proteins, each of which makes many second messenger molecules, so a handful of hormone molecules produce a large response. Amplification is the point of having a cascade at all.

Answer: B

Multiplying 100 by 1,000 gives 100,000, which shows how a very small signal produces a very large internal response. Adding the two numbers instead is the usual error. Amplification is multiplicative at every stage of a cascade.

Answer: C

Each stage of the cascade multiplies the number of active molecules, so the effect grows geometrically. This is why hormones are effective at concentrations far below those of ordinary metabolites. Nanomolar concentrations suffice where metabolites need millimolar.

Answer: D

Correct answer: adding phosphate groups to target proteins

A kinase transfers a phosphate from ATP onto a serine, threonine or tyrosine residue of a target protein, changing its shape and so its activity. Phosphatases remove those phosphates, and the balance between the two is what makes phosphorylation a reversible switch rather than a permanent modification - which is exactly what a signalling pathway needs.

Answer: A

A pathway that could only be switched on would stay on permanently, so termination matters as much as activation. Many diseases arise from a signal that fails to be turned off. Constitutively active signalling proteins are common in tumours.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason states the consequence. A pathway with no off switch would respond once and never recover, so it could not report a change in conditions. Termination is built in at every level - the ligand is degraded or removed, the receptor is desensitised and internalised, GTP is hydrolysed on the G protein, phosphatases reverse the kinases, and second messengers are broken down.

Answer: C

The unregulated rise in cyclic AMP drives chloride and water out of the intestinal cells. It is a clear demonstration of what happens when a signal cannot be switched off. Rehydration therapy treats the consequence rather than the cause.

Answer: D

It does the opposite, jamming the pathway permanently on so that cyclic AMP accumulates. The resulting loss of fluid, rather than any tissue damage, is what makes the disease dangerous. Replacing the lost fluid and salts is therefore the essential treatment.