Cell Signalling Pathways Questions and Answers

Answer: A

The signal diffuses only a short distance and is usually degraded or taken up quickly, which keeps its effect local. Endocrine signals travel in the bloodstream to distant targets. The distinction is one of range rather than of chemistry.

Answer: B

Hormones reach the whole body but affect only cells carrying the right receptor. This is why one circulating molecule can produce quite different effects in different tissues. Adrenaline dilates some blood vessels and constricts others for this reason.

Answer: C

Correct answer: responds to a signal it released itself

Autocrine signalling is a cell talking to itself: it secretes a molecule for which it carries the receptor. It is used to reinforce a decision a cell has already made - which is why it is common in development, where a group of neighbouring cells all releasing and receiving the same signal commits the whole group to one fate, and why it is a frequent feature of cancer cells driving their own proliferation.

Answer: D

Both are true and the reason gives the advantage to the tumour: self-sufficiency in growth signalling is one of the recognised hallmarks of cancer, and autocrine loops are one route to it. Independence from external growth signals is what the loop buys.

Answer: A

Only cells bearing the appropriate receptor can respond, so the same molecule passes most tissues without effect. Specificity resides in the receptor rather than in the distribution of the signal. A cell without the receptor is simply blind to the message.

Answer: B

Being lipid-soluble it crosses the membrane unaided, and the receptor complex then acts directly on the DNA as a transcription factor. Water-soluble signals must instead be received at the surface. The route a signal takes follows directly from its solubility.

Answer: C

Both are true and the reason accounts for both halves: making new protein takes time, whereas phosphorylating an enzyme already present is nearly immediate, and the newly made protein persists after the signal has gone.

Answer: D

The receptor spans the membrane and relays the message inward without the signal itself entering. This is why the external molecule is called the first messenger and the internal relay the second. The message is thus handed across the membrane rather than carried through it.

Answer: A

Made from ATP by adenylyl cyclase when a receptor is activated, it spreads the signal through the cell by activating protein kinase A. Calcium ions and inositol trisphosphate are other examples. A second messenger amplifies the signal as well as spreading it.

Answer: B

Correct answer: activating a G protein that relays the signal

Binding of the ligand changes the receptor’s conformation so that it acts on an associated G protein, causing it to exchange GDP for GTP and dissociate. The activated subunit then acts on an effector such as adenylate cyclase or phospholipase C. The family is enormous - several hundred receptors in humans and the target of a large share of all prescribed drugs.