Cell Signalling Pathways Questions and Answers

Answer: C

Ten to the fourth power is 10,000, since the stages multiply rather than add. This is why a cascade of only a few steps can produce so enormous a response. Each additional stage multiplies rather than adds to the amplification.

Answer: D

Correct answer: cells in direct physical contact

A gap junction is a channel joining the cytoplasms of two adjacent cells, so ions and small molecules pass only between cells that touch. That makes it the shortest-range signalling there is, and the fastest - which is why it synchronises the contraction of cardiac and smooth muscle, where the coupling must be essentially instantaneous.

Answer: A

Both are true and the reason gives the mechanism: direct cytoplasmic continuity spreads the depolarisation far faster than any released signal could, so the tissue behaves electrically as though it were a single cell rather than millions of separate ones.

Answer: B

Correct answer: immune recognition and development

Contact-dependent signalling requires the two cells to touch, so it is used where the identity of the particular neighbour matters. A T cell inspecting an antigen-presenting cell and the Notch-Delta interaction that decides which of two adjacent cells adopts which fate are the two classic cases - in both, the signal must be addressed to one specific cell and no other.

Answer: C

Correct answer: False

Two important modes involve no release at all. Contact-dependent signalling works through membrane-bound molecules on cells that touch, and gap junction signalling passes small molecules directly between joined cytoplasms without either ever entering the extracellular space. Both are used precisely because they are addressed to particular neighbours rather than broadcast.

Answer: A

The calcium is released from the Endoplasmic reticulum.

When a cell is stimulated through receptors coupled to phospholipase C, the second messenger inositol 1,4,5-trisphosphate (IP₃) binds IP₃ receptors, which are calcium channels in the ER membrane. The ER (the sarcoplasmic reticulum in muscle) is the main intracellular calcium store, so opening these channels raises cytosolic Ca²⁺ within seconds.

Mitochondria take up calcium and can buffer it, but they are not the store released on stimulation. The nucleus and peroxisomes are not calcium-release stores in signalling.

Answer: C

The exception is the receptor for Estrogen.

Estrogen is a steroid: small and lipid-soluble, it diffuses through the plasma membrane and binds its receptor inside the cell, in the cytoplasm or nucleus. The hormone-receptor complex then acts as a transcription factor, binding estrogen response elements in DNA.

The other ligands are water-soluble and cannot cross the membrane, so their receptors sit on the cell surface: GABA acts on GABA_A (ion channel) and GABA_B receptors, acetylcholine on nicotinic and muscarinic receptors, and luteinizing hormone on a G protein-coupled receptor.

Answer: D

The one that is not a second messenger is the potassium ion.

A second messenger is a small intracellular signal whose concentration changes sharply when a hormone or neurotransmitter binds a cell-surface receptor, and which then relays the signal inside the cell. Potassium is the main intracellular cation and sets the resting membrane potential, but its level is not used to carry receptor signals in this way.

The other three are classic second messengers. Cyclic AMP is made by adenylyl cyclase and activates protein kinase A. Diacylglycerol is released from PIP₂ by phospholipase C and activates protein kinase C. Calcium ions, released from the endoplasmic reticulum by IP₃, act through calmodulin and many other proteins.

Answer: B

The exception is an extracellular ligand binding site.

Intracellular (nuclear) receptors, such as those for steroid hormones, thyroid hormone, retinoic acid and vitamin D, bind ligands that cross the plasma membrane because they are lipid-soluble. Their ligand-binding domain is therefore inside the cell, in the cytoplasm or nucleus.

The other features are typical: a DNA-binding domain (zinc fingers that recognise hormone response elements), signalling by lipid ligands such as steroids, and a transcription-activating domain that recruits coactivators. An extracellular binding site belongs to cell-surface receptors.

Answer: A

When the signal binds, the G protein gets activated.

Binding of the ligand changes the shape of the receptor, which then acts as a guanine nucleotide exchange factor: the alpha subunit of the trimeric G protein releases GDP and binds GTP. The GTP-bound alpha subunit separates from the beta-gamma pair, and both go on to regulate effectors such as adenylyl cyclase or phospholipase C. The signal ends when the alpha subunit hydrolyses its GTP.

The G protein does not bind calcium or calmodulin; calcium and calmodulin act further downstream, after phospholipase C has produced IP₃. Since activation is correct, 'None of the above' is wrong.