Cholera toxin causes severe diarrhoea by locking a G protein in its
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.
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State whether the following is true or false.
Cholera toxin acts by blocking a signalling pathway entirely.
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.
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Calcium ions serve well as a second messenger because their concentration in the cytosol is normally
Answer: A
Pumps hold cytosolic calcium about ten thousand times below the level outside and in the endoplasmic reticulum, so opening a channel produces a sharp and unmistakable rise. Restoring the low resting level afterwards costs the cell energy.
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Inositol trisphosphate acts by
Answer: B
Produced when a membrane lipid is cleaved, it diffuses to the reticulum and opens calcium channels there. The other product of that cleavage, diacylglycerol, stays in the membrane and activates protein kinase C. One cleavage event thus produces two distinct signals at once.
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Consider the following statements about second messengers.
1. Cyclic AMP is produced from ATP by adenylyl cyclase.
2. Calcium is normally kept at low concentration in the cytosol.
3. Second messengers carry the signal across the plasma membrane.
Answer: C
The first two are correct. The third misplaces their role, since the receptor carries the message across the membrane and the second messenger then spreads it within the cell. The two roles are sequential rather than interchangeable.
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State whether the following is true or false.
Nitric oxide can act as a signalling molecule despite being a gas.
Answer: D
It diffuses through membranes directly and acts on an enzyme inside the target cell, relaxing smooth muscle in blood vessels. Its very short life limits its action to the immediate neighbourhood. Nitroglycerin relieves angina by releasing it in the vessel wall.
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A receptor's specificity for its signal molecule depends chiefly on
Answer: A
Correct answer: the shape of its binding site
Specificity is molecular complementarity: the binding site’s shape and its distribution of charge and hydrogen-bonding groups fit one molecule and not others. That is why a cell is deaf to every signal for which it lacks a receptor, however concentrated, and why a drug designed to fit the same site can mimic or block the natural ligand.
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An antagonist drug is one that
Answer: B
Occupying the site blocks the natural signal from binding, so the pathway is silenced. Beta blockers work this way on the receptors that adrenaline would otherwise activate. An agonist would do the opposite and stimulate the receptor.
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State whether the following is true or false.
An agonist mimics the natural signal while an antagonist blocks it.
Answer: C
Correct answer: True
An agonist binds the receptor and produces the same conformational change as the natural ligand, so it triggers the response - morphine at the opioid receptor. An antagonist binds without activating and so occupies the site, preventing the natural ligand from acting - a beta blocker at the adrenergic receptor. Both depend on fitting the same binding site, and they differ only in what happens after binding.
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Assertion (A): Beta blockers slow the heart rate.
Reason (R): They occupy adrenaline receptors without activating them.
Answer: D
Both are true and the reason gives the mechanism: with the receptors blocked, circulating adrenaline cannot exert its usual stimulating effect, so the heart is left responding to a weaker signal than it otherwise would.
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