Question

Nicotinic ACh receptors have ____ units, with each unit crossing the membrane ____ times. The receptors may be homomeric or heteromeric.

a) 4, 1
b) 5, 2
c) 3, 3
d) 2, 4

Answer 1

Nicotinic ACh receptors have five subunits, with each subunit crossing the membrane four times. The correct answer to the student's question is therefore composed of an option that would indicate '5, 4', which is not provided in the options a) through d).

Step-by-step explanation:

Nicotinic acetylcholine receptors (AChRs) are a type of ionotropic receptor found in various locations including the neuromuscular junction (NMJ) and autonomic ganglia. Answering the question posed, these nicotinic ACh receptors have five subunits, with each subunit crossing the membrane four times. The receptors can be either homomeric, consisting of identical subunits, or heteromeric, consisting of a combination of different subunits.

This structural arrangement allows them to function as ligand-gated ion channels that, when activated by the neurotransmitter acetylcholine, or exogenous agonists like nicotine, will facilitate the passage of cations across the cell membrane leading to a depolarizing postsynaptic potential. The correct answer to the given question is 5, 4, so none of the given options a) through d) are correct as they do not match this description.

Each subunit has a membrane-spanning domain composed of four transmembrane segments, totaling twenty transmembrane segments for the entire pentameric receptor.