7.
Of the graphs shown below, which is most representative of the kinetics of ion transport
through a membrane channel?
a. Graph (A)
b. Graph (B)
c. Graph (C)
d. Graph (D)
The correct answer is Graph (D), which shows a hyperbolic increase in transport rate that gradually plateaus as external ion concentration rises, reflecting saturable kinetics of ion transport through a membrane channel.
Concept: ion channel transport kinetics
Ion channels are integral membrane proteins that allow specific ions to cross the lipid bilayer down their electrochemical gradient, generating a flux that initially rises with ion concentration but then saturates at a maximum rate. This saturation occurs because only a finite number of channels or channel states are available; once all conducting channels are occupied or fully open, further increases in ion concentration do not increase the transport rate.
Analysis of Graph (A)
Graph (A) shows a strictly linear increase of transport rate with external ion concentration, with no sign of a plateau. Such a straight-line relationship is typical of simple diffusion through the lipid bilayer, where flux is directly proportional to concentration and does not involve a limited number of binding or gating sites. Because ion transport through a membrane channel displays saturable, enzyme‑like kinetics rather than a purely linear one, Graph (A) does not represent the true kinetics of ion channels.
Analysis of Graph (B)
Graph (B) depicts a high transport rate at low ion concentration that quickly rises and then becomes completely flat, effectively suggesting immediate saturation even at very low concentrations. Real ion channels exhibit an initial concentration‑dependent rise in flux before reaching saturation; the curve is not flat across almost the entire concentration range. Therefore, Graph (B) misrepresents the characteristic gradual approach to a maximal rate observed in ion channel kinetics.
Analysis of Graph (C)
Graph (C) forms a bell‑shaped curve, where transport rate increases with concentration up to a peak and then declines as concentration increases further. Such behavior might occur in special inhibitory or toxic conditions, but it is not the typical kinetic profile of standard ion channels under physiological conditions, where increased ion concentration does not reduce flux once channels are saturated. Since normal ion transport through channels does not show a declining phase with higher concentration, Graph (C) is not the appropriate representation.
Why Graph (D) is correct
Graph (D) displays a hyperbolic curve: the transport rate rises steeply with increasing external ion concentration and then gradually levels off into a plateau, indicating a maximal rate Vmax. This behavior mirrors Michaelis–Menten–type saturable kinetics, where a finite number of channel or carrier sites become fully utilized at high substrate (ion) concentrations, so further increases in concentration cannot increase the rate. Thus, Graph (D) is most representative of the kinetics of ion transport through a membrane channel and correctly matches the expected saturable transport profile.
