Q.38 Enzyme activity profiles as a function of time in the absence or presence of different
types of feedback mechanisms are shown in the figure below. Match the following
feedback mechanisms with the corresponding profiles in the figure.
(p) No feedback mechanism
(q) Negative feedback mechanism with short delay
(r) Negative feedback mechanism with long delay
(s) Positive feedback mechanism
(A) (p) – (i); (q) – (ii); (r) – (iii); (s) – (iv)
(B) (p) – (iv); (q) – (iii); (r) – (ii); (s) – (i)
(C) (p) – (iv); (q) – (ii); (r) – (iii); (s) – (i)
(D) (p) – (i); (q) – (iii); (r) – (ii); (s) – (iv)

The correct matching is:
(p) No feedback mechanism – profile (iv)
(q) Negative feedback, short delay – profile (iii)
(r) Negative feedback, long delay – profile (ii)
(s) Positive feedback mechanism – profile (i)​

So, the right option is (B).

Understanding each feedback profile

Profile (iv) – No feedback mechanism

• In profile (iv), enzyme activity rises quickly when the signal is applied and then stays at a constant elevated plateau as long as the signal remains.

• There is no overshoot, oscillation, or decay while the signal is present, which is characteristic of a system with no feedback control; activity simply follows the input signal.​

So, (p) No feedback mechanism = (iv).

Profile (iii) – Negative feedback with short delay

• In profile (iii), the enzyme activity initially increases when the signal appears, but a small overshoot is followed by a rapid damping back toward a lower, near‑steady level.

• Such a quick correction of the overshoot indicates negative feedback with a short delay, where the inhibitory effect is felt almost immediately and quickly stabilizes the system without sustained oscillations.​

So, (q) Negative feedback, short delay = (iii).

Profile (ii) – Negative feedback with long delay

• In profile (ii), after the signal is applied, enzyme activity shows large, sustained oscillations (repeated up‑and‑down swings) instead of settling quickly.

• Long delays in negative feedback are known to produce oscillatory behavior because the correction always arrives “late”, leading to continuous over‑ and undershooting.​

So, (r) Negative feedback, long delay = (ii).

Profile (i) – Positive feedback mechanism

• In profile (i), enzyme activity rises after the signal is applied and then continues to ramp upwards even after the external signal is removed, moving toward a high, self‑sustained state.

• This self‑amplifying, runaway increase is the hallmark of positive feedback, where the output enhances its own production and can maintain high activity independent of the original signal.​

So, (s) Positive feedback mechanism = (i).

Checking all options

Use the correct mapping (p‑iv, q‑iii, r‑ii, s‑i):

• (A) (p‑i; q‑ii; r‑iii; s‑iv) – does not match.

• (B) (p‑iv; q‑iii; r‑ii; s‑i) – matches the reasoning above and is correct.

• (C) (p‑iv; q‑ii; r‑iii; s‑i) – interchanges short‑ and long‑delay negative feedback, contradicting the oscillatory nature of profile (ii).​

• (D) (p‑i; q‑iii; r‑ii; s‑iv) – swaps “no feedback” and “positive feedback”, inconsistent with the sustained, self‑amplifying rise in profile (i).​

Hence, the answer is Option (B).