Question

If microbial growth occurs under substrate unlimited conditions, according to Monod’s kinetics, choose the correct option(s) from the following:

• A. Microbial growth follows zero order with respect to substrate concentration.
• B. Microbial growth follows first order with respect to substrate concentration.
• C. Specific growth rate is half of maximum specific growth rate.
• D. Specific growth rate is almost equal to maximum specific growth rate.

Hint:

Under substrate-unlimited conditions (\( S \gg K_s \)), the specific growth rate becomes independent of substrate, leading to zero-order kinetics with \( \mu \approx \mu_{{max}} \).

Answer 1

The Correct Option is A, D

Step 1: Understand Monod's equation. 
Monod's equation is: \[ \mu = \mu_{{max}} \cdot \frac{S}{K_s + S} \] Where \( \mu \) is the specific growth rate, \( S \) is substrate concentration, \( K_s \) is the half-saturation constant, and \( \mu_{{max}} \) is the maximum specific growth rate. 
Step 2: Apply substrate-unlimited condition. 
Under substrate-unlimited conditions, \( S \gg K_s \), hence: \[ \mu \approx \mu_{{max}} \] This implies that microbial growth is at its maximum rate and no longer dependent on substrate concentration — i.e., zero-order kinetics with respect to substrate. 
Step 3: Evaluate options. 
(A) is correct: zero-order with respect to substrate. 
(B) is incorrect: not first-order when substrate is abundant.
(C) is incorrect: that applies when \( S = K_s \), not substrate-unlimited. 
(D) is correct: growth rate is almost equal to \( \mu_{{max}} \).