Monod kinetics: In the Monod equation, a higher Ks value indicates what about the microorganism's affinity for the limiting substrate?

Introduction:
The Monod equation relates specific growth rate to substrate concentration using two parameters: mu_max and Ks. Understanding the physical meaning of Ks is essential for interpreting culture performance and designing feeding strategies.

Given Data / Assumptions:

• Monod form: mu = mu_max * S / (Ks + S).
• Ks is the substrate concentration giving mu = mu_max / 2.
• Affinity is inversely related to the magnitude of Ks.

Concept / Approach:
A larger Ks means a higher substrate concentration is required to reach half of mu_max, which implies lower apparent affinity for the substrate. Conversely, a small Ks indicates high affinity because the organism approaches high growth rates at low S.

Step-by-Step Solution:
Write mu = mu_max * S / (Ks + S).Set mu = mu_max / 2 to interpret Ks: mu_max / 2 = mu_max * S / (Ks + S) gives S = Ks.Conclude that larger Ks shifts the half-saturation point to higher S, indicating lower affinity.

Verification / Alternative check:
Plotting mu versus S for different Ks values shows that curves with larger Ks rise more slowly with S, consistent with reduced substrate affinity.

Why Other Options Are Wrong:
Greater affinity: contradicts the definition of Ks as half-saturation concentration.

Unaffected by substrate binding: Ks directly reflects the system response to S.

Lower dissociation constant value: a higher Ks is not lower; this option reverses the direction.

Common Pitfalls:

• Confusing Ks with Km of enzymes without recalling the half-saturation interpretation.
• Assuming affinity is proportional, rather than inversely related, to Ks.

Final Answer:
Lower affinity for the substrate