# DBT JRF 2022 — Question 107

Exam: DBT JRF 2022 · Section: B · Topic: Bioprocess Engineering › Batch Culture & Monod Kinetics · Difficulty: Medium

## Question

In a batch culture of cells following Monod Kinetics, the declining log phase lasts longer when:

## Options

- **A.** Initial substrate concentration is high
- **B.** Initial substrate concentration is low
- **C.** $K_s$ values are large
- **D.** $K_s$ values are small

## Answer

Option C

## Explanation

Effect of substrate affinity constant Ks on growth kinetics: In the Monod equation: $\mu = \mu_{max} \cdot \frac{[S]}{K_s + [S]}$ $K_s$ = substrate concentration at half-maximal growth rate (analogous to enzyme $K_M$ ). Effect of a LARGE $K_s$ : • The organism has LOW affinity for the substrate (more substrate needed to reach half-maximal $\mu$ ). • As substrate is consumed in the bioreactor, $\mu$ drops steeply as [S] falls. • This results in a longer, more pronounced decline in growth rate as substrate approaches exhaustion. • The 'declining log phase' (transition from log growth to stationary) is more extended when $K_s$ is large. Effect of a SMALL $K_s$ : • HIGH affinity: $\mu$ remains near $\mu_{max}$ until substrate is nearly completely exhausted. • Abrupt transition to stationary phase. Answer: Large $K_s$ → low substrate affinity → $\mu$ drops faster as substrate depletes → longer declining log phase.

Source: https://mypyqbuddy.com/questions/in-a-batch-culture-of-cells-following-monod-6a104cd5920734408bc0c77a