GATE 2024 BT Q.52: solution and answer

Official answer: A, B, CSource: Official answer key, GATE 2024 BT (IISc Bengaluru)

1. What a batch process is: all the medium and the inoculum go into a closed vessel at $t=0$, and nothing is added or removed afterwards except gases (air in, ${\text{CO}}_2$ out) and small amounts of acid or base for pH control. So the only things that can affect growth are the conditions inside the vessel.

2. (A) pH affects growth. Every organism has an optimum pH and a range it can tolerate. pH changes the ionisation of groups in enzyme active sites, the proton-motive force across the membrane and how nutrients are taken up. It also drifts during a batch run as organic acids or ammonia build up, so the specific growth rate $\mu$ (biomass made per unit biomass per unit time, in ${\text{h}}^{-1}$) changes with it.

3. (B) Osmolarity affects growth. A high solute concentration (salt or sugar) draws water out of the cell by osmosis and lowers water activity. The cell then has to spend energy making or taking up compatible solutes, so $\mu$ drops and the lag phase gets longer. A very low osmolarity can make cells without a rigid wall swell and burst. Either way, growth changes.

4. (C) Substrate concentration affects growth. Monod kinetics gives $\mu =\frac{{\mu }_{max}S}{K_s+S}$. Here $S$ is the concentration of the limiting substrate (${\text{g L}}^{-1}$), ${\mu }_{max}$ is the maximum specific growth rate (${\text{h}}^{-1}$) and $K_s$ is the saturation constant, the value of $S$ at which $\mu ={\mu }_{max}/2$ (${\text{g L}}^{-1}$). As the batch uses up $S$, $\mu$ falls and the culture enters stationary phase. At very high $S$, substrate inhibition can also slow growth.

5. (D) Substrate feed rate does not apply. A batch reactor has no feed stream, so the feed flow rate is $F=0$. Feed rate is a variable only in fed-batch and continuous culture. In a chemostat, for example, the dilution rate is $D=F/V$ and $\mu =D$ at steady state. A batch culture has no feed rate that could change, so D is not a factor.

6. Correct options: A, B, C.