GATE 2019 BT Q.15: solution and answer

Official answer: DSource: Official answer key, GATE 2019 BT (IIT Madras)

1. The Monod model: $\mu =\frac{{\mu }_{max}S}{K_s+S}$. Here $\mu$ is the specific growth rate (${\text{h}}^{-1}$), ${\mu }_{max}$ is its maximum value, $S$ is the concentration of the limiting substrate and $K_s$ is the saturation constant (the value of $S$ at which $\mu ={\mu }_{max}/2$).

2. In the Monod model, $S$ is the concentration of the substrate in the medium around the cells. The model is empirical and unstructured: it treats the cell as a black box with no internal state. Growth rate therefore depends only on the concentration of a nutrient the cell takes up from outside.

3. (A) An extracellular carbon source such as glucose is the classic Monod limiting substrate. It can be limiting.

4. (B) An extracellular nitrogen source such as ${\text{NH}}_4^{+}$ can limit growth in the same way, with its own $K_s$. It can be limiting.

5. (C) Dissolved oxygen is a substrate taken up from the liquid phase. When aerobic growth is limited by oxygen, the same form applies: $\mu =\frac{{\mu }_{max}{C}_{O_2}}{K_{O_2}+C_{O_2}}$, where $C_{O_2}$ is the dissolved-oxygen concentration and $K_{O_2}$ is its saturation constant. It can be limiting.

6. (D) An intracellular carbon source, such as a stored reserve, is inside the cell. It is not a concentration in the medium, so it cannot stand in for $S$, and the unstructured Monod model has no variable for anything inside the cell. It cannot be the limiting substrate.

7. Answer: (D).