GATE 2019 BT Q.17: solution and answer

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

1. Definition of endogenous metabolism: the cell catabolises its own material, meaning stored polymers first and then other cell constituents, to get the energy it needs to stay alive (maintenance). Exogenous metabolism is the opposite case, where the cell catabolises substrate taken up from the medium.

2. When it matters in a batch culture: endogenous metabolism goes on all the time. It becomes the cell's main energy source in the stationary and death phases, once the limiting extracellular substrate has run out. The cells still need maintenance energy, for example to hold the membrane potential and ion gradients and to repair proteins and DNA. With nothing left in the medium, they draw on reserves they built up earlier.

3. What the internal reserves are: intracellular storage polymers such as glycogen, poly-$\beta$-hydroxybutyrate (PHB) and polyphosphate. When these run low, the cell starts to break down its own RNA and proteins as well.

4. Kinetic view (Herbert's endogenous-metabolism model): $$\frac{dX}{dt}=({\mu }_g-k_e)X$$ Here $X$ (${\text{g L}}^{-1}$) is the biomass concentration, ${\mu }_g$ (${\text{h}}^{-1}$) is the gross specific growth rate and $k_e$ (${\text{h}}^{-1}$) is the rate constant of endogenous metabolism. The $-k_{e}X$ term is biomass being used up by the cell itself. That is why the net specific growth rate ${\mu }_g-k_e$ is lower than ${\mu }_g$, and why $X$ falls once ${\mu }_g\to 0$ after the substrate is gone.

5. Eliminate the other options: (B) Catabolising extracellular substrates is exogenous metabolism, which is the normal growth mode. (C) Extracellular products are metabolites the cell has secreted. If a cell does take one back up and use it (acetate, for example), it takes it from the medium, so that is exogenous metabolism too. (D) Degrading a toxic substrate also means using a compound from outside the cell, so it is exogenous and has nothing to do with the definition.

6. So the material catabolised during endogenous metabolism is the cell's internal reserves, option (A).