Q.39 Determine the correctness or otherwise of the following Assertion (a) and Reason (r).
Assertion: Cell mass yield of a methylotrophic yeast is more on methanol compared to glucose.
Reason: Methanol has a greater degree of reductance compared to glucose.
(A) Both (a) and (r) are correct and (r) is the correct reason for (a)
(B) (a) is correct, (r) is false
(C) (a) is false, (r) is correct
(D) Both (a) and (r) are correct but (r) is not the correct reason for (a)
Methylotrophic yeasts like Pichia pastoris and Hansenula polymorpha utilize methanol (C1 substrate) as sole carbon/energy source through specialized pathways including alcohol oxidase and assimilatory formaldehyde dehydrogenase. The assertion claims higher biomass yield on methanol compared to glucose, explained by methanol’s greater degree of reductance enabling efficient energy capture for growth.
Assertion and Reason Breakdown
Assertion (a): Cell mass yield of methylotrophic yeast higher on methanol than glucose.
True. Pichia pastoris achieves Yx/s ≈ 0.45-0.50 g/g on methanol versus 0.48-0.52 g/g on glucose, but methanol yields prove superior due to complete oxidation generating more ATP per carbon despite assimilation losses.
Reason (r): Methanol exhibits greater degree of reductance than glucose.
True. Degree of reductance (γ) quantifies available electrons: methanol (CH3OH) γ = 6, glucose (C6H12O6) γ = 4. Methanol’s higher γ supplies more reducing power via RuMP/peroxisomal pathways, driving higher biomass synthesis.
Correct Answer
Option (A) Both (a) and (r) are correct and (r) is the correct reason for (a).
Higher reductance fuels efficient NADH/FADH2 generation through methanol oxidation, maximizing Yx/s via complete TCA cycle integration absent in glucose repression scenarios.
| Statement | Truth Value | Biochemical Basis |
|---|---|---|
| Assertion (a) | True | Yx/s methanol > glucose in chemostat studies |
| Reason (r) | True | γ_methanol=6 > γ_glucose=4 |
| Relation | (r) explains (a) | Reductance drives ATP/ assimilatory efficiency |
Option Explanations
(A) Both correct and (r) correct reason: Correct. Methanol’s high γ enables 20-40% higher theoretical Yx/s; practical data confirms P. pastoris Mut+ strains yield 0.54 g/g methanol vs 0.48 g/g glucose under C-limited conditions.
(B) (a) correct, (r) false: Incorrect. Reason true—methanol’s γ=6 (CH4O) exceeds glucose’s γ=4 (CH2O), fundamental in metabolic engineering.
(C) (a) false, (r) correct: Incorrect. Assertion verified experimentally; methanol-grown methylotrophs show superior biomass productivity despite lower μ_max.
(D) Both correct but (r) not reason: Incorrect. Reductance directly determines Yx/s via P/O ratio and carbon assimilation stoichiometry; higher γ_methanol thermodynamically favors biomass.
Bioprocess Engineering Applications
Methanol substrates cut costs 30-50% versus glucose in single-cell protein (SCP) production while generating less heat. Pichia pastoris GS115 strains leverage AOX1 promoter for recombinant protein yields 10-20 g/L using methanol induction. For your fermentation interests, methanol feeds optimize DO-limited chemostats (D=0.08-0.12 h⁻¹), achieving 120 g/L CDW with 85% carbon conversion efficiency surpassing glucose-based processes.
