Section 4: Bioprocess Engineering and Process Biotechnology
Bioreactor types, scale-up
Subtopic: Fundamentals of Bioreactor types, scale-up
Question Statement:
Saccharomyces cerevisiae is cultured in a chemostat (continuous fermentation) at a dilution rate of
Q.5 h
-1. The feed substrate concentration is 10 g.L-1. The biomass concentration in the chemostat at
steady state will be _____________ g.L-1.
Assumptions: Feed is sterile, maintenance is negligible and maximum biomass yield with respect to
substrate is 0.4 (g biomass per g ethanol).
Microbial growth kinetics is given by
m
s
s
Ks
where µ is specific growth rate (h-1), µm = 0.7 h-1, Ks = 0.3 g.L-1 and s is substrate concentration
(g.L-1).
Options:
(A) Surface immunoglobulins ̶ B cell antigen receptor
(B) Affinity maturation ̶ isotype switching
(C) Fc region of antibodies ̶ binding to complement proteins
(D) Spleen, the secondary lymphoid organ ̶ no connection with the lymphatic system
Official Answer Key:
B ; D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.47)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Immunology (Antibody Function, Secondary Lymphoid Organs & B Cell Development)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each proposed combination to identify which ones are **INCORRECT**:
- **Combination A**: *Surface immunoglobulins — B cell antigen receptor (BCR)*: **CORRECT**. Membrane-bound IgM/IgD paired with $\text{Ig}\alpha/\text{Ig}\beta$ heterodimers form the functional BCR.
- **Combination B**: *Affinity maturation — isotype switching*: **INCORRECT**. Affinity maturation occurs via **somatic hypermutation (SHM)** of V region genes followed by antigen selection in germinal centers, whereas isotype (class) switching is mediated by **class-switch recombination (CSR)** between heavy chain constant region switch regions ($S_\mu \to S_\gamma, S_\alpha$). They are distinct molecular processes.
- **Combination C**: *Fc region of antibodies — binding to complement proteins*: **CORRECT**. The $C_H2$ domain of IgG and $C_H3$ domain of IgM bind the C1q globular heads to trigger the classical complement pathway.
- **Combination D**: *Spleen, the secondary lymphoid organ — no connection with the lymphatic system*:
- The spleen receives blood via the splenic artery and cleanses blood-borne antigens; unlike lymph nodes, it possesses **no afferent lymphatic vessels**. However, the spleen does possess **efferent lymphatic vessels** that drain lymphocytes from the white pulp into the thoracic duct. Saying it has "no connection with the lymphatic system" can be viewed as incorrect, or alternatively, contrasting its systemic blood filtration.
- The official GATE key identified both **B and D** as valid INCORRECT combinations.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT COMBINATION / NOT ANSWER]**: Accurately defines the BCR.
- **Option (B) [INCORRECT COMBINATION / CORRECT ANSWER]**: Conflates affinity maturation (somatic hypermutation) with isotype switching (class switch recombination).
- **Option (C) [CORRECT COMBINATION / NOT ANSWER]**: The Fc region directly binds C1q.
- **Option (D) [INCORRECT COMBINATION / CORRECT ANSWER]**: Spleen connects to the lymphatic system via efferent lymphatics.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B ; D**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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