Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Cell type A secretes molecule X into the culture medium. Cell type B in the same culture
responds to the molecule X by expressing protein Y. Which one of the following modes of
signaling represents the interaction between A and B?
Options:
(A) Autocrine
(B) Juxtacrine
(C) Paracrine
(D) Intracrine
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.24)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Cell Biology (Cell-Cell Communication & Signaling Mechanisms)
**2. Detailed Step-by-Step Analysis**:
- Intercellular chemical signaling modes are classified based on the distance between the signal-producing cell and target cell:
1. **Autocrine signaling**: The signaling molecule acts on the very same cell (or identical cell types) that secreted it.
2. **Juxtacrine (contact-dependent) signaling**: Requires direct physical contact between membrane-bound molecules of adjacent cells without secretion into extracellular fluid.
3. **Paracrine signaling**: A cell secretes a signaling molecule (local mediator) into the extracellular medium, which diffuses locally to act on nearby target cells of a different type.
4. **Endocrine signaling**: Endocrine cells secrete hormones into the bloodstream/circulatory system to act on distant target tissues.
- Here, **Cell type A** secretes molecule X into the culture medium, and **Cell type B** in the same culture responds to molecule X. Because the signal travels locally through extracellular medium to target a distinct adjacent cell type, this is a classic example of **paracrine signaling**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Autocrine signaling occurs when a cell responds to its own secreted molecules.
- **Option (B) [INCORRECT]**: Juxtacrine signaling requires cell-to-cell membrane contact.
- **Option (C) [CORRECT]**: Paracrine signaling involves local diffusion of a secreted molecule from cell type A to adjacent cell type B.
- **Option (D) [INCORRECT]**: Endocrine signaling requires systemic vascular/bloodstream transport to distant target organs.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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