Q.56 Given below are two statements :
Statement I: Phototrophs and chemotrophs derive their energy from sunlight and chemical fuels.
respectively.
Statement II : Phototrophs and chemotrophs cannot be autotrophs but only heterotrophs.
In the light of the above statements, choose the correct answer from the options given below :
1. Both Statement I and Statement II are true
2. Both Statement I and Statement II are false
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
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Understanding Phototrophs and Chemotrophs in Biology
Phototrophs and chemotrophs classify organisms by their energy sources, a core concept in microbial physiology and biochemistry. Phototrophs harness sunlight via photosynthesis, while chemotrophs oxidize chemical compounds like glucose or inorganic molecules for energy. This distinction often appears in exams like NEET, CSIR NET, or GATE Biotechnology.
Let’s analyze the given statements to determine the correct option.
The Two Statements
Statement I: Phototrophs and chemotrophs derive their energy from sunlight and chemical fuels, respectively.
Statement II: Phototrophs and chemotrophs cannot be autotrophs but only heterotrophs.
Options:
-
Both Statement I and Statement II are true
-
Both Statement I and Statement II are false
-
Statement I is true but Statement II is false
-
Statement I is false but Statement II is true
Correct Answer: Option 3 (Statement I True, Statement II False)
Why? Statement I accurately describes energy acquisition—phototrophs use light (e.g., plants, cyanobacteria), chemotrophs use chemicals (e.g., bacteria oxidizing sulfur). Statement II is wrong: both can be autotrophs (self-feeding via inorganic sources) or heterotrophs (needing organics).
Breakdown of Key Terms with Examples
| Term | Energy Source | Nutrition Mode | Examples |
|---|---|---|---|
| Photoautotroph | Sunlight | Autotroph (CO₂ to organics) | Cyanobacteria, algae |
| Photoheterotroph | Sunlight | Heterotroph (needs organics) | Purple non-sulfur bacteria |
| Chemoautotroph | Chemical oxidation | Autotroph (CO₂ to organics) | Nitrifying bacteria (Nitrosomonas) |
| Chemoheterotroph | Chemical oxidation | Heterotroph (needs organics) | Humans, E. coli (on glucose) |
This table shows phototrophs/chemotrophs span autotrophic and heterotrophic nutrition.
Detailed Option Analysis
-
Option 1: Both true
Incorrect. Statement I holds, but II fails—many phototrophs (e.g., plants) and chemotrophs (e.g., Thiobacillus) are autotrophs, fixing CO₂ independently. -
Option 2: Both false
Incorrect. Statement I is factually correct per standard microbiology texts like Brock Biology of Microorganisms. -
Option 3: Statement I true, II false
Correct. Energy classification (I) is distinct from carbon nutrition (II). Autotrophy depends on Calvin cycle or similar, not energy source alone. -
Option 4: Statement I false, II true
Incorrect. I is true, and II wrongly excludes autotrophs.
Exam Tip: Energy vs Nutrition Axes
Think of it as two axes:
-
Energy: Light (photo) or chemicals (chemo).
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Carbon: Inorganic (auto) or organic (hetero).
Most plants are photoautotrophs; animals chemoheterotrophs. This orthogonality trips up many students.
Master this for questions on microbial metabolism or ecological roles. For practice, recall: Can a phototroph be heterotrophic? Yes—Rhodospirillum rubrum grows on organics under light.
