Q.6 Given below are two statements:
Statement I: On morphological grounds, scientists speculated that mitochondria and chloroplasts might be intracellular parasites that has established symbiotic relationship with the eukaryotic cells.
Statement II:Blue-green algae thought to have originated the mitochondria, and bacteria have originated the chloroplasts.
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
Mitochondria Chloroplast Origin: Statement I True, II False
The endosymbiotic theory revolutionized cell biology by proposing that mitochondria and chloroplasts originated from ancient prokaryotes engulfed by early eukaryotic cells. The correct answer is 3. Statement I is true but Statement II is false.
This classic question tests understanding of the endosymbiotic hypothesis, first popularized by Lynn Margulis. Let’s verify each statement and explain all options.
Statement I Analysis: Morphological Symbiosis Speculation
Scientists observed that mitochondria and chloroplasts have double membranes, their own circular DNA, 70S ribosomes, and divide independently—features resembling bacteria. These morphological similarities led to early speculation of intracellular parasites evolving into symbiotic organelles within eukaryotic cells.
This statement holds true, as historical evidence from the 19th century (e.g., Schimper) and modern support confirm endosymbiosis via engulfed prokaryotes establishing mutual benefit: energy production for host protection.
Statement II Analysis: Origins Reversed and Incorrect
Statement II claims “Blue-green algae thought to have originated the mitochondria, and bacteria have originated the chloroplasts.” This reverses reality.
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Mitochondria evolved from aerobic bacteria (likely alpha-proteobacteria or Rickettsia-like).
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Chloroplasts originated from cyanobacteria (blue-green algae).
Blue-green algae (cyanobacteria) gave rise to chloroplasts, not mitochondria; bacteria (aerobic) originated mitochondria. Thus, Statement II is false.
Evaluation of All MCQ Options
| Option | Description | Correct? | Reason |
|---|---|---|---|
| 1 | Both statements true | No | Statement II reverses origins |
| 2 | Both statements false | No | Statement I matches endosymbiotic evidence |
| 3 | Statement I true, II false | Yes | Morphological basis correct; origins swapped in II |
| 4 | Statement I false, II true | No | Statement I proven; II factually wrong |
Endosymbiotic Theory Evidence Summary
Key proofs include bacterial-like replication, genomes (mtDNA, cpDNA), and protein synthesis using fMet start codon. Eukaryotes acquired mitochondria first, then chloroplasts in plant lineages.
Correct Answer: 3. Statement I is true but Statement II is false
