The endosymbiotic theory explains the origins of which of the following cellular structures?

• A. Mitochondria and chloroplasts
• B. The nucleus
• C. The plasma membrane
• D. Ribosomes

Answer: (A)

Verified step by step guidance

1. Understand the endosymbiotic theory: This theory proposes that certain organelles in eukaryotic cells originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells and formed a symbiotic relationship.
2. Identify the cellular structures involved: The theory specifically explains the origins of mitochondria and chloroplasts, which are double-membrane-bound organelles found in eukaryotic cells.
3. Examine the evidence supporting the theory: Mitochondria and chloroplasts have their own DNA, which is circular like prokaryotic DNA, and they replicate independently of the host cell. They also have ribosomes similar to those found in prokaryotes.
4. Compare the other options: The nucleus, plasma membrane, and ribosomes are not explained by the endosymbiotic theory. The nucleus is thought to have evolved through invagination of the plasma membrane, while ribosomes and the plasma membrane are universal structures found in both prokaryotic and eukaryotic cells.
5. Conclude that the correct answer is mitochondria and chloroplasts, as these are the organelles whose origins are explained by the endosymbiotic theory.