How did mitochondria and chloroplasts most likely arise?
Multiple Choice
They budded off from other organelles like the endoplasmic reticulum.
They arose spontaneously from compounds within the cells.
They arose from viruses that were engulfed and became incorporated into the cell.
They arose from bacteria that were engulfed and not digested.
The Correct Answer and Explanation is:
Correct Answer:
They arose from bacteria that were engulfed and not digested.
Explanation
The most widely accepted explanation for the origin of mitochondria and chloroplasts is the endosymbiotic theory. According to this theory, these organelles originated from free-living bacteria that were engulfed by a primitive eukaryotic cell over a billion years ago. Instead of being digested, the engulfed bacteria formed a symbiotic relationship with the host cell, eventually evolving into the organelles we recognize today.
Mitochondria are believed to have originated from aerobic (oxygen-using) proteobacteria, while chloroplasts are thought to have evolved from photosynthetic cyanobacteria. These engulfed bacteria provided the host cell with advantages—mitochondria enhanced energy production through aerobic respiration, and chloroplasts allowed cells to harness light energy via photosynthesis.
Several lines of evidence support the endosymbiotic theory:
- Double Membranes: Both mitochondria and chloroplasts have double membranes, consistent with the engulfing mechanism.
- Own DNA: They possess their own circular DNA, which is similar to bacterial genomes, not the linear DNA found in the nucleus of eukaryotic cells.
- Ribosomes: Their ribosomes resemble those found in bacteria (70S) rather than the eukaryotic cytoplasmic ribosomes (80S).
- Reproduction: Mitochondria and chloroplasts replicate by binary fission, a process characteristic of bacteria.
- Phylogenetic evidence: Genetic analyses show that mitochondrial DNA is closely related to alpha-proteobacteria, and chloroplast DNA resembles that of cyanobacteria.
This symbiotic relationship was a major evolutionary event, enabling the rise of complex eukaryotic cells and, ultimately, multicellular life. It illustrates how cooperation between different species can lead to major biological innovations and diversification over evolutionary time.
