1. The endosymbiotic theory is a scientific hypothesis that explains the origin of certain organelles within cells. Specifically, the endosymbiotic theory suggests that certain organelles, such as mitochondria and chloroplasts, were once free-living prokaryotic cells that were engulfed by larger host cells. These smaller cells were eventually able to survive and reproduce within the host cell, forming a symbiotic relationship. Over time, the host cell and the engulfed cell became increasingly dependent on each other, and the engulfed cell lost its ability to survive and reproduce on its own.
2. The functions of endosymbiotic organelles are closely related to the functions of their free-living ancestral counterparts. For example, mitochondria are responsible for generating energy within cells through the process of cellular respiration, a function that is also performed by certain types of prokaryotes. Chloroplasts, which are found in plant cells and some other eukaryotes, are responsible for photosynthesis, a process that is also carried out by certain types of photosynthetic prokaryotes.
There is strong evidence that supports the endosymbiotic theory. For example, mitochondria and chloroplasts have their own DNA and ribosomes, which are similar to those found in prokaryotes. Additionally, the genetic code used by mitochondria and chloroplasts is more similar to the genetic code used by prokaryotes than it is to the genetic code used by the host cell. These and other observations support the idea that these organelles were once independent prokaryotic cells that became integrated into larger host cells through a process of endosymbiosis.