Endosymbiotic Theory
Endosymbiotic theory explained in an easy to understand way:
Imagine you move into a new apartment and bring along a friend who was supposed to stay for just a few days but ends up becoming your permanent roommate because they help with rent and chores. This situation is similar to the endosymbiotic theory, which suggests that mitochondria and chloroplasts started as independent organisms that entered into a host cell and ended up staying because they provided valuable benefits. Just as your friend becomes an integral part of your living arrangement by contributing to household needs, mitochondria and chloroplasts were once independent cells that became permanent residents within prokaryotic cells, forming a mutually beneficial relationship that resulted in the complex eukaryotic cells we see today.
Practice Version
Endosymbiotic Theory: States that mitochondria and chloroplasts evolved from eukaryotic cells that entered prokaryotic cells. Endosymbiotic theory. This theory suggests that some organelles in eukaryotic cells originated from ancient symbiotic relationships where one cell lived inside another.