What does the endosymbiont theory suggest about organelles like chloroplasts?

The endosymbiont theory posits that certain organelles, particularly chloroplasts and mitochondria, originated from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells. According to this theory, instead of being digested, these engulfed prokaryotes formed a symbiotic relationship with the host cell, providing benefits such as the ability to perform photosynthesis or aerobic respiration. Over time, these prokaryotes became an integral part of the eukaryotic cell, evolving into the organelles we observe today.

This concept is supported by several pieces of evidence, including the presence of double membranes around these organelles, their own circular DNA similar to that of bacteria, and their ability to replicate independently of the eukaryotic cell cycle through a process similar to binary fission. Thus, the assertion that chloroplasts were once free-floating prokaryotes that were engulfed by other cells accurately reflects the key idea behind the endosymbiont theory.