Final answer:
Structures such as chloroplasts and mitochondria in plant and animal cells may have originated from free-living bacteria through endosymbiosis. The relative sizes of prokaryotic cells, eukaryotic cells, and eukaryotic organelles support this theory.
Step-by-step explanation:
Scientists believe that structures such as chloroplasts and mitochondria found in plant and animal cells may have originally been free-living bacteria. Evidence supports the hypothesis that these organelles were acquired through endosymbiosis, in which a eukaryotic cell engulfed and maintained a symbiotic relationship with a prokaryotic cell. The relative sizes of prokaryotic cells, eukaryotic cells, and eukaryotic organelles provide further support for this argument. For example, mitochondria, which are derived from aerobic bacteria, are similar in size and structure to certain group of bacteria, with a length of 1 to 10 micrometers.