Why are mitochondria absent in bacteria?

Mitochondria are believed to have originated from a symbiotic relationship between an ancestral eukaryotic cell and an engulfed bacterium. This theory, known as endosymbiotic theory, proposes that a eukaryotic host cell engulfed a free-living bacterium, forming a mutually beneficial relationship. Over time, the engulfed bacterium evolved into the mitochondrion, becoming an essential organelle within eukaryotic cells.

However, bacteria themselves do not have mitochondria. This is because bacteria are prokaryotes, which are simple, single-celled organisms that lack membrane-bound organelles, including mitochondria. Instead, bacteria generate energy through various metabolic processes, such as anaerobic respiration or fermentation, occurring within their cytoplasm and cell membrane.

Bacteria possess their own specialized structures for energy production and other cellular processes. For example, some bacteria have infoldings of their plasma membrane called mesosomes, which play a role in energy production and cellular respiration. Other bacterial species may have specialized compartments called carboxysomes that aid in carbon dioxide fixation during photosynthesis.

While mitochondria are absent in bacteria, they possess their own unique cellular structures and mechanisms to carry out essential metabolic functions. The presence of mitochondria is a distinguishing feature of eukaryotic cells and is thought to have played a crucial role in the evolution and complexity of eukaryotes.