Hypothesis regarding mitochondria

In fact, the most accredited (相信) hypothesis traces the origin of the mitochondrion to a sort of infection of the ancient eukaryotic cell, which millions of years ago managed not only to survive but even to swallow, without destroying it, an aerobic bacterium that knew how to deal with oxygen, i.e., being able to use oxygen to produce energy, establishing a symbiotic relationship of mutual benefit.

Mitochondria are surrounded by a double-membrane system, consisting of inner and outer mitochondrial membranes separated by an intermembrane space. The inner membrane forms numerous folds (cristae), which extend into the interior (or matrix) of the organelle.

Source: https://www.ncbi.nlm.nih.gov