if there is no mitochondria in bacteria then from which it gets energy to perform its function
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The conversation is a bit confusing but maybe this will help a bit (1) optimizing *efficiency* of energy production is not the single most important factor driving evolution of single cell organisms or functionality of cells in complex organisms. Efficiency requires complexity and additional energy expenditure. Sometimes all you need is a certain level of efficiency. If all you do is multiply in a reasonably nutrient rich environment, why bother with unneeded genetic complexity? (2) mitochondria vary from organism to organism and tissue to tissue. They evolved to adapt to their cell environment over hundreds of millions of years. Some are more efficient than others. ATP production may not always be their prime or most demanding function under certain circumstances. Stem cells are glycolytic. Adipose tissue during hibernation are uncoupled. Etc. Aerobic bacteria do just fine in their environments. Protoeukaryotic host of protomitochondria was probably more concerned with detoxifying O2 than having more ATP. That was a bonus