Question

fill in the blank question
the endosymbiotic theory for mitochondria has been modified to suggest that the host became dependent on the

gas produced by
the symbiont.
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Explanation:

The modified endosymbiotic theory for mitochondria proposes that the ancestral symbiont (the precursor to mitochondria) was an aerobic bacterium that produced oxygen, and the host cell became dependent on this oxygen for its metabolic processes.

Answer:

Oxygen