What concept suggests that small bacteria were engulfed by larger cells to become mitochondria and help to self flourish and become a eukaryotic cell

Endosymbiotic theory

Explanation:

The concept that hypothesized that small bacteria were engulfed by larger cells to become the mitochondrion  and eventually become a eukaryotic cell is known as symbiogenesis or the endosymbiotic theory.

The endosymbiotic theory argues that organelles like mitochondrion and the chloroplast found in modern-day eukaryotic cells were formerly prokaryotic microbes. These microbes got ingested by primitive, amoeba-like eukaryotic cells that were first formed but instead of dying, the engulfed microbes found a way to symbiotically coexist with their hosts, eventually forming the organelles - aerobic microbes forming the mitochondrion and photosynthetic microbes formed the chloroplast.