The endosymbiotic theory

The endosymbiotic theory explains the evolution of the eukaryotic cell and eukaryotic organelles by phagocytocis of small prokaryotic cells. This theory states that some of the organelles in today’s eukaryotic cells were once prokaryotic bacteria.

In this theory, the first eukaryotic cell was probably an amoeba-like nucleated (probably DNA in a nucleoid region not an actual nucleus) prokaryotic cell that got nutrients by phagocytosis (engulfing nutrients or other cells) Some of these unicellular amoeba-like organisms engulfed prokaryotic cells that somehow were not digested within the organism.

In the process of being engulfed the smaller cells would have been wrapped in membrane from the larger cell, today we see double membranes in mitochondria and chloroplasts. The symbiotic relationship was beneficial because the host cell would have provided essential nutrients to the engulfed prokaryotic cell in exchange the smaller prokaryotic cell used these nutrients to synthesize ATP molecules, this ATP was used as an energy source by the host cell. The smaller prokaryotic cell was given a safe environment as well as receiving nutrients from the larger host cell.