One piece of evidence supporting the endosymbiotic theory comes from examination of membranes surrounding the chloroplasts of microbial eukaryotes that have a history of secondary and/or tertiary endosymbiosis. How might we expect the membranes in these cases to be different?

The endosymbiotic theory suggests that certain organelles, particularly mitochondria and chloroplasts, within eukaryotic cells were once free-living bacteria that were engulfed by a precursor to the modern eukaryotic cell. Over time, a symbiotic relationship developed, and these bacteria evolved into the organelles we see today.

One piece of evidence supporting this theory comes from the examination of the membranes surrounding the chloroplasts of microbial eukaryotes that have a history of secondary and/or tertiary endosymbiosis.

In primary endosymbiosis, a eukaryotic cell engulfs a free-living prokaryote, which then becomes an organelle within the host cell. This is thought to be how the first mitochondria and chloroplasts came to be. The engulfed prokaryote is surrounded by a double membrane – its own membrane and the portion of the host's membrane that was pinched off during engulfment.

In secondary endosymbiosis, a eukaryotic cell engulfs another eukaryotic cell that has already undergone primary endosymbiosis. The engulfed cell is then also surrounded by a double membrane, resulting in a total of four membranes around the organelle.

In tertiary endosymbiosis, the process is repeated again, with a eukaryotic cell engulfing a eukaryotic cell that has undergone secondary endosymbiosis. This results in even more layers of membranes.

Therefore, we would expect the membranes of chloroplasts in microbial eukaryotes that have undergone secondary or tertiary endosymbiosis to have more than two membranes. The exact number would depend on how many rounds of endosymbiosis the cell has undergone. This is one way in which the membranes in these cases might be different.