Symbiosis: two organisms live closely together (Example: Nemo living with a sea anemone)

Endosymbiosis: one organism lives inside another (bacteria in us)

Theory explaining why mitochondria and chloroplasts are found in cells:

It's thought that mitochondria (m) and chloroplasts (c) were types of prokaryotic cells that lived inside other cells. The m & c helped keep the cell alive and so were retained.

Evidence on relationship of mitochondria and chloroplasts to prokaryote cells:

a. mitochondria and chloroplasts are both surrounded by double membranes (like bacteria are).

b. mitochondria and bacteria have similar size.

c. mitochondrial ribosomes resemble bacterial ribosomes.

d. mirochondria and chloroplast DNA is circular like bacteria.

e. mitochondria divides by simple fission like bacteria.