Theory about the origin of several major organelles inside eukaryotic cells such as normal plant and animal cells. It is believed that mitochondria (which cell respiration occurs) and chloropasts (where photosynthesis occurs in plants) were originally prokaryotic organisms that entered an eukaryotic cell and formed a symbiotic relation ship, with the host providing suitable conditions for the organelle, and the organelle enabling the cell to use new energy sources (aerobic respiration in mitochondria, photosynthesis in chloroplasts). There are several indications that this occurs, such as the fact that both bacteria(thought to be the closest relation to the ancestors of mitochondria) and mitochondria both have their own circular DNA, and that several other characteristics are similar (ribosomes, size etc) Also, looking at prokaryotic photosythesising organisms such as algae, the photosyntyhesising enzymes have been found in the cytoplasm, not in a seperate organelle. Cyanobacteria,or Blue-green algae are thought to be a close relative of the organism that became the chloroplast.