membrane, this serve to increase the surface area of membrane on which membrane-bound reactions can take place. The existence of this double membrane has led many biologists to theorize that mitochondria are the descendants of some bacteria that was endocytosed by a larger cell billions of years ago, but not digested. This fascinating theory of symbiosis, which might lend an explanation to the development of eukaryotic cells, has additional supporting evidence. Mitochondria have their own DNA and their own ribosome's; and those ribosomes are more similar to bacterial ribosomes than to eukaryotic ribosomes.
             These organelles are the site of photosynthesis in plants and other photosynthesizing organisms. They also have a double membrane. There is a more complete description of the chloroplast here, in the chapter on photosynthesis.
             The ER is the transport network for molecules targeted for certain modifications and specific final destinations, as opposed to molecules that are destined to float freely in the cytoplasm. There are two types of ER, rough and smooth. Rough ER has ribosomes attached to it, and smooth ER does not.
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