Eukaryotic Cells

Cells can be divided into two distinct groups, those with nuclei and those without. Those with are called eukaryotes and those without are prokaryotes. Prokaryotic cells arose about 3.8 million years ago, when Earth was only 750 million years old (Cooper). During this time, the Earth's atmosphere was composed mostly of carbon dioxide and nitrogen, due to the abundance of carbon dioxide, prokaryotic cells began to use it in addition to sunlight to generate energy in a process called photosynthesis (Cooper). The using of carbon dioxide resulted in it being converted into oxygen, which was then released from the cells as a waste product, and this free oxygen floated towards the atmosphere, eventually allowing a great abundance of it (Cooper).

Endosymbiosis is a process of which an organism enters into the body of another organism for the sake of survival (Archibald). While eukaryotic and prokaryotic cells share a basic makeup of a membrane, DNA, and cytoplasm, eukaryotic cells are larger and much more complex, as they contain small specialized structures called organelles (Cooper). These organelles are actually prokaryotic cells, or descendants of prokaryotic cells, that entered into the membrane of another, larger prokaryote and was not expelled or digested, instead it gained shelter from the other cell as they both managed to benefit from each other (Archibald). Because of this beneficial relationship, both were able to reproduce unimpeded (Cooper). This theory is supported by two organelles in particular, mitochondria and chloroplasts (Cooper). They both contain their own genome and cell membrane that separates them from the eukaryotic cells they inhabit, and they reproduce just like prokaryotes do and have the same structure (Cooper). These organelles can no longer survive on their own however, as they have become completely dependent on their eukaryotic hosts (Cooper). Just as their eukaryotic hosts are incapable of properly functioning without them and cannot replace their functions with other organelles (Archibald). It is possible that the other organelles were prokaryotes that have become so integrated with their hosts that they can no longer act like prokaryotes