Eukaryota

The Eukaryota are one of the two major groups of biological organisms. The other is the Prokaryota, which contains the eubacteria and archae-bacteria.

Differences Between Eukaryotic and Prokaryotic Cells

The key feature of all eukaryotes is that they possess eukaryotic cells. These differ from prokaryotic cells in several important respects. Eukaryotic cells are more complex and highly organized than prokaryotic cells. They are also, on average, ten times larger. Only eukaryotic cells have membrane-bound organelles. The organelles are separated from the cytoplasm by plasma membranes.

In eukaryotic cells, the DNA is contained within a nucleus, an organelle bound by a double membrane. Eukaryotic DNA is linear, with a beginning and an end, and is divided into a number of separate chromosomes. Prokaryotic cells, on the other hand, have only a single circular-shaped chromo-some of DNA that is not contained in a nucleus. One consequence of this difference is that eukaryotes can have much larger amounts of DNA than prokaryotes, which is necessary for the evolution of complex organisms.

Other important eukaryotic organelles include the mitochondria, which are responsible for the cell's metabolism (the conversion of food into usable energy resources in the form of ATP), and the chloroplasts, which allow those species engaging in photosynthesis to use light energy to fix carbon, that is, to take in atmospheric carbon in the form of carbon dioxide and incorporate it into organic molecules for use by the organism. While both of these functions are also performed by prokaryotes, the machinery for these processes is not organized into organelles.

Eukaryotic cells also have an extensive system of internal membranes, including the endoplasmic reticulum (responsible for the synthesis of proteins) and the Golgi apparatus (responsible for processing and packaging proteins from the endoplasmic reticulum), which are not found in prokaryotic cells. Finally, unlike prokaryotes, not all eukaryotes have cell walls, and the cell walls of eukaryote species are composed of different materials from prokaryotic cell walls.

The Endosymbiotic Theory of the Origin of Eukaryotic Cells

Because of the relative simplicity of prokaryotic cells, it has long been supposed that they preceded eukaryotic cells in time. The endosymbiotic hypothesis for the origin of eukaryotic cells was first proposed by Lynn Margulis in her 1981 book, Symbiosis in Cell Evolution. The endosymbiotic hypothesis ("endo" means "within," "symbiosis" is a situation in which organisms live together in close association) suggests that eukaryotic cells arose when certain prokaryotic cells acquired endosymbionts, in this case other prokaryotic cells that lived within them. It is believed that the endosymbionts derived benefits from the host such as protection and organic nutrients, while the host obtained ATP (from the prokaryotes which evolved into mitochondria) or access to the products of photosynthesis (from the prokaryotes which evolved into chloroplasts).

What were these prokaryotic symbionts? Likely candidates have been identified: mitochondria may have originated from endosymbiotic aerobic bacteria, while chloroplasts probably arose from endosymbiotic cyanobacteria, a prokaryotic group that had already evolved photosynthesis. There is considerable evidence supporting this hypothesis. Within eukaryotic cells, mitochondria and chloroplasts both have their own genetic material, separate from that of the nuclear DNA. It is in the form of a single circular chromosome, much like that seen in prokaryotes. Also, mitochondria and chloroplasts possess their own protein-synthesizing machinery, which again resembles that found in prokaryotes.

The evolution of the eukaryotic cell represented an advance in the degree of complexity present in cells. It allowed for the evolution of further complexity of organization, including multicellular organisms. span>

see also Prokaryota.

Jennifer Yeh

Bibliography

Curtis, Helena. Biology. New York: Worth Publishers, 1989.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.

Margulis, Lynn. Symbiosis in Cell Evolution: Life and Its Environment on the Early Earth. San Francisco: W. H. Freeman, 1981.

ATP, or adenosine-triphosphate, is the organic molecule that forms the basis of energy in all living organisms. ATP is used in all cellular processes that require energy.