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Prokaryotes are single-celled organisms such as bacteria that have no distinct nucleus. In addition to the lack of a nucleus, prokaryotes lack many of the other small organelles found in the larger eukaryotic cells.

A typical prokaryote is bound by a plasma membrane and a cell wall. Within this double boundary, the fluid material inside the cell (the cytoplasm) is studded with small, rounded bodies called ribosomes. The ribosomes are composed of nucleic acids and proteins, and function in protein synthesis. The chromosomes containing the hereditary material of prokaryotes are concentrated within a region called the nucleoid. Because the nucleoid is not separated from the rest of the cytoplasm by a membrane, it is not considered a true nucleus. Dissolved in the cytoplasm of prokaryotes are the various chemicals needed by the cell to function.

Prokaryotes were the first organisms to evolve on Earth, predating eukaryotes in the fossil record by about one billion years. Appearing on Earth 3.5 billion years ago, the first prokaryotes were probably bacteria that performed photosynthesis (cyanobacteria), which is a process that produces carbohydrates from sunlight, water, and carbon dioxide.

Eukaryotes are thought to have evolved when cells engulfed prokaryotic cells, and incorporated them into their cytoplasm. Some of the eukaryotic organelles, particularly mitochondria (the organelle that contains energy-producing enzymes) and chloroplasts (the organelle that contains photosynthetic enzymes in photosynthetic cells) resemble individual free-living prokaryotic cells. Supporting this theory (called the endosymbiotic theory) is the fact that mitochondria and chloroplasts have their own DNA sequences, as if they were once separate organisms in their own right.

Prokaryotes are divided taxonomically into two large goups: the archaebacteria and the eubacteria. Archaebacteria are probably little changed from the organisms that first evolved billions of years ago. They are capable of living in extremely harsh environments, such as salt marshes, hot springs, or even beneath the ice. Eubacteria evolved later. Some are photosynthetic bacteria; some are chemosynthetic bacteria, making carbohydrates from other chemicals besides carbon dioxide; and some are heterotrophic bacteria, deriving nutrients from the environment. Heterotrophic prokaryotes include some pathogens, bacteria that cause diseases, such as pneumonia, food poisoning, and tuberculosis.

Two typical prokaryotic cells: a blue-green alga and a bacteria. Illustration by Hans & Cassidy. Courtesy of Gale Group.

See also Eukaryotae.

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