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Eukaryotae, or eukaryotic cells, are large and complex cells bounded by an outer plasma membrane. They contain many organelles within their cytoplasm and a nucleus separated from the cytoplasm by the nuclear membrane. Fossils of eukaryotic cells are present in rocks dated as 1.5 billion years old. All living things on Earth, except bacteria and blue-green algae (cyanobacteria), which are Prokaryotae, are composed of eukaryotic cells.

The nucleus of a eukaryotic cell is a membrane-bound compartment containing genetic information in the form of DNA organized into chromosomes. The nuclei of eukaryotic cells divide by mitosis, a process which results in two daughter nuclei that are identical to the parent cell. The cell's nucleus directs its overall functioning, while the membrane-bound organelles in the cytoplasm carry out a variety of specialized jobs in a coordinated fashion.

In plants, organelles called chloroplasts trap the energy from sunlight in a process called photosynthesis. Plants then use that energy to drive metabolic pathways. In both animal and plant eukaryotic cells, the cellular energy is generated by organelles called mitochondria. Other organelles, the lysosomes, are membrane-bound packages of digestive enzymes. These digestive enzymes, and other proteins, are manufactured in the ribosomes located on the rough endoplasmic reticulum, a kind of cellular highway. An organelle called the Golgi complex then moves the enzymes—and other proteins—into the membranes and distributes them.

Some eukaryotic cells have a flagellum, whip-like projection from the cell membrane that aids in the cell's locomotion. Others may have cilia, shorter, hair-like strands arranged around the perimeter of the cell in a characteristic way. The cilia of prokaryotic cells are less complex than those of eukaryotic cells.

The types and arrangement of a cell's organelles enable eukaryotic cells of multicellular organisms to perform specialized functions. In humans, the eukaryotic cells of a number of organs are highly specialized, but nevertheless maintain most of the defining features of the eukaryotic cell. For example, the cells of the brain, liver, bone, muscle of a growing baby divide by mitosis under the control of the DNA in the nucleus, with the liver cells producing more liver cells, and bone cells producing other bone cells.

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Science EncyclopediaScience & Philosophy: Ephemeris to Evolution - Historical Background