Eukaryotae

Eukaryotae, or eukaryotic cells, are cells that have their genetic material contained within a specialized membrane (the nuclear membrane) that is located inside the cell.

Eukaryotic cells are ancient. 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 that 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, whiplike projection from the cell membrane that aids in the cell’s locomotion. An example of such a cell are sperm. Other cells may have cilia, shorter, hairlike strands arranged around the perimeter of the cell in a characteristic way. Cilia function to create a wave of movement over the surface, which can direct nutrients and wastes to an intended location.

Eukaryotic cells also contain structures that perform a certain function. Each of these represents an organelle. An example of an organelle is the mitochondrion, which generates energy. 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.