Eukaryote

Eukaryotes are organisms consisting of one or more eukaryotic cells, the most distinctive feature of which is the presence of one or more membrane-bound nuclei. Animals, plants, fungi, and various other groups collectively referred to as Protista are all eukaryotes, varying from single-celled organisms to truly multicellular forms, in which different cells are specialized for different tasks and in general do not survive when isolated. In the three-domain system, eukaryotes are treated as one domain, referred to as the Eukaryota or Eukarya. Members of the other two domains, the Bacteria and Archaea, are microorganisms with prokaryotic cells.

Cell structure

Eukaryotic cells are generally larger than prokaryotes, with a difference in volume on the order of a thousand times. The cell includes a variety of different structures, collectively referred to as organelles. The nucleus is characteristically surrounded by a double membrane, with pores allowing materials to pass in and out. It contains the genetic material, which is separated into a series of DNA strands called chromosomes. The remainder of the cell, called the cytosol, is supported by a cytoskeleton.

Tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, or ER. This includes regions that are smooth and others that are rough, where some of the ribosomes are attached, rather than all being distributed throughout the cell as in prokaryotes. Proteins synthesized by attached ribosomes enter the ER, and are thereby distributed throughout the cell, generally budding out into vesicles. In most eukaryotes, there is also a Golgi apparatus composed of stacked vesicles where proteins are modified. There may also be various vacuoles and other structures. Collectively these make up the endomembrane system.

Various other structures occur. Aside from a few protists, all eukaryotes have mitochondria, which are involved in catabolism and in particular in using oxygen. Plants and the various lines of algae also have chloroplasts, which conduct photosynthesis. Both of these are believed to represent derived prokaryotic endosymbionts, in the case of chloroplasts probably incorporated at several different times in different groups (see perhaps endosymbiotic hypothesis). Many eukaryotes have flagella or cilia, projections of the cell surface supported by microtubules that are typically motile or sensory. These arise from basal bodies or kinetosomes, which have a characteristic structure, also found in centrioles which in some groups assist in nuclear division. A few other microtubule supported structures are found, such as the axopodia of some amoeboids.

Reproduction

Nuclear division is often coordinated with cell division. This generally takes place by mitosis, a process which allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes there is also a process of sexual reproduction, typically involving an alternation between haploid generations, where only one copy of each chromosome is present, and diploid generations, where two are present, occurring through nuclear fusion (syngamy) and meiosis. There is considerable variation in this pattern, however.