eukariotid

A eukariotid, or eukaryote, is any organism whose cells contain a nucleus enclosed by a membrane and a range of other membrane-bound organelles. Eukariotids include animals, plants, fungi, and numerous protists, as well as many microscopic and colonial forms. They are distinguished from prokaryotes by their cellular complexity, internal compartmentalization, and larger cell size.

Key features of eukariotids include a defined nucleus that houses linear chromosomes organized with histones, and an extensive endomembrane system that includes the endoplasmic reticulum and Golgi apparatus. The energy factories, mitochondria, are present in most lineages, and chloroplasts are found in photosynthetic groups through plastid ancestry. The cytoskeleton provides shape and motors for movement and division. Most eukariotids exhibit organized cell division via mitosis, and many also undergo meiosis as part of sexual reproduction. While some lineages have lost mitochondria or chloroplasts through evolution, these organelles or their descendants often remain essential.

Diversity among eukariotids is vast. They range from single-celled organisms to complex multicellular beings. Nutritional modes are varied, including photoautotrophy, heterotrophy, and mixotrophy. Traditional classifications group eukariotids into the kingdoms Animalia, Plantae, Fungi, and numerous protists, though modern taxonomy emphasizes clades and genetic relationships, with protists forming a paraphyletic assemblage that spans several lineages.

Origin and evolution of eukariotids are linked to endosymbiosis. Mitochondria are widely accepted to originate from an ancestral alpha-proteobacterium, and plastids from cyanobacteria in photosynthetic relatives. This endosymbiotic origin, alongside innovations such as the nuclear envelope and cytoskeleton, enabled the diverse and complex biology characteristic of eukariotids.