eukaryotesorganisms

Eukaryotes are organisms whose cells contain a nucleus bounded by a nuclear envelope and a suite of membrane-bound organelles. This cellular organization distinguishes them from prokaryotes, whose cells lack a defined nucleus. Eukaryotes include animals, plants, fungi, and a diverse array of protists. They can be unicellular or multicellular and range from microscopic algae to large land animals.

Key features of eukaryotic cells include an internal cytoskeleton, an endomembrane system, and specialized compartments such as mitochondria where energy is produced. In many groups, plastids such as chloroplasts enable photosynthesis. The genome is organized into multiple linear chromosomes within the nucleus, and gene expression is regulated by complex processes across the nucleus and cytoplasm.

Most eukaryotes reproduce sexually, through meiosis and fertilization, which creates genetic diversity. Asexual reproduction via mitosis is also common. Life cycles vary by lineage and can include stages with alternating generations, as seen in plants and some algae and protists.

Traditional broad groups include animals, plants, fungi, and protists, though modern phylogenies recognize major divisions, or clades, within the eukaryotic tree. Eukaryotes occupy nearly every habitat on Earth and play central roles in ecosystems as producers, consumers, decomposers, and symbionts.

Origin and evolution date eukaryotes to at least 1.6–2.1 billion years ago, with mitochondria arising from a mitochondrial bacterial endosymbiont and, in many lineages, chloroplasts arising from cyanobacterial endosymbionts. This endosymbiotic origin explains many core eukaryotic features, including energy metabolism and cellular complexity.