Eukaryote: organisms with complex, membrane-bound cells

Cellular organization and organelles

Eukaryotic cells commonly contain a suite of membrane-bound organelles: mitochondria for energy metabolism, an endoplasmic reticulum and Golgi apparatus for synthesis and trafficking of proteins and lipids, and, in photosynthetic lineages, chloroplasts. The cytoskeleton of microtubules, actin filaments and intermediate filaments supports cell shape, intracellular transport and mitotic chromosome movements. Some organelles originated by endosymbiosis, a process in which once-free bacteria became permanent intracellular partners; this explains the bacterial ancestry of mitochondria and chloroplasts.

Genetics and reproduction

Control of cellular function in eukaryotes depends on nuclear transcription, RNA processing and regulated protein synthesis. Many eukaryotes reproduce asexually by mitotic division; others also undergo meiosis and sexual reproduction, which reshuffles genetic variation and can facilitate adaptation. Horizontal gene transfer and other genetic exchanges occur in some groups, contributing to genomic complexity.

Diversity: major groups and examples

The eukaryotic domain includes multicellular lineages and numerous single-celled forms. Major familiar groups are animals, plants, fungi and a broad collection of mostly unicellular protists, which include many algae. By contrast, bacteria and archaea lack nuclei and most membrane-bound organelles and are referred to collectively as prokaryotes. Eukaryotes occupy nearly every habitat on Earth and fulfill roles as primary producers, consumers and decomposers.

Origins and fossil evidence

Fossil and molecular evidence indicates that eukaryotes arose during the Proterozoic eon, well after the origin of prokaryotic life. One of the oldest candidate eukaryote fossils is the filamentous form Grypania, originally dated near 2.1 billion and later revised to roughly 1.87 billion years. Microfossils called acritarchs appear in Mesoproterozoic sediments (Mesoproterozoic era) and are often interpreted as resting cysts or reproductive stages of single-celled algae and plankton; they are associated in discussions with algal plankton and the early diversification of photosynthetic eukaryotes.

Classification and phylogeny

Eukaryotes are commonly considered a separate superkingdom or domain distinct from bacteria and archaea. Deeper relationships within Eukaryota are the subject of active research: molecular phylogenies and genomic comparisons have led to competing proposals about major classification schemes and the number and boundaries of higher taxa. Various proposed taxonomies differ in whether they recognize a few broad clades or many traditional kingdoms and subkingdom groups.

Ecological and practical importance

Eukaryotes include species essential for ecosystems and human societies: plants and algae produce oxygen and form the base of many food webs; fungi decompose organic matter and form symbioses with plants; animals include pollinators, predators and humans themselves. Many eukaryotic microbes are pathogens of plants, animals and people, while others are sources of beneficial products, model organisms for biomedical research and tools in biotechnology.

Further reading and resources

• General overview of eukaryotes: organism entry.
• Cell structure and organelles: cellular architecture, genome organization.
• Fossils and deep time: entries on Grypania and acritarchs and Proterozoic context (Proterozoic, eon).
• Taxonomy and classification debates: classification resources and alternative taxonomies.
• Comparisons with other life forms: contrasts with bacteria, archaea and the collective term prokaryotes.

This summary introduces central concepts but does not replace detailed texts and primary research. For deeper study consult specialized reviews, textbooks and curated databases cited above (classification, general overview, Grypania, acritarchs).