eukaryote

An eukaryote is an organism whose cells contain a nucleus and other membrane-bound organelles. Eukaryotes form a major lineage in taxonomy, commonly grouped under Eukarya (or Eukaryota). Their cells are characterized by compartmentalization, linear chromosomes housed within a nucleus, and an extensive endomembrane system that includes the endoplasmic reticulum and the Golgi apparatus.

A key feature is the presence of mitochondria, which generate most cellular ATP. In plants, algae, and some other groups, chloroplasts enable photosynthesis, reflecting ancient endosymbiosis with cyanobacteria. The cytoplasm is enclosed by a plasma membrane, and many plants and fungi possess a rigid cell wall (cellulose in plants; chitin in fungi); animals generally do not.

The nucleus is surrounded by a double membrane with nuclear pores for controlled exchange. The cytoskeleton, composed of actin filaments, intermediate filaments, and microtubules, provides structural support, drives movement, and organizes intracellular transport. Ribosomes are 80S in the cytoplasm, while organelles such as mitochondria and chloroplasts contain their own smaller ribosomes.

Eukaryotes reproduce through mitosis, yielding two genetically identical daughter cells, and many lineages also use meiosis and fertilization to generate genetic diversity. They range from single-celled protists to the multicellular plants, animals, and fungi that dominate most ecosystems.

Eukaryotes are believed to have evolved from prokaryotic ancestors more than a billion years ago, with endosymbiotic events giving rise to mitochondria and, in the lineage leading to plants and some algae, chloroplasts. This compartmentalized cellular architecture distinguishes eukaryotes from prokaryotes.