The eukaryotes () are cellular organisms of the domain Eukaryota or Eukarya, which have a distinct cell nucleus (formed by a nuclear membrane), membrane-bound organelles and organization supported by a cytoskeleton. Extant eukaryotes are traditionally grouped into four kingdoms: animals, plants, fungi and protists.
"Protists" are a basal paraphyletic group that includes all algae, amoebae, flagellates and ciliates; however, this is now considered a wastebasket taxon and has been replaced with newer classifications that better represent the phylogeny of these organisms. As part of the prevailing three-domain system, eukaryotes constitute a major group of organisms alongside the two domains of prokaryotes: Bacteria and Archaea.
Cladistically, eukaryotes emerged within the archaeal phylum Promethearchaeota, colloquially also known as "Asgard archaea". Ignoring mitochondrial DNA (which comes from a bacterial lineage rather than an archaeal one), this would imply only two domains of life, Bacteria and Archaea, with eukaryotes incorporated as a clade under the Archaea.
Eukaryotes first emerged during the Paleoproterozoic, and the leading evolutionary theory is that they were the result of a metabolically mutualistic symbiogenesis between an anaerobic Promethearchaeota archaeon and an endosymbiont aerobic alphaproteobacterium (i.e. proto-mitochondrion) in response to the oxidative stress following the Great Oxygenation Event. A second symbiogenetic event between an eukaryotic host (a protist) and a beta-cyanobacteria endosymbiont during the Mesoproterozoic led to the latter becoming chloroplasts, and resulted in the evolution of the photosynthetic clade Archaeplastida, which includes all green algae, red algae, glaucophytes and land plants.
Eukaryotes can be unicellular or multicellular, and their cell sizes are significantly (hundreds of times) larger than prokaryotes. They have biomembranes enclosing their nucleus and organelles such as the endoplasmic reticulum, Golgi apparatus, ribosome, vacuole and lysosome, as well as endosymbiont-derived organelles (which are bound by two or more layers of membrane) such as mitochondria and various plastids. Eukaryotes can reproduce both asexually through mitosis, and sexually through meiosis and gamete fusion (fertilization). Due to their larger size and more complex cellular architecture, eukaryotes are capable of morphogenesis and phagocytosis, which enables microscopic predation of other unicellular organisms. Multicellularity in some eukaryotes led to cell differentiation and functional specialization, which led to the formation of various organs and tissues and the development of organ systems. Eukaryotes represent only a small minority of the total number of organisms on Earth, but due to their generally much larger sizes, their collective global biomass is much greater than that of prokaryotes.
Etymology The word eukaryote is derived from the Greek words "eu" (εὖ) meaning "true" or "good" and "karyon" (κάρυον) meaning "nut" or "kernel", referring to the nucleus of a cell.
Diversity
Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometers across, to animals like the blue whale, weighing up to 190 tonnes and measuring up to long, or plants like the coast redwood, up to tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to long. The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species. Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms, but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth.
The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms. Multicellularity in some form has evolved independently at least 25 times within the eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds, have evolved within only six eukaryotic lineages: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.
Distinguishing features
Nucleus The defining feature of eukaryotes is their membrane-bound nucleus, distinguishing them from prokaryotes that lack a nucleus. Eukaryotic cells are often much larger and contain more genetic information than bacterial cells. This genetic information is divided into multiple linear molecules of DNA called chromosomes, which are contained inside the nucleus.
Chromosomes are packaged into a highly organized structure called chromatin by histone proteins. Chromatin is further organized by other DNA binding proteins, which regulate the folded shape of chromosomes. During mitotic division, these chromosomes compacted and separated into two matching sets by a microtubular spindle.
Internal membranes
Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis.
The nucleus is surrounded by a double membrane known as the nuclear envelope, with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, which is involved in protein transport and maturation. It includes the rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (cisternae), the Golgi apparatus.
Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in the cytoplasm.
Mitochondria
Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell", for its function providing energy by oxidizing sugars or fats to produce the energy-storing molecule ATP. Mitochondria have two surrounding membranes, each a phospholipid bilayer, the inner of which is folded into invaginations called cristae where aerobic respiration takes place.
Mitochondria contain their own DNA, which is circular like bacterial DNA, from which it originated, and which encodes rRNA and tRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.
Some eukaryotes, such as the metamonads Giardia and Trichomonas, and the amoebozoan Pelomyxa, appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes, having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. It is thought that mitochondria developed from prokaryotic cells which became endosymbionts living inside eukaryotes.
Plastids
Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from other eukaryotes through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.
Cytoskeletal structures
The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins. These include α-actinin, fimbrin, and filamin in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, make the network dynamic.
Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or multiple shorter structures called cilia. These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin, and are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in many stramenopiles. Their interior is continuous with the cell's cytoplasm.
Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division.
Cell wall
The cells of plants, algae, fungi and most chromalveolates, but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell.
The major polysaccharides making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.
Sexual reproduction
Eukaryotes have a life cycle that involves sexual reproduction, alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa, to form a zygote; this may grow into a body, with its cells dividing by mitosis, and at some stage produce haploid gametes through meiosis, a division that reduces the number of chromosomes and creates genetic variability. There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases. Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio.