The bacterium Escherichia coli (E. coli), is a single-celled prokaryote
False-color scanning electron micrograph of the unicellular coccolithophore Gephyrocapsa oceanica
An amoeba is a single-celled eukaryote
title page of Gmelin's Historia Fucorum, dated 1768
Polypore fungi and angiosperm trees are large multicellular eukaryotes.
The kelp forest exhibit at the Monterey Bay Aquarium: A three-dimensional, multicellular thallus
Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, a paper in the scientific journal Nature suggested that these 3.5 Gya (billion years old) geological formations contain fossilized cyanobacteria microbes. This suggests they are evidence of one of the earliest known life forms on Earth.
Rock lichens in Ireland
LUCA may have used the Wood–Ljungdahl or reductive acetyl–CoA pathway to fix carbon.
Floridian coral reef
Algae on coastal rocks at Shihtiping in Taiwan
Phytoplankton, Lake Chūzenji
Harvesting algae
Seaweed-fertilized gardens on Inisheer
Dulse, a type of edible seaweed
Algae bladder

Algae (singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms.

- Algae

Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles (such as mitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived from endosymbiotic bacteria).

- Organism

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A Caenorhabditis elegans stained to highlight the nuclei of its cells

Multicellular organism

A Caenorhabditis elegans stained to highlight the nuclei of its cells

A multicellular organism is an organism that consists of more than one cell, in contrast to a unicellular organism.

All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.


The endomembrane system and its components
Simplified structure of a mitochondrion
Longitudinal section through the flagellum of Chlamydomonas reinhardtii
Structure of a typical animal cell
Structure of a typical plant cell
Fungal Hyphae cells: 1 – hyphal wall, 2 – septum, 3 – mitochondrion, 4 – vacuole, 5 – ergosterol crystal, 6 – ribosome, 7 – nucleus, 8 – endoplasmic reticulum, 9 – lipid body, 10 – plasma membrane, 11 – spitzenkörper, 12 – Golgi apparatus
This diagram illustrates the twofold cost of sex. If each individual were to contribute the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation.
Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes
One hypothesis of eukaryotic relationships – the Opisthokonta group includes both animals (Metazoa) and fungi, plants (Plantae) are placed in Archaeplastida.
A pie chart of described eukaryote species (except for Excavata), together with a tree showing possible relationships between the groups
The three-domains tree and the Eocyte hypothesis
Phylogenetic tree showing a possible relationship between the eukaryotes and other forms of life; eukaryotes are colored red, archaea green and bacteria blue
Eocyte tree.
Diagram of the origin of life with the Eukaryotes appearing early, not derived from Prokaryotes, as proposed by Richard Egel in 2012. This view implies that the UCA was relatively large and complex.

Eukaryotes are organisms whose cells have a nucleus enclosed within a nuclear envelope.

Eukaryotic cells typically contain other membrane-bound organelles such as mitochondria and Golgi apparatus; and chloroplasts can be found in plants and algae.

Modern stromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.

Unicellular organism

Modern stromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.
A bottom-dwelling community found deep in the European Arctic.
Paramecium tetraurelia, a ciliate, with oral groove visible
A scanning electron microscope image of a diatom
Transmission electron microscope image of budding Ogataea polymorpha

A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells.

Many eukaryotes are multicellular, but some are unicellular such as protozoa, unicellular algae, and unicellular fungi.


Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes

A protist is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus.

Therefore, some protists may be more closely related to animals, plants, or fungi than they are to other protists; however, like the groups algae, invertebrates, and protozoans, the biological category protist is used for convenience.