The bacterium Escherichia coli (E. coli), is a single-celled prokaryote
An amoeba is a single-celled eukaryote
Polypore fungi and angiosperm trees are large multicellular eukaryotes.
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.
LUCA may have used the Wood–Ljungdahl or reductive acetyl–CoA pathway to fix carbon.

Organism is any organic, living system that functions as an individual entity.

- Organism
The bacterium Escherichia coli (E. coli), is a single-celled prokaryote

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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.

Diagram of a fly from Robert Hooke's innovative Micrographia, 1665


Scientific study of life.

Scientific study of life.

Diagram of a fly from Robert Hooke's innovative Micrographia, 1665
In 1842, Charles Darwin penned his first sketch of On the Origin of Species.
In the Bohr model of an atom, electrons (blue dot) orbit around an atomic nucleus (red-filled circle) in specific atomic orbitals (grey empty circles).
Model of hydrogen bonds (1) between molecules of water
Organic compounds such as glucose are vital to organisms.
A phospholipid bilayer consists of two adjacent sheets of phospholipids, with the hydrophilic tails facing inwards and the hydrophobic heads facing outwards.
The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of a hemoglobin protein
Structure of an animal cell depicting various organelles
Structure of a plant cell
Example of an enzyme-catalysed exothermic reaction
Respiration in a eukaryotic cell
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
In meiosis, the chromosomes duplicate and the homologous chromosomes exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploid gametes.
Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
Bases lie between two spiraling DNA strands.
The extended central dogma of molecular biology includes all the processes involved in the flow of genetic information.
Regulation of various stages of gene expression
Composition of the human genome
Construction of recombinant DNA, in which a foreign DNA fragment is inserted into a plasmid vector
Model of concentration gradient building up; fine yellow-orange outlines are cell boundaries.
Natural selection for darker traits
Comparison of allopatric, peripatric, parapatric and sympatric speciation
Bacteria – Gemmatimonas aurantiaca (-=1 Micrometer)
Archaea – Halobacteria
Diversity of protists
Diversity of plants
Diversity of fungi. Clockwise from top left: Amanita muscaria, a basidiomycete; Sarcoscypha coccinea, an ascomycete; bread covered in mold; chytrid; Aspergillus conidiophore.
Bacteriophages attached to a bacterial cell wall
Root and shoot systems in a eudicot
The xylem (blue) transports water and minerals from the roots upwards whereas the phloem (orange) transports carbohydrates between organs.
Reproduction and development in sporophytes
Negative feedback is necessary for maintaining homeostasis such as keeping body temperature constant.
Diffusion of water and ions in and out of a freshwater fish
Different digestive systems in marine fishes
Respiratory system in a bird
Circulatory systems in arthropods, fish, reptiles, and birds/mammals
Asynchronous muscles power flight in most insects. a: Wings b: Wing joint c: Dorsoventral muscles power upstrokes d: Dorsolongitudinal muscles power downstrokes.
Mouse pyramidal neurons (green) and GABAergic neurons (red)
Sexual reproduction in dragonflies
Cleavage in zebrafish embryo
Processes in the primary immune response
Brood parasites, such as the cuckoo, provide a supernormal stimulus to the parenting species.
Terrestrial biomes are shaped by temperature and precipitation.
Reaching carrying capacity through a logistic growth curve
A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.
Fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of the slow carbon cycle, such as volcanic and tectonic activity, are not included.
Efforts are made to preserve the natural characteristics of Hopetoun Falls, Australia, without affecting visitors' access.

For instance, all organisms are made up of cells that process hereditary information encoded in genes, which can be transmitted to future generations.


False-color scanning electron micrograph of the unicellular coccolithophore Gephyrocapsa oceanica
title page of Gmelin's Historia Fucorum, dated 1768
The kelp forest exhibit at the Monterey Bay Aquarium: A three-dimensional, multicellular thallus
Rock lichens in Ireland
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.

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.

The Tasmanian tiger (Thylacinus cynocephalus) is an example of an extinct species.


The Tasmanian tiger (Thylacinus cynocephalus) is an example of an extinct species.
External mold of the extinct Lepidodendron from the Upper Carboniferous of Ohio
Skeleton of various extinct dinosaurs; some other dinosaur lineages still flourish in the form of birds
The dodo of Mauritius, shown here in a 1626 illustration by Roelant Savery, is an often-cited example of modern extinction.
The passenger pigeon, one of the hundreds of species of extinct birds, was hunted to extinction over the course of a few decades.
Scorched land resulting from slash-and-burn agriculture
The golden toad was last seen on May 15, 1989. Decline in amphibian populations is ongoing worldwide.
The large Haast's eagle and moa from New Zealand
Tyrannosaurus, one of the many extinct dinosaur genera. The cause of the Cretaceous–Paleogene extinction event is a subject of much debate amongst researchers.
Georges Cuvier compared fossil mammoth jaws to those of living elephants, concluding that they were distinct from any known living species.
A great hammerhead caught by a sport fisherman. Human exploitation now threatens the survival of this species. Overfishing is the primary driver of shark population declines, which have fallen over 71% since 1970.

Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species.

All adult Eurasian blue tits share the same coloration, unmistakably identifying the morphospecies.


All adult Eurasian blue tits share the same coloration, unmistakably identifying the morphospecies.
A region of the gene for the cytochrome c oxidase enzyme is used to distinguish species in the Barcode of Life Data Systems database.
The cladistic or phylogenetic species concept is that a species is the smallest lineage which is distinguished by a unique set of either genetic or morphological traits. No claim is made about reproductive isolation, making the concept useful also in palaeontology where only fossil evidence is available.
A chronospecies is defined in a single lineage (solid line) whose morphology changes with time. At some point, palaeontologists judge that enough change has occurred that two species (A and B), separated in time and anatomy, once existed.
A cougar, mountain lion, panther, or puma, among other common names: its scientific name is Puma concolor.
The type specimen (holotype) of Lacerta plica, described by Linnaeus in 1758
Ernst Mayr proposed the widely used Biological Species Concept of reproductive isolation in 1942.
Palaeontologists are limited to morphological evidence when deciding whether fossil life-forms like these Inoceramus bivalves formed a separate species.
Horizontal gene transfers between widely separated species complicate the phylogeny of bacteria.
John Ray believed that species breed true and do not change, even though variations exist.
Carl Linnaeus created the binomial system for naming species.
Blackberries belong to any of hundreds of microspecies of the Rubus fruticosus species aggregate.
The butterfly genus Heliconius contains many similar species.
The Hypsiboas calcaratus–fasciatus species complex contains at least six species of treefrog.
Carrion crow
Hybrid with dark belly, dark gray nape
Hybrid with dark belly
Hooded crow
Seven "species" of Larus gulls interbreed in a ring around the Arctic.
Opposite ends of the ring: a herring gull (Larus argentatus) (front) and a lesser black-backed gull (Larus fuscus) in Norway
A greenish warbler, Phylloscopus trochiloides
Presumed evolution of five "species" of greenish warblers around the Himalayas

In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity.

Many of the internal organs of the human body

Organ (biology)

Organ is a collection of tissues joined in a structural unit to serve a common function.

Organ is a collection of tissues joined in a structural unit to serve a common function.

Many of the internal organs of the human body
The liver and gallbladder of a sheep
Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.
The flower is the angiosperm's reproductive organ. This Hibiscus flower is hermaphroditic, and it contains stamen and pistils.
Strobilus of Equisetum telmateia
Human viscera

Organs exist in most multicellular organisms.

Martinus Beijerinck in his laboratory in 1921


Martinus Beijerinck in his laboratory in 1921
Antigenic shift, or reassortment, can result in novel and highly pathogenic strains of human flu
A typical virus replication cycle
Some bacteriophages inject their genomes into bacterial cells (not to scale)
The Baltimore Classification of viruses is based on the method of viral mRNA synthesis
Overview of the main types of viral infection and the most notable species involved
Transmission electron microscope image of a recreated 1918 influenza virus
Two rotaviruses: the one on the right is coated with antibodies that prevent its attachment to cells and infecting them.
The structure of the DNA base guanosine and the antiviral drug acyclovir
Peppers infected by mild mottle virus
Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall
Scientist studying the H5N1 influenza virus

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.

Simplified view of the cellular metabolism


Simplified view of the cellular metabolism
Structure of adenosine triphosphate (ATP), a central intermediate in energy metabolism
Structure of a triacylglycerol lipid
This is a diagram depicting a large set of human metabolic pathways.
Glucose can exist in both a straight-chain and ring form.
Structure of the coenzyme acetyl-CoA.The transferable acetyl group is bonded to the sulfur atom at the extreme left.
The structure of iron-containing hemoglobin. The protein subunits are in red and blue, and the iron-containing heme groups in green. From.
A simplified outline of the catabolism of proteins, carbohydrates and fats
Mechanism of ATP synthase. ATP is shown in red, ADP and phosphate in pink and the rotating stalk subunit in black.
Plant cells (bounded by purple walls) filled with chloroplasts (green), which are the site of photosynthesis
Simplified version of the steroid synthesis pathway with the intermediates isopentenyl pyrophosphate (IPP), dimethylallyl pyrophosphate (DMAPP), geranyl pyrophosphate (GPP) and squalene shown. Some intermediates are omitted for clarity.
Effect of insulin on glucose uptake and metabolism. Insulin binds to its receptor (1), which in turn starts many protein activation cascades (2). These include: translocation of Glut-4 transporter to the plasma membrane and influx of glucose (3), glycogen synthesis (4), glycolysis (5) and fatty acid synthesis (6).
Evolutionary tree showing the common ancestry of organisms from all three domains of life. Bacteria are colored blue, eukaryotes red, and archaea green. Relative positions of some of the phyla included are shown around the tree.
Metabolic network of the Arabidopsis thaliana citric acid cycle. Enzymes and metabolites are shown as red squares and the interactions between them as black lines.
Aristotle's metabolism as an open flow model
Santorio Santorio in his steelyard balance, from Ars de statica medicina, first published 1614

Metabolism (, from μεταβολή metabolē, "change") is the set of life-sustaining chemical reactions in organisms.

(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.


Organelle is a specialized subunit, usually within a cell, that has a specific function.

Organelle is a specialized subunit, usually within a cell, that has a specific function.

(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.
Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

In biology organs are defined as confined functional units within an organism.