
An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
* Cellphone, a phone connected to a cellular network
* Clandestine cell, a penetration-resistant form of a secret or outlawed organization
* Electrochemical cell, a de ...
or
unicellular organism
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. Organisms fall into two general categories: prokaryotic organisms and ...
with the ability to alter its shape, primarily by extending and retracting
pseudopods. Amoebae do not form a single
taxonomic group; instead, they are found in every major
lineage of
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
organisms. Amoeboid cells occur not only among the
protozoa
Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...
, but also in
fungi
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
,
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
, and
animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...
.
Microbiologist
A microbiologist (from Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of par ...
s often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits
amoeboid movement
Amoeboid movement is the most typical mode of locomotion in adherent eukaryotic cells. It is a crawling-like type of movement accomplished by protrusion of cytoplasm of the cell involving the formation of pseudopodia ("false-feet") and posterio ...
.
In older classification systems, most amoebae were placed in the
class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
or
subphylum
In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum.
The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...
Sarcodina, a grouping of
single-celled organisms
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. Organisms fall into two general categories: prokaryotic organisms and ...
that possess pseudopods or move by
protoplasm
Protoplasm (; ) is the part of a cell that is surrounded by a plasma membrane. It is a mixture of small molecules such as ions, monosaccharides, amino acids, and macromolecules such as proteins, polysaccharides, lipids, etc.
In some definitions ...
ic flow. However,
molecular phylogenetic
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
studies have shown that Sarcodina is not a
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
group whose members share
common descent
Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...
. Consequently, amoeboid organisms are no longer classified together in one group.
The best known amoeboid
protists
A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...
are ''
Chaos carolinense
''Chaos'' is a genus of single-celled amoeboid organisms in the family Amoebidae. The largest and most-known species, the so-called "giant amoeba" (''Chaos carolinensis''), can reach lengths up to 5 mm, although most specimens fall betwee ...
'' and ''
Amoeba proteus
''Amoeba proteus'' is a large species of amoeba closely related to another genus of giant amoebae, ''Chaos (genus), Chaos''. As such, the species is sometimes given the alternative scientific name ''Chaos diffluens''.
This protozoan uses extensi ...
'', both of which have been widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" ''
Naegleria fowleri
''Naegleria fowleri'', also known as the brain-eating amoeba, is a species of the genus ''Naegleria''. It belongs to the phylum Percolozoa and is classified as an amoeboflagellate Excavata, excavate, an organism capable of behaving as both an ...
'', the intestinal parasite ''
Entamoeba histolytica
''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mil ...
'', which causes
amoebic dysentery
Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba '' Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, coloni ...
, and the multicellular "social amoeba" or
slime mould
Slime mold or slime mould is an informal name given to a polyphyletic assemblage of unrelated eukaryotic organisms in the Stramenopiles, Rhizaria, Discoba, Amoebozoa and Holomycota clades. Most are near-microscopic; those in the Myxogastria ...
''
Dictyostelium discoideum
''Dictyostelium discoideum'' is a species of soil-dwelling Amoeboid, amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, ''D. discoideum'' is a eukaryote that transitions from a collection of unic ...
''.
Biology
Pseudopods and movement

Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges of
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
formed by the coordinated action of
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...
microfilaments
Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other p ...
pushing out the
plasma membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
that surrounds the cell. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another.
Amoebozoan species, such as those in the genus ''
Amoeba
An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...
'', typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section.
Cercozoan amoeboids, such as ''
Euglypha
''Euglypha'' is a genus of cercozoa. It includes the species ''Euglypha rotunda''.
References
External links
*
*
Imbricatea
Rhizaria genera
{{Cercozoa-stub ...
'' and ''
Gromia
''Gromia'' is a genus of protists, closely related to foraminifera, which inhabit marine and freshwater environments. It is the only genus of the family Gromiidae. ''Gromia'' are ameboid, producing filose pseudopodia that extend out from the cel ...
'', have slender, thread-like (filose) pseudopods.
Foraminifera
Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the
Radiolaria
The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...
and
Heliozoa
Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes (protists) with stiff arms (Pseudopodia#Morphology, axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule- ...
, have stiff, needle-like, radiating
axopodia
A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...
(actinopoda) supported from within by bundles of
microtubules
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
.
Free-living amoebae may be "
testate
Intestacy is the condition of the estate of a person who dies without a legally valid will, resulting in the distribution of their estate under statutory intestacy laws rather than by their expressed wishes. Alternatively this may also apply ...
" (enclosed within a hard shell), or "naked" (also known as
gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including
calcium
Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
,
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
,
chitin
Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
, or agglutinations of found materials like small grains of sand and the
frustules of
diatoms
A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
.
To regulate
osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a Solution (chemistry), solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a soluti ...
, most freshwater amoebae have a contractile
vacuole
A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
which expels excess water from the cell. This
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
is necessary because freshwater has a lower concentration of
solutes
In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...
(such as salt) than the amoeba's own internal fluids (
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
). Because the surrounding water is
hypotonic
In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective memb ...
with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane, selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of ...
. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the
tonicity
In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two Solution (chemistry), solutions separated by a Semipermeable membrane, partially-permeable cell Cell membrane, membrane. Tonicity ...
of the surrounding water.
Nutrition

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other
protist
A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...
s. Some are
detritivore
Detritivores (also known as detrivores, detritophages, detritus feeders or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...
s and eat dead organic material.
Amoebae typically ingest their food by
phagocytosis
Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or
cytostome
A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacu ...
, and there is no fixed place on the cell at which phagocytosis normally occurs.
Some amoebae also feed by
pinocytosis
In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...
, imbibing dissolved nutrients through
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...
formed within the cell membrane.
Size range
The size of amoeboid cells and species is extremely variable. The marine amoeboid ''
Massisteria voersi'' is just 2.3 to 3
micrometre
The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
s in diameter,
within the size range of many bacteria. At the other extreme, the shells of deep-sea
xenophyophores can attain 20 cm in diameter. Most of the free-living freshwater amoebae commonly found in
pond water, ditches, and lakes are
microscopic
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale betwe ...
, but some species, such as the so-called "giant amoebae"
''Pelomyxa palustris'' and
''Chaos carolinense'', can be large enough to see with the naked eye.
Sexual reproduction
Recent evidence indicates that several Amoebozoa lineages undergo
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
.
Orthologs
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...
of genes employed in
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
of sexual
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s have recently been identified in the ''
Acanthamoeba
''Acanthamoeba'' is a genus of amoeboid, amoebae that are commonly recovered from soil, fresh water, and other habitat (ecology), habitats.
The genus ''Acanthamoeba'' has two stages in its life cycle, the metabolically active trophozoite stage a ...
''
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
. These genes included ''
Spo11
Spo11 is a protein that in humans is encoded by the ''SPO11'' gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic recombination. Its active site contains a tyrosine which ligates and dissociates with DNA to p ...
,
Mre11,
Rad50
DNA repair protein RAD50, also known as RAD50, is a protein that in humans is encoded by the ''RAD50'' gene.
Function
The protein encoded by this gene is highly similar to ''Saccharomyces cerevisiae'' Rad50, a protein involved in DNA double- ...
,
Rad51
DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to t ...
,
Rad52, Mnd1,
Dmc1,
Msh'' and ''
Mlh''.
This finding suggests that the ‘'Acanthamoeba'’ are capable of some form of meiosis and may be able to undergo sexual reproduction.
The meiosis-specific
recombinase
Recombinases are genetic recombination enzymes.
Site specific recombinases
DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria ( ...
,
Dmc1, is required for efficient meiotic
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
, and ''Dmc1'' is expressed in ''
Entamoeba histolytica
''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mil ...
''.
The purified Dmc1 from ''E. histolytica'' forms
presynaptic
In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
filaments and catalyses
ATP-dependent
homologous DNA pairing and DNA strand exchange over at least several thousand
base pairs
A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
.
The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.
These processes are central to meiotic recombination, suggesting that ''E. histolytica'' undergoes meiosis.
Studies of ''
Entamoeba invadens'' found that, during the conversion from the
tetraploid
Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...