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Amoeba

Amoeba

An amoeba (pronounced uh-MEE-buh) is any of several tiny, one-celled protozoa in the phylum (or primary division of the animal kingdom) Sarcodina. Amoebas live in freshwater and salt water, in soil, and as parasites in moist body parts of animals. They are composed of cytoplasm (cellular fluid) divided into two parts: a thin, clear, gel-like outer layer that acts as a membrane (ectoplasm); and an inner, more watery grainy mass (endoplasm) containing structures called organelles. Amoebas may have one or more nuclei, depending upon the species.

The word amoeba comes from a Greek word meaning "to change." The amoeba moves by continually changing its body shape, forming extensions called pseudopods (false feet) into which its body then flows. The pseudopods also are used to surround and capture foodmainly bacteria, algae, and other protozoafrom the surrounding water. An opening in the membrane allows the food particles, along with drops of water, to enter the cell, where they are enclosed in bubblelike chambers called food vacuoles. There the food is digested by enzymes and absorbed into the cell. The food vacuoles then disappear. Liquid wastes are expelled through the membrane.

Water from the surrounding environment flows through the amoeba's ectoplasm by a process called osmosis. When too much water accumulates in the cell, the excess is enclosed in a structure called a contractile vacuole and squirted back out through the cell membrane. The membrane also allows oxygen to pass into the cell and carbon dioxide to pass out.

The amoeba usually reproduces asexually by a process called binary fission (splitting in two), in which the cytoplasm simply pinches in half

and pulls apart to form two identical organisms (daughter cells). This occurs after the parent amoeba's genetic (hereditary) material, contained in the nucleus, is replicated and the nucleus divides (a process known as mitosis). Thus, the hereditary material is identical in the two daughter cells. If an amoeba is cut in two, the half that contains the nucleus can survive and form new cytoplasm. The half without a nucleus soon dies. This demonstrates the importance of the nucleus in reproduction.

Some amoebas protect their bodies by covering themselves with sand grains. Others secrete a hardened shell that forms around them that has a mouthlike opening through which they extend their pseudopods. Certain relatives of the amoeba have whiplike organs of locomotion called flagella instead of pseudopods. When water or food is scarce, some amoebas respond by rolling into a ball and secreting a protective body covering called a cyst membrane. They exist in cyst form until conditions are more favorable for survival outside.

Words to Know

Asexual reproduction: Any reproductive process that does not involve the union of two individuals in the exchange of genetic material.

Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.

Organelle: A functional structure within the cytoplasm of a cell, usually enclosed by its own membrane.

Osmosis: The movement of water across a semipermeable membrane from an area of its greater concentration to an area of its lesser concentration.

Protozoan: A single-celled, animal-like organism.

Pseudopod: From pseudo, meaning "false," and pod, meaning "foot"; a temporary extension from a cell used in movement and food capture.

Some common species of amoebas feed on decaying matter at the bottom of freshwater streams and stagnant ponds. The best-known of these, Amoeba proteus, is used for teaching and cell biology research. Parasitic species include Entamoeba coli, which resides harmlessly in human intestines, and Entamoeba histolytica, which is found in places where sanitation is poor and is carried by polluted water and sewage. Infection with Entamoeba histolytica causes a serious intestinal disease called amoebic dysentery, marked by severe diarrhea, fever, and dehydration.

[See also Cell; Protozoa; Reproduction ]

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Amoeba

Amoeba A genus of protoctists (see also protozoa) of the phylum Rhizopoda, members of which have temporary body projections called pseudopodia. These are used for locomotion and feeding and result in a constantly changing body shape (see amoeboid movement). Most species are free-living in soil, mud, or water, where they feed on smaller protoctists and other single-celled organisms, but a few are parasitic. The best known species is the much studied A. proteus.

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amoeba

amoeba Microscopic, almost transparent, single-celled protozoan animal that has a constantly changing, irregular shape. Found in ponds, damp soil and animal intestines, it consists of a thin outer cell membrane, a large nucleus, food and contractile vacuoles and fat globules. It reproduces by binary fission. Length: up to 3mm (0.lin). Class Sarcodina; species include the common Amoeba proteus and Entamoeba histolytica, which causes amoebic dysentery.

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amoeba

amoeba (ă-mee-bă) n. (pl. amoebae) any protozoan of irregular and constantly changing shape. Some amoebae cause disease in humans (see Entamoeba).
amoebic adj. —amoeboid adj.

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amoeba

a·moe·ba • n. (pl. -bas or -bae / -bē/ ) variant spelling of ameba. DERIVATIVES: a·moe·bic adj.a·moe·boid adj.

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amoeba

amoeba (zool.) microscopic animalcule of the class Protozoa, the shape of which is perpetually changing. XIX. — modL. — Gr. amoibḗ change.

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amoeba

amoeba (ameba) Any single-celled eukaryote that is naked and changes shape due to the irregular extension and retraction of pseudopodia.

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amoeba

amoeba: see ameba.

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amoeba

amoebaabba, blabber, dabber, grabber, jabber, stabber, yabber •Alba, Galbaamber, camber, caramba, clamber, Cochabamba, gamba, mamba, Maramba, samba, timbre •Annaba, arbor, arbour, barber, Barbour, harbour (US harbor), indaba, Kaaba, Lualaba, Pearl Harbor, Saba, Sabah, Shaba •sambar, sambhar •rebbe, Weber •Elba •Bemba, December, ember, member, November, Pemba, September •belabour (US belabor), caber, labour (US labor), neighbour (US neighbor), sabre (US saber), tabor •chamber • bedchamber •antechamber •amoeba (US ameba), Bathsheba, Bourguiba, Geber, Sheba, zariba •cribber, dibber, fibber, gibber, jibba, jibber, libber, ribber •Wilbur •limber, marimba, timber •winebibber •calibre (US caliber), Excalibur •briber, fibre (US fiber), scriber, subscriber, Tiber, transcriber •clobber, cobber, jobber, mobber, robber, slobber •ombre, sombre (US somber) •carnauba, catawba, dauber, Micawber •jojoba, Manitoba, October, sober •Aruba, Cuba, Nuba, scuba, tuba, tuber •Drouzhba • Toowoomba • Yoruba •Hecuba

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amoeba

amoeba(ameba) Any single-celled eukaryote that is naked and changes shape by the irregular extension and retraction of pseudopodia.

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Amoeba

Amoeba

Amoebas are protozoans characterized by cytoplasmic extensions that can change the overall shape of the organism. They consist of a mass of cellular fluid surrounded by a membrane, and contain one or more nuclei (depending upon the species), as well as other cell organelles, such as food vacuoles.

The word amoeba is derived from the Greek word ameibein (to change), which describes the amoebas most easily distinguishable feature, the continuous changing of shape by repeated formation of pseudopodia (Greek: false feet).

Pseudopodal movement is based on a continual change in the state of protoplasm flowing into the foot-like appendage. An interior fluid (endoplasm), under pressure from an exterior gel (ectoplasm), flows forward in the cell. When the endoplasm reaches the tip of a developing pseudopod, the fluid is forced backward against the ectoplasm, and is turned into a gel. After returning to the body of the cell, the newly formed ectoplasm gel is converted back to fluid endoplasm, and again flows forward under the pressure of the exterior gel.

Pseudopodia serve two important functions locomotion and food capture, activities that are often interrelated. Amoebas use their pseudopodia to ingest food by a method called phagocytosis (Greek: phagein, to eat). The streaming of protoplasm inside the pseudopodia moves the amoeba forward. When the organism contacts a food particle, the pseudopodia surround the particle. After the amoeba corrals the food, the food passes into the cell via food vacuoles. Inside the cell, the food is digested by enzymes within food vacuoles and assimilated by the amoeba. Later, the amoeba expels parts of the food that are not acceptable.

Organisms referred to by the term amoeba belong to the Domain Eukarya, Kingdom Protista, Subkingdom Protozoa, which almost exclusively includes single-celled eukaryotes that get nutrition from other organisms. Amoebas belong to the class Sarcodina, which has as its principle-distinguishing feature the presence of pseudopodia.

The Rhizopoda (in some classifications a superclass of Sarcodina) contains all organisms that form lobopodia, which are broad pseudopodia, or filopodia, which are thread-like psuedopodia. These organisms move by means of pseudopodial movement. Rhizopoda includes the subclass Lobosea. Lobosea includes both naked amoebas and testaceous forms (those with tests, or shells).

Amoebas that have tests are sometimes referred to as shelled. These tests include coatings of various kinds, such as scales, mucoid layers called glycocalyces, or complex filaments much smaller than scales.

Amoebas are further classified within the order Amoebida. Amoebida are characterized as free-living in fresh or saltwater or soil. Most Amoebida ingest bacteria, but larger members also feed on algae and other protozoans. Amoebida is further divided into several families including Amoebidae, Hartmannellidae, Entamoebidae, and Acanthamoebidae.

The family Amoebidae includes mostly freshwater species of amoeba, whose pseudopodal movement is either monopodial (the entire protoplasmic mass moves forward) or polypodial (several pseudopodia advance simultaneously). One member, Amoeba proteus is commonly used for teaching and cell biology research. Chaos carolinense, one of the larger species of amoeba, has multiple nuclei and can reach a length of 0.12 in (3 mm).

The Hartmannellidae family includes small and medium-sized amoebas that move forward monopodially, advancing by means of a steady cytoplasmic flow. They feed on bacteria, although some species of the genus Saccamoeba also feed on unicellular algae.

The family Entamoebidae includes most of the obligately endozoic (parasitic inside a host) Amoebida organisms, including Entamoeba histolytica. Amebiasis (infection with E. histolytica ) is a serious intestinal disease also called amoebic dysentery. It is characterized by diarrhea, fever, and dehydration. Although amebiasis is usually limited to the intestine, it can spread to other areas of the body, especially the liver.

E. histolytica exists as either a trophozoite or cyst. The trophozoite is motile, possesses a single nucleus, and lives in the intestine. It is passed from the body in diarrhea, but cannot survive outside the host. The cyst form, consisting of condensed protoplasm surrounded by a protective wall, is produced in the intestine, can survive outside the host, and even withstands the acid of the stomach when it is ingested with food or contaminated water. Once inside the intestine, E. histolytica multiplies by means of binary fission.

Acanthamoebidae, another family of Amoebida, includes the genus Acanthamoeba, members of which are often isolated from fresh water and soil. Acanthamoeba cause primary amebic meningoencephalitis (PAM, inflammation of the brain and its protective membranes), especially in individuals who are ill and whose immune systems are weakened. Acanthamoeba infections have been traced to fresh water, hot tubs, soil, and homemade contact lens solutions. In the latter case, contamination of contact lens solution with the organism has caused keratitis, an inflammation of the cornea accompanied by pain and blurred vision. Severe cases can require a corneal transplant or even removal of the eye.

An amoeba of the order Schizopyrenida, Naegleria fowleri is an especially dangerous human parasite, causing rapidly fatal PAM in people swimming in heated water, or warm, freshwater ponds and lakes, mainly in the southern United States. Both Naegleria and Acanthamoeba enter through the nasal mucosa and spread to the brain along nerves.

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Amoeba

Amoeba

Amoebas are single-celled protozoans of the order Amoebida. They consist of a mass of cellular fluid surrounded by a membrane , and containing one or more nuclei (depending upon the species ), as well as other cell organelles, such as food vacuoles.

The word amoeba is derived from the Greek word ameibein (to change), which describes the amoeba's most easily distinguishable feature, the continuous changing of shape by repeated formation of pseudopods (Greek: false feet).

Pseudopodal movement is based on a continual change in the state of protoplasm flowing into the foot-like appendage. An interior fluid (endoplasm), under pressure from an exterior gel (ectoplasm), flows forward in the cell. When the endoplasm reaches the tip of a developing pseudopod, the fluid is forced backward against the ectoplasm, and is turned into a gel. After returning to the body of the cell, the newly formed ectoplasm gel is converted back to fluid endoplasm, and again flows forward under the pressure of the exterior gel.

Pseudopods serve two important functions—locomotion and food capture, activities that are often interrelated. Amoebas use their pseudopods to ingest food by a method called phagocytosis (Greek: phagein, to eat).

The streaming of protoplasm inside the pseudopods moves the amoeba forward. When the organism contacts a food particle, the pseudopods surround the particle. After the food is corralled by the amoeba, an opening in the membrane allows the food particle to pass into the cell. Inside the cell, the food is enclosed within food vacuoles, digested by enzymes, and assimilated by the amoeba. The amoeba expels particles that are not acceptable as food.

The organisms generally implied by the term "amoe ba" belong to the phylum Protozoa , class Mastigophora, which includes organisms with flagellae (whip-like organs of locomotion) such as Chlamydomonas angulosa, as well as those with pseudopods. The class Sarcodina, which has as its principle distinguishing feature the almost universal presence of pseudopods, includes Amoeba proteus, the best-known protozoan.

The Rhizopoda (in some classifications a subclass of Sarcodina) contains all common "naked amoebas," which are either tubular or somewhat flattened. They move by means of protoplasmic flow, by producing pseudopodia, or by advancing as a single mass. Rhizopoda also includes sarcodinids known as giant amoebas and testaceous forms (those with tests, or shells). Some apparently "naked" amoebas have coatings of various kinds, such as scales, mucoid layers called glycocalyces, or complex filaments much smaller than scales.

In addition to the naked forms, many species of amoeba have tests (hard coverings), and are referred to as shelled amoebas. Most of these shelled amoebas are classified in the order Arcellinida. They have a test with a single opening, and are predominantly freshwater organisms. Shelled amoebas feed on a variety of organisms, such as bacteria , algae , and other protozoans.

Most members of the order of Amoebida are free-living in fresh or salt water or soil , and ingest bacteria. Larger members also feed on algae and other protozoans. Several amoebas of this group are pathogenic to humans.

The family Amoebidae includes mostly freshwater species, whose pseudopodal movement is either monopodial (the entire protoplasmic mass moves forward) or polypodial (several pseudopods advance simultaneously). One member, Amoeba proteus is commonly used for teaching and cell biology research. Chaos carolinense, one of the larger species, has multiple nuclei and can reach a length of 0.12 in (3 mm).

The Hartmannellidae family includes small and medium-sized amoebas that move forward monopodially, advancing by means of a steady flow. They feed on bacteria, although some species of the genus Saccamoeba also feed on unicellular algae.

The family Entamoebidae includes most of the obligately endozoic (parasitic inside a host) Amoebida organisms, including Entamoeba histolytica. Amebiasis (infection with E. histolytica) is a serious intestinal disease also called amoebic dysentery . It is characterized by diarrhea, fever, and dehydration. Although amebiasis is usually limited to the intestine, it can spread to other areas of the body, especially the liver.

E. histolytica exists as either a trophozoite or cyst. The trophozoite is motile, possesses a single nucleus, and lives in the intestine. It is passed from the body in diarrhea, but cannot survive outside the host. The cyst form, consisting of condensed protoplasm surrounded by a protective wall, is produced in the intestine, can survive outside the host, and even withstands the acid of the stomach when it is ingested with food or contaminated water. Once inside the intestine, E. histolytica multiplies by means of binary fission.

Another family, Acanthamoebidae (in the Amoebida suborder Acanthopodina), includes the genus Acanthamoeba genera, which are often isolated from fresh water and soil. Acanthamoeba cause primary amebic meningoencephalitis (PAM, inflammation of the brain and its protective membranes), especially in individuals who are ill and whose immune systems are weakened. Acanthamoeba infections have been traced to fresh water, hot tubs, soil, and homemade contact lens solutions. In the latter case, contamination of contact lens solution with the organism has caused keratitis, an inflammation of the cornea accompanied by pain and blurred vision . Severe cases can require a corneal transplant or even removal of the eye .

A member of the order Schizopyrenida, Naegleria fowleri is an especially dangerous human parasite, causing rapidly fatal PAM in people swimming in heated water, or warm, freshwater ponds and lakes, mainly in the southern United States. Both Naegleria and Acanthamoeba enter through the nasal mucosa and spread to the brain along nerves.

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Amoeba

Amoeba


An amoeba is a single-celled organism that has no fixed shape. As a protozoan and a member of the kingdom Protista that has animal-like qualities, an amoeba has to find and eat its food (since it is unable to make its own food as plantlike Protists do). Of the many species of amoeba, some live in water and soil, while others are parasites and live inside plants or animals.

Many biologists believe that the first protozoans were similar to today's amoeba. As members of the phylum (or primary division of the animal kingdom) Sarcodina, amoebas are distinct among protozoa in that they have no definite shape. Although they consist of a single cell surrounded by a membrane, they move by changing the shape of that membrane and therefore do not have one particular shape that makes them immediately recognizable. Some would say that they are recognizable because of this formlessness, while others say they look like a tiny bag of jelly. An amoeba actually moves in a very strange manner using pseudopods or "false feet."

When it wants to move, an amoeba turns its jelly-like body solid in a certain spot to form a temporary "foot" which it stretches in the direction it wants to move. The rest of the amoeba then flows into the pseudopod and basically changes its position to where its pseudopod had reached. This type of movement is called "streaming." Every time an amoeba does this it changes its shape. Since this is not the fastest way to move about, an amoeba only can move at a top speed of about 1 inch (2.54 centimeters) an hour.

An amoeba eats its food in much the same manner as it moves. It flows around and surrounds the food and then the amoeba totally engulfs it. The food is then held in a food vacuole, which is a specialized structure that digests the food. The amoeba mainly eats bacteria, algae, and other protozoans. Any waste that remains after the food is digested is released from a contractile vacuole (one that can open and close). Water

also flows into the amoeba by the process of osmosis (in which water flows through a membrane until the solutions on either side of it are at equal strength). When the amoeba needs to expel some water, it squeezes its contractile vacuole and squirts some out.

There are four types of protozoans that are called amoeboid protozoans. All use pseudopods to move about and to capture their food, and three of them are considered shelled amoeba since they are at least partially covered by a shell made up of the minerals they secrete. The first of the amoeba is the familiar "naked" amoeba called Amoeba proteus. While most of this type of amoeba live in water, some are parasitic and live in the human gut. The infected human then contracts the disease called amoebic dysentery. One of the shelled amoeba that lives in a tiny chambered sea shell and uses threadlike pseudopods to move about is called a foraminiferan. This amoeba can barely be seen with the naked eye. A second shelled amoeba is the freshwater heliozoan or "sun animal." It has thin pseudopods that look like needles that radiate from its body like rays from the Sun. The third and most intricate of the shelled amoebas is the radiolarian. These sea creatures have skeletons of silica, a mineral that does not dissolve in deep water, so that the deepest seabeds are covered with a thick layer of what is called radiolarian ooze.

Amoebas usually reproduce asexually by a process known as binary fission. In this form of reproduction, an amoeba splits in two after pinching in half and forms two smaller but identical cells. This occurs after the cell's nucleus duplicates its hereditary material and divides in two.

[See alsoCell; Protozoa; Reproduction, Asexual ]

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