external image 743px-Amoeba_(PSF).svg.png(11) (SR)

Classification/Diagnostic Characteristics:

Kingdom: Protozoa
Phylum: Protozoa
Subphylum: Sarcodina
Superclass: Rhizopoda
Class: Lobosa (Amoebas, Amibes)
Order: Amoebida
Family: Amoebidae
Genus: Amoeba
(SL 22)

  • The amoeba is a protist, meaning it is a eukaryote that is not a plant, animal or fungus. Protist does not describe a related group of organisms, rather it refers to all eukaryotic organisms that cannot be classified as part of a larger group. Protists are characterized by their diversity as eukaryotes. Amoebas are often are characterized by their lobe shaped pseudopods. They are often single celled organisms as are protists, but protists can also be multicellular.
  • Amoeba are part of the kingdom of protists. Split from this kingdom they are in the phylum of protozoa; the class Rhizopoda; the order of amoebida; the family of amoebidae; and the genus of amoeba (1). The protist kingdom includes organisms that have single, complex cells. The protozoa phylum includes microscopic organisms that have flagella. Some Protozoa like the Amoeba eat algae (3). The class of Rhizopoda includes unicellular animals that have no definite shape or form. These organisms move by pseudopods or false feet, which are temporary extensions of the cell, and can move in any direction. These organisms feed by engulfing the prey and incorporating it into their cytoplasm, where the prey remains inside a food vacuole until it is digested. The order Amoebida includes naked amoeba that never have a flagellate stage (2). (SM)

Relationship to Humans:
  • Many types of protists become natural resources such as limestone or oil that are used by humans.
  • Amoebas can be harmful to humans. A common example of a harmful amoeba is E. histolytica, which is the cause of amoebic dysentery. This amoeba travels through water contaminated by sewage. By eating food that has been washed with contaminated water, humans are exposed to it. Although it affects the intestine and causes diarrhea and pain, the disease is treatable (9).
  • Another amoeba, N. fowleri, has the ability to enter humans through the nasal passage by inhalation. It can then travel to the brain, and within two weeks, the victim can die. This organism thrives in warm freshwater ponds, lakes, rivers, and even soil. This amoeba does not intentionally look to kill humans; instead, it usually goes for bacteria in soil or water. However, this disease is very rare, and the amoeba is not threatening to humans if swallowed according to the CDC (8). (RS)

Habitat and Niche:
  • Amoebas can live both in the water and on land. They have a variety of functions from being food and making soil, to being pathogens or parasites.
  • Amoebas strive in salt or freshwater, but are often found in mud beneath the water than floating in the water itself. The common species, Amoeba Proteus and other aerobic amoeba's are found attached to the underside of leaves of water plants such as mosses. The parasitic amoeba species that live inside human beings and most animals are usually found in the digestive tract. (7) (NU)
  • Amoeba prefer moister environments, though the lack of oxygen in overly saturated environments can be deleterious to amoebas that use aerobic processes. The also favor environments with more organic substances, and will often live on the surfaces of macroalgae, where they can feed on bacteria, a primary source of food for the amoeba. Overall, amoeba can be found in a wide variety of environments, from soil to lagoons, and from freshwater to saltwater, wherein the amoeba's habitat diversifies further, as it can be found both at the water surface or at the bottom. Water films play an integral role in the locomotion of the creature, and large amoeba are unable to exist if the space is too small (10). (SS)

Predator Avoidance:
  • Some amoebas live in shells or secrete hard casings that cover their bodies. Others are poisonous to predators.
  • Amoebas secrete a chemical that keeps other amoebas away, preventing cannibalism. This chemical secretion also aids the amoeba in protecting itself from predators. When amoebas are in threatening situations, (or there is a shortage of food or water) they will roll into a ball and form a cyst membrane around themselves. In some cases, amoebas will surround themselves with sand grains for protection. Those amoebas who live in shells have mouth-like openings in their shells through which they stick out their pseudopods (12,13). (JM)

Amoeba Shell (
Amoeba Shell (

Nutrient Acquisition:
  • Some amoebas feed on other microorganisms or particles using phagocytosis. This process allows single-celled organisms to envelop food in a pocket within their membranes and digest the food through the use of lysosomes. Others have a digestive vacuole where they digest food.
  • Amoebas feed on microscopic organisms such as bacteria or single-celled algae. As previously mentioned, the amoeba uses phagocytosis to envelope its food with its cytoplasm and engulf it into the amoeba's body. The food enters the food vacuole, and the cytoplasm secretes enzymes that digest parts of the prey, while the soluble parts are absorbed back into the cytoplasm. If there is any part or residue that the enzymes or cytoplasm left untouched, it is left behind as the amoeba moves on (20). (AG)
  • Phagocytosis is able to occur due to the amoeba's movable and shape-shifting nature and ability to form pseudopods, or false feet. These pseudopods are makeshift extensions of the amoeba's body and commonly encircle the amoeba's prey. There are four kinds of pseudopodia that amoeba may use for food acquisition or locomotion, but reticulopodia are primarily used in the feeding process (17). (AC)

  • This video shows an amoeba feeding on a microorganism. In the video, the amoeba's pseudopods quickly extend and encircle the microorganism and subsequently engulf it. The vacuole that is formed by the joined pseudopod ends is commonly referred to as the phagosome and is in communication with the cell's lysosomes, which, after the initial engulfment, sends enzymes into the phagosome to digest the microorganism captured inside (18, 19). (AC)

Reproduction and Life Cycle:

  • Protists can reproduce both asexually and sexually. Asexual processes include binary fission, multiple fission, budding, and sporulation. Sexual reproduction also takes many forms. Sometimes the gametes are the only haploid cells. Other times the zygote is the only diploid cell. In addition, both diploid and haploid cells can undergo mitosis and undergo haploid and diploid stages. Many protists can also have sex that never results in offspring. Conjugation allows organisms to exchange DNA without any offspring production. Organisms who engage in conjugation line up next to each other and fuse their bodies together, allowing DNA to mix and mingle. Protists cell cycles also vary. Some multicellular engage in a process called an alternation of generations. This process allows multicellular, diploid, spore-producing organisms to create a haploid, gamete-producing organism that can then fuse with other haploid organisms. These two organisms are created in different generations and the structures may or may not differ morphologically.

Examples of the Amoeba Life (21)(MT)
Examples of the Amoeba Life (21)(MT)

  • This image displays two types of asexual reproduction: Binary and Multiple Fission. Under harsh conditions, the Amoeba develops a cyst, or protective covering in which multiple mitosis takes place. Once under favorable conditions the cyst ruptures and several daughter amoebas are released (21). (MT)

Growth and Development:
  • Amoebas are single celled organisms and do not grow or develop further than their meiotic stage.

  • All unicellular protists are surrounded by a membrane. Some have protective outer coatings.
  • The amoeba membrane is porous and flexible, allowing it to change shape easily (21). (MT)

Amoeba integument.jpg
Difflugia Coronata, a species of ameoba, can construct a portable shelter about 150 microns in diameter. (YR) (14)

  • Some species of amoeba, like those in the Difflugia genus, construct an outer shell from sand grains for added protection. Others naturally produce a thin, hardened shell that encases them, with openings for their pseudopods (15, 16). (YR)

  • Amoebas under water move with the current. Many protists also have cilia that act like flagella to help them move.
  • Amoebas have false feet called pseudopodia that help it move. The amoeba stretches bulges of cytoplasm out and pulls itself with its pseudopodia. Flexible microfilaments near the membrane power the pseudopodia. (4) (LK)


Sensing the Environment:
  • The amoeba's complex membrane is covered in proteins that help sense its environment and set off signals within the cell.

Gas Exchange:
  • Amoebas are typically unicellular organisms and thus are able to have gasses diffuse through their outer membranes. The surface area of the amoeba's cell membrane is very large compared to its small cytoplasm, so only diffusion of carbon dioxide between the cytoplasm and the surrounding water is necessary. Oxygen comes in, carbon dioxide diffuses out (6). (SP)

Waste Removal:
  • Since they are unicellular organisms, waste can diffuse through the outer membrane.
  • Amoeba do not have a stomach, but they do have a vacuole in which food is digested and then expelled from the amoeba through a process called exocytosis. These contractile vacuoles perform the task of excreting water and waste from the amoeba. Once filled to the maximum, vesicles are able to fuse with the cellular membrane of the amoeba to release the contents of the vacuole. With this system, amoeba are able to rid their bodies of wastes and toxic substances (5). (CM)

Environmental Physiology:
  • Amoebas have no way to control their temperature and thus are the same temperature as their surroundings. Certain protists cannot exist at certain temperatures though.
  • Amoebas control their water intake through osmosis, which dictates the amount of water that the Amoeba should have in its body in comparison to its environment. Hypertonic environments will result in water flowing into the organism. Hypotonic environments will result in water flowing out of the organism.
  • Salt regulation is dictated and regulated by protein channels embedded in the membrane.

Internal Circulation
  • Since they are unicellular organisms, amoebas have no need for an internal circulatory system. Amoeba are small enough so that all substances are able to diffuse within the cell; amoeba cells are able to absorb nutrients through their semipermeable membrane. (SL)

Chemical Control (endocrine system)
  • Since amoebas are single celled organisms, they do not have the ability to or need to create and secrete hormones.

Review Questions:
1). How do amoeba take in the nutrients and resources they need to survive? (SF)

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