Amoeba

Amoeba

Structure, Classification, Examples, and FAQs of Amoeba:

All living beings on the earth are made up of structures called cells. Down at the microscopic level, this is the very basis of life. Every organism, including humans, begins their lives as a single cell and then branches into millions of these cells. Some organisms, however, remain single-celled for all their lives. They are microscopic organisms and they require only one cell to survive. One of the most common of this variety is an amoeba.

What is Amoeba?:

Amoeba is a unicellular organism that can only be viewed under a microscope. It is a simple eukaryotic organism that moves around through Pseudopodia (cytoplasm pushing the cell membrane); the word pseudopodia means ‘false feet’.

They are commonly found on freshwater surfaces such as ponds and rivers. The ability to change its body shape as required is the key feature of amoebae (plural). The scientific name of Amoeba is “Amoeba proteus”, also termed as the genus name. Amoeboid cells are generally found within animal species, some forms of algae, and even fungi. Asexual reproduction is the typical nature of amoebic cells and the cells multiply through a biological process called binary fission (or multiple fission).

What Does the Structure Of Amoeba Look Like?:

Accounting to the amoeba’s characteristics, its cytoplasm along with cellular contents enclose within a cell membrane. The DNA is bundled into the main cellular compartment, called the nucleus, and is covered by the plasmalemma. The nucleus comprises other several membrane-bound genetic organelles that help in protein transportation and produce energy.

To name a few:
Contractile vacuoles (osmoregulation subcellular structures)
Golgi apparatus (a folded membrane with vesicles)
Fat globules (the hydrophobic intracellular pieces)Food vacuoles (a storage unit for every food)

Water globules (small, colorless, spherical vacuole water elements)

Mitochondria (the powerhouse of a cell)Speaking of its cytoplasmic entity, an amoeba has 2 layers namely the inner endoplasm (granulated) and outer ectoplasm (non-granulated).

The entire body of an amoeba is transparent and looks similar to gelatin. When it comes to the food consumption process of an amoeba, prey is usually ingested either through pinocytosis or phagocytosis. Adding to this, an amoeba can turn itself into a microbial cyst, when its food and living food conditions are abnormal. Bacteria, plant cells, metazoa, algae, and protozoa are some of the common examples of what an amoeba eats. Yet, it does not have a well-defined mouth or anus for secretion or excretion. Since every amoeboid cell is a pseudopod, it does not have a definite shape. However, the size of an amoeba cell is around 250 and 750 microns. There are even amoebae that are visible to a human’s naked eye. Moreover, A. Proteus is the biggest known cell in the unicellular kingdom of organisms.

Amoeba Classification:

The classification of amoeboid cells has a few critics since it possesses only a true nucleus and is neither an animal nor a plant. To maintain consistency, ameboid cells are classified commonly under the Protista Kingdom. Following below is the scientific representation of the amoeba classification

Domain: Eukaryota
Kingdom: Amoebozoa
Phylum: Tubuline
Order: Tubulinida
Family: Amoebidae
Genus: Amoeba
Species: Proteus, animalcule, dubia, animalcule, etc.

Current research studies focus on classifying amoeboid cells using their subunit ribosomal RNA (SSU rRNA) genes. ‘Sarcodina’ is the single-most acceptable taxonomic group for the classification of amoeba cells. This is classified based on certain observable characteristics and after studying its morphology. Note that this classification is not based on any evolutionary relationship and hence it is not to be considered as a family tree.

Some Examples Of Amoeba:

Many varieties of amoeba can be found in the surroundings, in places that are damp and watery. They are mostly seen in wet soil and freshwater environments. The most common example of free-living amoeba is the Amoeba proteus. This can grow up to a size of 2 mm, and unlike other varieties, this is not pathogenic and can not cause any diseases in humans or animals.

Can Amoeba be dangerous?:

Some other varieties of amoeba are opportunistic pathogens. This means that they can be free-living, however, they prefer to infect and live inside a host and derive nutrition from them. These parasites can cause problems like amoebic dysentery and amoebiasis in human beings. These pathogens can be ingested with food and/or water and they can form a cyst-like structure inside the host’s body. These can be significantly harmful since sometimes the pathogens can invade the bloodstream and reach other vital organs.

Another kind of dangerous amoeba that lives in freshwater, is Naegleria fowleri. It can also be free-living but can invade the bodies of hosts and derive nutrition from them. The severity of this infection lies in the fact that this pathogen can enter the nasal mucosa and travel upwards toward the brain. It feeds on specific brain tissues such as neurons. This can be fatal for human beings if not detected promptly, since it may lead to primary amoebic meningoencephalitis.

Amoeba Movement:

As a class of organisms, amoebae are defined by their unique movement patterns. This movement strategy produces forward movement via the following three steps: ballooning the plasma membrane forward. This distinct rearrangement is known as a pseudopodium or “false foot”, which is very similar in nature to that of the lamellipodium generated in higher vertebrates.
the pseudopodium attaches to the substrate, and is filled with cytosol;
the rear portion of the amoeba releases its attachment to the substrate and is propelled forward.

During amoeboid movement, the viscosity of the cytosol cycles between a fluid-like sol, which flows from the central region of the cytoplasm known as the endoplasm into the pseudopodium at the front of the cell. Once this occurs, the endoplasm becomes an ectoplasm containing a gel-like substance that forms the cortex under the plasma membrane.
As the amoeba moves forward, the ectoplasmic gel is converted once again into the endoplasmic sol, and the cycle is repeated as the cell continues to move. This transition between the gel and sol states occurs following the collapse and reassembly of networks of actin microfilaments located in the cytosol. In particular, cofilin is responsible for the disassembly of actin filaments to form the sol, whereas profilin leads to actin polymerization, and the gel is formed by α-actinin.

Amoeba Size and Shape:

Size:

Amoebae differ in both size and shape, and even members of the same species can be highly morphologically distinct.

While the earliest identified amoebae were approximately 400 to 600 microns in size, both extremely small (between 2 and 3 microns) as well as exceptionally large amoebae (20 cm; visible to the naked eye) have been documented to date.
Therefore, amoeba species exhibit a wide range of sizes. Indeed, when scientists study amoebae, the samples are typically passed through a filter approximately 0.45 microns in size, and the remnants on the filter are used for culturing.

Shape:

Since amoebae both move and eat using pseudopods, they are classified based on the morphology and internal structure of their pseudopods. For example, Amoebozoan species (e.g., amoeba) exhibit bulbous pseudopods with a tubular mid-section and rounded ends; Cercozoan amoeboids, (e.g., Euglypha and Gromia) have pseudopods which appear thin and thread-like; Foraminifera produces slender pseudopods that branch and merge with one another to form net-like structures; others are characterized by rigid, needle-like pseudopods with a complex network of microtubules.

Free-living amoebae (which do not require a host) are either “testate” or “naked”. Testate amoebae contain a hard shell, whereas naked amoebae do not. Testate amoebae shells are typically composed of calcium, silica, chitin, or other components (e.g., sand granules). Another component typically found in freshwater amoebae is a contractile vacuole. This vacuole is required to expel excess water from the cell and maintain an osmotic balance. Since the concentration of solutes in freshwater is lower than the amoeba’s internal cytosol, water flows across the cell membrane via osmosis. Therefore, the contractile vacuole pumps this excess water out of the cell to ensure that the cell does not burst. In contrast, most marine amoebae do not possess a contractile vacuole as the cytosol and the water outside of the amoeba are balanced.

Amoeba Reproduction:

Due to the extremely diverse nature of amoebae, the various species of amoebae reproduce using a variety of different methods. These methods include spores, binary fission, and even sexual.

Binary Fission:

By far the most common form of asexual reproduction employed by amoebae is binary fission. In preparation for reproduction, the amoeba will withdraw its pseudopodia and form a spherical shape.


Mitosis is observed in the nucleus, and the cytoplasm divides at the center of the cell and separates, forming two daughter cells. Since this process involves simply copying the genetic information to form a second cell, the two resulting daughter cells are identical clones of the parent cell. Thus, the nucleus is absolutely essential for this form of reproduction. This has been verified in experiments involving slicing an amoeba in half or extracting the nucleus from the amoeba. In both situations, the cell eventually dies without a nucleus.

Multiple Fission and Encystment:

Under conditions of food shortages, amoebae will reproduce via multiple fission. This process involves the production of multiple daughter cells by: the pseudopodia being retracted and the amoeba forming a spherical shape;
the amoeba secretes a substance that hardens and encapsulates the cell, forming a cyst (encystment);
the amoeba, protected by the cyst, will undergo mitosis several times, producing multiple daughter cells;
when favorable conditions return, the cyst wall bursts, releasing the daughter cells. Within a host, the amoeba will undergo encystment as a means of protection against desiccation as it travels through the colon, which ensures its survival outside of the host.

Spore Formation:
Solitary haploid amoebae (known as myxamoebae or “social amoebae”) reside on decaying vegetation (e.g., logs), eat bacteria, and reproduce asexually via binary fission as described above. However, unlike the amoebae, which undergo encystment when the food supply becomes exhausted, tens of thousands of myxamoebae will fuse, forming a moving stream of cells converging at a central location. It is at this region that the cells stack on top of each other and form a conical mound termed a “tight aggregate”. Next, a tip rises from the top of the conical mound, and the tight aggregate folds to produce a mobile “grex” (also termed a pseudoplasmodium or slug), 2–4 mm in length and surrounded by a slimy substance. The grex will then migrate towards an illuminated area, where it will differentiate into a fruiting body composed of a tubed stalk (approximately 15%–20% of the entire cellular population) and spore cells. This process involves the secretion of an extracellular coating and the extension of a tube through the grex by prestalk cells located in the anterior of the grex. As the prestalk cells differentiate into stalk cells, they create vacuoles and become enlarged. This serves to lift the prespore cells in the posterior section of the grex. The elevated prespore cells differentiate into spore cells and disperse, each representing a new myxamoeba, while the stalk cells die.

Sexual Reproduction:

Myxamoebae are also unique in that they can also reproduce sexually. This occurs when two myxamoebae fuse to create a giant cell. This giant cell will then engulf all other cells in a myxamoebae aggregate. After ingesting all of its neighbors, the giant cell will encyst itself, and undergo meiosis and mitosis division several times under the protective cover of the cyst. When appropriate environmental conditions are met, the cyst will burst, releasing new myxamoebae. Since this process involves meiosis and the genetic information from two amoebae, the resulting daughter cells will be genetically distinct from the parent cells.

Temperature and Reproduction:

Temperature is a critical factor that impacts the amoebae growth. While several amoebae species have been found to grow at a wide range of temperatures ranging from 10°C to 37°C, pathogenic strains have been found to survive more efficiently at higher temperatures (between 32°C and 37°C). This indicates that amoebae are highly resistant to temperature fluctuations and most are adapted to survival within humans. Therefore, this may have pathogenic implications, as amoeboid cysts are extremely resistant to microbicides and can infect humans via contaminated drinking water.

Quiz:

1. What is a “grex”?
A. A method of phagocytosis.
B. The joining of myxamoebae to form a giant slug.
C. An intracellular organelle.
D. An amoeboid daughter cell.

2. Which is NOT a method of reproduction exhibited by amoebae?
A. Sexual
B. Binary fission
C. Spore Formation
D. All are forms of amoebae reproduction.

3. The purpose of cyst formation is:
A. Protection
B. Dormancy until favorable conditions are achieved.
C. Migration
D. A and B only
E. All of the above

Give the answer to the Question in the Comment section: