Amoeba Order

An amoeba is a unicellular, highly motile eukaryotic organism that can be found in its free-living form in various water bodies and as a parasite in both vertebrates and invertebrates. The organism belongs to the kingdom protozoa.

The general scientific name of amoeba is Amoeba proteus. There is some parasitic amoeba such as the intestinal parasite Entamoeba histolytica and the brain-eating amoeba Naegleria fowleri.

Classification of Amoeba

Historically, the amoeba has been classified under a single taxonomic group known as Sarcodina. Within this group, the species was subdivided based on their pseudopodia. However, this classification was not specific and the introduction of molecular phylogenetics took a different approach to the classification of this group. 

Molecular phylogenetics remodeled the eukaryotic taxonomic classification. The classification was carried out based on comparison analyses between regular DNA sequences and 18S subunit ribosomes or SSU rDNA. The first level of groups that rose from this new phylogenetic tree was called supergroups and a well-known research scholar classified these supergroups in  categories

• Phiskontha

• Amoebozoa

• Excavata

• Archaeplastida

• and SAR (comprised of three groups: Alveolata, Stramenopiles and Rhizaria)

As the name suggests Amoeba comes under the subgroup Amoebozoa. Below is the scientific classification of amoeba.

Domain: Eukaryota

Kingdom: Amoebozoa

Phylum: Tubulinea

Order: Tubulinida

Family: Amoebidae

Genus: Amoeba

Species: proteus, animalcule, dubia, animalcule, etc.

The Structure of Amoeba

The amoeba cell consists of cytoplasm, cell membrane, gas vacuoles nuclei, mitochondria and pseudopodia: the locomotory organ of the organism. 

The clear, gel-like outer part of the cytoplasm is known as ectoplasm whilst the granular part which contains the organelles of the cell is known as endoplasm.

A unique aspect of the amoeba anatomy is the formation of the temporary extensions of the cytoplasm called pseudopodia. It is used for location of the cell as well as a  device to capture food. More than one pseudopodia can be observed at one time in an amoeba. 

The Size of the Cell

Amoeboid cell size has a range between 400-600micros and exceptionally large cells can be up to 20 cm long which makes them visible to the naked eye. The cell size makes all species of amoeba morphologically distinct.  

Cell Shape

The shape of amoeba cells depends on the pseudopods. Some amoebozoa species show bulbous pseudopods with rounded ends and a tubular midsection whilst Cercozoan amoeboids have thin thread-like pseudopods. The group foraminifera shows slender branched pseudopods that merge with each other to form net-like structures.

The Characteristics of the Amoeba Cell

• Amoeba was first discovered by Rösel von Rosenhof, in 1755 and the name was proposed by B.D.S. Vincent. The first detailed description of amoeba was done by Leidy.

• The shape of an amoeba cell is not fixed due to the appearance and disappearance of the pseudopodia. If there are multiple pseudopodia observed in an amoeba cell, it is called polypoidal.

• Pseudopodia assists in locomotion and the ingestion of food.

• A plasmalemma is observed in the cells which is made of a lipid bi-layer: Lipid, protein and lips.

• Microvilli are observed on the cell surface and they are mucoprotein. These help in the attachment of the amoeboid cell to any surface. The adhesive layer of the microvilli is composed of glycolipids and glycoproteins.

• Osmosis and diffusion take place through plasmalemma.

• The amoeba cell is mononucleated. The shape of the nucleus for a young amoeba cell is biconcave whilst for an adult amoeba is biconvex. A net-like structure can be observed below the cell membrane and it is called the honeycomb lattice due to its structure.

• Amoeba cell chromosomes are known as chromidia.

• The cortical layer of the cytoplasm is called the ectoplasm and it is composed of two layers; an inner sticky, granular jelly-like layer, and an outer clear, glossy fluid layer.

• The endoplasm is fluid, granular, and semitransparent.

• There are three types of vacuoles observed

• Contractile vacuole

• Water vacuole

• Food vacuole

They help in osmoregulation, food digestion, and food storage.

• Other cell organelles include mitochondria, Golgi bodies, ribosome, lysosome, and bi and tri-urate crystals.

• Nutrition in amoeba takes place in four steps:

• Ingestion

• Digestion

• Absorption and assimilation

• Egestion

They show holozoic nutrition and are omnivorous.

• Respiration takes place through body surface diffusion.

• Reproduction in amoeba always takes place through asexual means and the various methods are 

• Binary fission

• Multiple fission

• Regeneration

• Sporulation

• Encystment

• Locomotion takes place with the help of pseudopodia.

Locomotion: Amoeboid movement

Pseudopodia is the primary locomotory organ in amoeba and the overall movement of the cell takes place in three steps.

The first step is ballooning of the plasma membrane: This rearrangement is known as the pseudopodia or the false foot. It attaches itself to any substrate and is filled with cytosol. Afterwards, the rear portion of the amoeba releases the attachment from the substrate and moves forward.

During this movement, the viscocity of the cytosol flows from the endoplasm into the pseudopodium, which is at the front of the cell. Once this takes place, the endoplasm becomes ectoplasm which contains the gel-like substance that forms the cortex under the cell membrane.

Again, as the cell moves forward, the ectoplasmic gel converts into an endoplasmic sol and the cycle repeats itself as the organism continues to move. The transition of gel to sol takes place due to the collapse and rearrangement of actin microfilamets found in the cytosol. 

It should be noted that cofinin is responsible for the collapse of actin filaments and forms the sol whilst profilin is utilised for actin polymerisation. The gel is formed by filamin and α-actinin.