Amoebas are single-celled protists most recognizable for their ability to constantly change shape. They exist across numerous environments, from freshwater ponds and moist soil to the bodies of other organisms. Amoebas are primarily united by their unique method of movement.
Defining Characteristics and Internal Structure
Amoebas are eukaryotic organisms, meaning their cellular contents include a membrane-bound nucleus containing genetic material. Lacking a cell wall, they possess a highly flexible outer membrane that allows for the constant alteration of their form. Beneath this membrane, the cytoplasm is divided into two layers: the inner, fluid endoplasm and the outer, gel-like ectoplasm.
The nucleus acts as the cell’s control center, directing growth and reproduction. A key component within the cytoplasm is the contractile vacuole, which functions as an osmoregulator. This organelle collects excess water that enters the cell from the hypotonic freshwater environment and expels it, preventing the cell from rupturing. Other organelles, such as mitochondria and food vacuoles, manage energy production and digestion.
Locomotion and Phagocytosis
The amoeba moves through the extension and retraction of temporary, finger-like projections called pseudopods. Locomotion is driven by the internal cytoplasm shifting between a fluid state (plasmasol) and a viscous, gel-like state (plasmagel) in a process known as gel-sol transformation. The fluid plasmasol flows toward the advancing edge of the cell, pushing the plasma membrane outward to form a new pseudopod.
As the pseudopod extends and anchors itself, the cytoplasm solidifies into plasmagel at the front. The plasmagel at the cell’s rear liquefies back into plasmasol, pulling the rest of the cell forward. This ability to form extensions is also used for feeding through phagocytosis, or “cell eating.” The amoeba extends its pseudopods to surround and engulf a food particle. The membrane then pinches off internally to form a food vacuole where digestive enzymes break down the contents.
Reproduction and Cyst Formation
Amoebas primarily multiply through asexual reproduction via simple cell division, known as binary fission. Under favorable conditions, the amoeba elongates and its nucleus divides through mitosis. The cytoplasm then splits, resulting in two genetically identical daughter cells.
When environmental conditions become unfavorable, such as during drought or nutrient scarcity, the amoeba enters a dormant state. The organism withdraws its pseudopods, becomes spherical, and secretes a thick, protective outer layer called a cyst. This process, known as encystment, allows the amoeba to survive harsh conditions until the cell emerges to resume its active lifestyle.
Environmental Roles and Pathogenic Forms
Most amoebas are free-living organisms found ubiquitously in moist environments, including freshwater, marine sediments, and damp soil. They play a significant role in the ecosystem as predators of microorganisms, primarily feeding on bacteria and algae. By consuming these organisms and decomposing organic matter, amoebas help recycle nutrients and maintain balance within microbial communities.
While the majority of species are harmless, a small number of amoebas are pathogenic to humans. Entamoeba histolytica is an intestinal parasite that causes amoebic dysentery. Naegleria fowleri, often called the “brain-eating amoeba,” causes a destructive infection of the central nervous system if introduced through the nose, frequently resulting in a high fatality rate. Other free-living amoebas, such as Acanthamoeba and Balamuthia, can also cause serious infections.