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Created by: Team Biology at, Last Updated: May 31, 2024


Amoebas are fascinating single-celled organisms that thrive in diverse environments. They belong to the kingdom Protista and exhibit remarkable adaptability. These microscopic entities move and feed using pseudopodia, which are temporary projections of their cytoplasm. Amoebas play crucial roles in ecosystems as both predators and prey. They contribute to nutrient cycling and maintain the balance of microbial communities. Studying amoebas offers insights into basic biological processes and evolutionary history, making them an essential subject in the field of microbiology.

What is Amoeba?

An amoeba is a type of single-celled organism belonging to the kingdom Protista. Amoebas are characterized by their ability to change shape due to their flexible cell membrane and the presence of pseudopodia, which are temporary projections of the cell. These pseudopodia allow the amoeba to move and capture food through a process called phagocytosis.

Structure of Amoeba

Structure of Amoeba

Amoebas possess a simple yet highly efficient structure that allows them to thrive in various environments. Here’s an explanation of the key components of an amoeba’s structure:

  1. Cell Membrane:
    • The cell membrane encloses the amoeba, providing shape and protection. It is flexible and allows the amoeba to change shape as it moves and engulfs food particles.
  2. Cytoplasm:
    • The cytoplasm is the gel-like substance within the cell membrane. It consists of two parts: the outer clear ectoplasm and the inner granular endoplasm. The cytoplasm contains all the organelles and is essential for the amoeba’s movement and metabolic activities.
  3. Pseudopodia:
    • Pseudopodia, or “false feet,” are temporary projections of the cytoplasm. Amoebas use pseudopodia for locomotion and to capture food. The formation of pseudopodia is driven by the flow of cytoplasm, allowing the amoeba to move and change shape.
  4. Nucleus:
    • The nucleus is the control center of the amoeba, containing its genetic material (DNA). It regulates cell functions, including growth, metabolism, and reproduction. The nucleus is typically centrally located and can be observed as a darker structure within the cytoplasm.
  5. Contractile Vacuole:
    • The contractile vacuole helps regulate water balance within the amoeba. It expels excess water that enters the cell by osmosis, maintaining the internal osmotic pressure and preventing the cell from bursting.
  6. Food Vacuoles:
    • Food vacuoles are temporary structures that form when the amoeba engulfs food particles. Enzymes within the food vacuoles break down the ingested material, allowing the amoeba to absorb nutrients.
  7. Plasma Gel and Plasma Sol:
    • The cytoplasm of the amoeba can exist in two states: the gel-like plasma gel and the more fluid plasma sol. The conversion between these states facilitates the movement of the amoeba by enabling the flow of cytoplasm into pseudopodia.

Classification of Amoeba

Kingdom: Protista

Amoebas are traditionally classified under the Kingdom Protista. They are distinct from plants, animals, and fungi, yet they share the common characteristic of being eukaryotic organisms, meaning they possess a true nucleus.

Domain: Eukaryota

As eukaryotes, amoebas have a defined nucleus encased within a membrane. This differentiates them from prokaryotes, which lack a true nucleus.

Phylum: Amoebozoa

Within the Protista kingdom, amoebas are placed in the phylum Amoebozoa. This phylum includes organisms that move and feed using pseudopodia, which are temporary projections of their cytoplasm.

Class: Lobosa

Amoebas typically belong to the class Lobosa, characterized by their lobose pseudopodia. These are blunt and finger-like extensions that assist in movement and ingestion of food.

Order: Various (e.g., Tubulinida, Arcellinida)

Amoebas can be further divided into different orders based on their morphology and genetic characteristics. Common orders include Tubulinida, which consists of free-living amoebas, and Arcellinida, which comprises testate (shelled) amoebas.

Genus and Species: Various

There are numerous genera and species of amoebas, each with unique features. Some well-known genera include:

  • Amoeba (e.g., Amoeba proteus)
  • Entamoeba (e.g., Entamoeba histolytica, which causes amoebic dysentery)
  • Naegleria (e.g., Naegleria fowleri, the brain-eating amoeba)

Special Note on Naegleria fowleri: Though not a true amoeba, Naegleria fowleri is an amoeboid organism that can cause severe infections in humans. It enters the body through the nose and travels to the brain, where it feeds on neurons, leading to brain tissue destruction. The infection it causes, known as primary amebic meningoencephalitis (PAM), has a high fatality rate of approximately 97%.

Functions of Amoebas

Amoebas perform several crucial functions that allow them to survive and thrive in various environments. Here are the main functions of amoebas:

  1. Locomotion:
    • Amoebas move using pseudopodia, which are extensions of their cytoplasm. This type of movement, called amoeboid movement, enables amoebas to navigate through their environment in search of food or favorable conditions.
  2. Feeding:
    • Amoebas are heterotrophic organisms that feed through a process called phagocytosis. They extend their pseudopodia to surround and engulf food particles, such as bacteria, algae, and other small organisms. Once engulfed, the food particles are enclosed in food vacuoles where they are digested by enzymes.
  3. Reproduction:
    • Amoebas primarily reproduce asexually through binary fission. In this process, the nucleus divides by mitosis, followed by the division of the cytoplasm, resulting in two genetically identical daughter cells. This method ensures rapid population growth in favorable conditions.
  4. Osmoregulation:
    • Amoebas maintain their internal water balance through the function of a contractile vacuole. This organelle expels excess water that diffuses into the cell, preventing the amoeba from bursting due to osmotic pressure.
  5. Respiration:
    • Amoebas perform respiration at the cellular level through the process of diffusion. Oxygen from the surrounding environment diffuses into the cell, while carbon dioxide, a metabolic waste product, diffuses out.
  6. Excretion:
    • Amoebas excrete waste products through diffusion across the cell membrane. Metabolic wastes, such as ammonia, are expelled directly into the surrounding environment.
  7. Response to Stimuli:
    • Amoebas exhibit simple responses to environmental stimuli. They can move toward favorable conditions (positive chemotaxis) or away from harmful substances (negative chemotaxis). This ability to respond to stimuli helps them find food and avoid dangers.
  8. Role in Ecosystems:
    • Amoebas play important roles in their ecosystems as both predators and prey. They help control bacterial populations and contribute to the decomposition process by breaking down organic matter. In turn, they serve as a food source for larger microorganisms and small invertebrates.

Reproduction in Amoeba

Amoebas primarily reproduce asexually through a process called binary fission. This method of reproduction is relatively simple and involves the following steps

  • Cell Growth:
    • Before reproduction, the amoeba grows in size and accumulates the necessary nutrients and energy required for the division process.
  • Nuclear Division (Mitosis):
    • The nucleus of the amoeba undergoes mitosis, a process where the genetic material (DNA) is replicated and divided into two identical sets. Mitosis involves several phases:
      • Prophase: The nuclear envelope begins to disintegrate, and the chromatin condenses into visible chromosomes.
      • Metaphase: The chromosomes align at the center of the cell (the metaphase plate).
      • Anaphase: The sister chromatids (identical halves of each chromosome) are pulled apart to opposite ends of the cell.
      • Telophase: New nuclear envelopes form around each set of chromosomes, resulting in two separate nuclei within the cell.
  • Cytoplasmic Division (Cytokinesis):
    • Following mitosis, the cytoplasm of the amoeba divides in a process called cytokinesis. The cell membrane pinches inward at the center, creating a cleavage furrow that eventually separates the cell into two daughter cells.
    • Each daughter cell receives one nucleus and a portion of the cytoplasm, including organelles such as mitochondria and vacuoles.
  • Formation of Daughter Cells:
    • The two newly formed daughter cells are genetically identical to the original parent cell. They continue to grow and eventually reach a size where they can undergo binary fission themselves.

Shape, Movement, and Nutrition of Amoeba

Shape of Amoeba

Amoebas are simple, single-celled organisms that belong to the kingdom Protista. They are characterized by their irregular shape, which is constantly changing. This shape is due to the absence of a rigid cell wall, allowing the amoeba to form extensions called pseudopodia (false feet). Pseudopodia are temporary projections of the cell membrane and cytoplasm that the amoeba uses for movement and capturing food.

  • Irregular and ever-changing due to lack of a fixed shape.
  • Pseudopodia: These are extensions of the cell membrane that aid in movement and feeding.
  • Flexible cell membrane: Allows the amoeba to change shape easily.

Movement of Amoeba

The primary method of movement in amoebas is known as amoeboid movement. This type of movement is facilitated by the formation of pseudopodia. Here’s how it works:

  1. Pseudopodium Formation: The amoeba extends a pseudopodium in the direction it wants to move.
  2. Cytoplasmic Flow: The cytoplasm (the internal fluid of the cell) flows into the pseudopodium, pushing the cell’s body forward.
  3. Anchoring and Pulling: The amoeba anchors the pseudopodium to a surface and pulls the rest of its body forward.
  • Pseudopodia: Key structures for movement, extending and retracting as needed.
  • Cytoplasmic streaming: The flow of cytoplasm into the pseudopodia enables movement.
  • Adhesion to surfaces: Helps in anchoring the pseudopodia to pull the cell body forward.

Nutrition of Amoeba

Amoebas are heterotrophic organisms, meaning they obtain their food by consuming other organisms. Their feeding process is known as phagocytosis. Here’s how amoebas feed:

  1. Detection of Food: The amoeba detects food particles (such as bacteria or smaller protists) through chemical signals in the environment.
  2. Engulfing Food: The amoeba extends pseudopodia around the food particle, encasing it within a part of the cell membrane.
  3. Formation of Food Vacuole: The food particle is enclosed within a food vacuole, a membrane-bound compartment inside the amoeba.
  4. Digestion: Enzymes are secreted into the food vacuole to break down the food particle into simpler substances.
  5. Absorption: The digested nutrients are absorbed into the cytoplasm, and any undigested material is expelled from the cell.
  • Heterotrophic: Consumes other organisms for food.
  • Phagocytosis: The process of engulfing food particles.
  • Food vacuole: Membrane-bound compartment where digestion occurs.
  • Enzymatic digestion: Breakdown of food particles for nutrient absorption.

Size Ranges of amoeba

Amoebas vary in size depending on the species. Their size can range from as small as a few micrometers to as large as several millimeters. Here is a general overview of the size range of amoebas:

  1. Small Amoebas:
    • Some amoeba species are very small, with sizes typically ranging from 10 to 20 micrometers (µm) in diameter. These small amoebas are often found in soil, water, and as parasites in other organisms.
  2. Medium-Sized Amoebas:
    • Many common amoeba species, such as Amoeba proteus, fall into this category. They typically range from 200 to 600 micrometers (µm) in size. These medium-sized amoebas are often studied in laboratories due to their relatively large size and ease of observation.
  3. Large Amoebas:
    • Some amoebas can grow to be quite large. For example, the species Pelomyxa palustris can reach sizes of up to 5 millimeters (mm) in diameter, making them visible to the naked eye. These larger amoebas are usually found in specific environments, such as freshwater habitats rich in organic material.


The origin of amoebas traces back to the early stages of life on Earth, approximately 1.5 to 2 billion years ago. Here is an overview of their origin and evolutionary history:

  1. Prokaryotic Ancestors:
    • The earliest life forms on Earth were simple prokaryotic cells, such as bacteria and archaea. These single-celled organisms dominated the planet for billions of years. It is believed that amoebas, like other eukaryotes, evolved from these prokaryotic ancestors.
  2. Emergence of Eukaryotes:
    • Around 1.5 to 2 billion years ago, eukaryotic cells emerged. These cells are characterized by a defined nucleus and membrane-bound organelles. The endosymbiotic theory suggests that eukaryotic cells arose when a prokaryotic cell engulfed another, forming a symbiotic relationship. This event led to the development of mitochondria and, in some lineages, chloroplasts.
  3. Protists and Early Amoeboid Forms:
    • Eukaryotic cells diversified into various forms, leading to the emergence of the Protista kingdom, which includes amoebas. The earliest amoeboid forms likely appeared during this time, characterized by their flexible shape and ability to form pseudopodia.
  4. Evolutionary Adaptations:
    • Over time, amoebas adapted to a wide range of environments, from freshwater and marine habitats to soil and as parasites in other organisms. Their simple yet versatile structure allowed them to survive and thrive in diverse ecological niches.
  5. Modern Amoebas:
    • Today, amoebas are classified within the phylum Amoebozoa, which includes various species with different sizes, shapes, and lifestyles. Some amoebas are free-living, while others are parasitic, such as Entamoeba histolytica, which causes amoebic dysentery in humans.

Habitat of Amoeba

Amoebas are highly adaptable and can thrive in a variety of environments. Their habitats include:

  1. Freshwater Environments:
    • Ponds, Lakes, and Streams: Amoebas are commonly found in still or slow-moving bodies of fresh water. They thrive in the nutrient-rich environments of ponds and lakes, where they feed on bacteria, algae, and other microorganisms.
    • Soil Moisture: Amoebas can also be found in moist soil, where they play a role in decomposing organic matter and recycling nutrients.
  2. Marine Environments:
    • Oceans and Seas: Some species of amoebas inhabit saltwater environments. They can be found in the intertidal zones, estuaries, and even deep-sea sediments, where they feed on plankton and other small marine organisms.
  3. Moist Terrestrial Environments:
    • Leaf Litter and Forest Floors: Amoebas can be found in the moist environments of leaf litter and decaying plant material on forest floors. They contribute to the breakdown of organic matter, helping to release nutrients back into the ecosystem.
  4. Host Organisms:
    • Parasitic Amoebas: Some amoebas are parasitic and can live inside host organisms. For example, Entamoeba histolytica resides in the human intestine and can cause amoebic dysentery. Similarly, Naegleria fowleri can inhabit the human brain if it enters through the nasal passages.
  5. Extreme Environments:
    • Thermal Springs: Certain amoebas can survive in extreme conditions, such as hot springs, where temperatures can be quite high.
    • Harsh Conditions: Some amoebas have adaptations that allow them to survive in environments with fluctuating temperatures, pH levels, and nutrient availability.

Diseases Caused by Ameba

Amebae are single-celled organisms that can cause various diseases in humans. Here are some notable diseases caused by ameba:

1. Amebiasis

Causative Agent: Entamoeba histolytica

Transmission: Fecal-oral route, primarily through contaminated food and water.


  • Diarrhea (can be bloody)
  • Abdominal pain
  • Fever
  • Weight loss

Complications: Severe cases can lead to liver abscesses, intestinal perforation, and peritonitis.

2. Primary Amebic Meningoencephalitis (PAM)

Causative Agent: Naegleria fowleri

Transmission: Entry of contaminated water into the nasal passages, often during swimming in warm freshwater bodies.


  • Severe headache
  • Fever
  • Nausea and vomiting
  • Stiff neck
  • Confusion
  • Seizures

Complications: Rapidly progresses to death if not treated promptly. PAM is almost always fatal.

3. Granulomatous Amebic Encephalitis (GAE)

Causative Agents: Acanthamoeba species and Balamuthia mandrillaris

Transmission: Through skin wounds or inhalation of cysts, can occur in immunocompromised individuals.


  • Headache
  • Fever
  • Neurological deficits (e.g., paralysis, speech difficulties)
  • Confusion
  • Seizures

Complications: Often fatal without early diagnosis and treatment.

4. Acanthamoeba Keratitis

Causative Agent: Acanthamoeba species

Transmission: Contact lens use, exposure to contaminated water.


  • Eye pain
  • Redness
  • Blurred vision
  • Sensitivity to light
  • Excessive tearing

Complications: Can lead to blindness if not treated effectively.

Prevention and Control

  • Good Hygiene: Regular handwashing, especially after using the bathroom and before handling food.
  • Safe Water Practices: Drinking and using safe, treated water. Avoid swimming in potentially contaminated water bodies.
  • Proper Contact Lens Care: Disinfecting contact lenses properly and avoiding the use of tap water for cleaning lenses.
  • Prompt Medical Attention: Seeking immediate medical care if symptoms of amebic infections appear, especially after exposure to contaminated water.

Benefits of Amoebas to Humans

While some amoebas are known for causing diseases, many others play beneficial roles in the ecosystem and can indirectly or directly benefit humans. Here are some ways amoebas are beneficial

  1. Nutrient Cycling:
    • Decomposition: Amoebas help decompose organic matter, breaking it down into simpler substances that can be reused by plants and other organisms. This process is crucial for nutrient cycling in ecosystems.
    • Soil Health: By decomposing organic material in the soil, amoebas contribute to soil fertility, which benefits agriculture and plant growth.
  2. Microbial Control:
    • Predation on Bacteria: Amoebas feed on bacteria, algae, and other microorganisms, helping to control microbial populations in various environments. This can prevent the overgrowth of harmful bacteria and maintain ecological balance.
    • Pathogen Reduction: By consuming pathogenic bacteria in natural environments, amoebas can help reduce the spread of certain diseases.
  3. Research and Biotechnology:
    • Model Organisms: Certain amoebas, such as Dictyostelium discoideum, are used as model organisms in scientific research. They provide insights into cellular processes, such as cell motility, differentiation, and signaling pathways, which can be applied to understand human biology and diseases.
    • Genetic Studies: Amoebas are used in genetic and evolutionary studies to explore the complexities of eukaryotic cells and the evolution of multicellularity.
  4. Bioremediation:
    • Pollutant Degradation: Some amoebas have the potential to be used in bioremediation, a process where living organisms are used to clean up polluted environments. Amoebas can help break down harmful substances, contributing to environmental restoration efforts.
  5. Symbiotic Relationships:
    • Symbiosis with Other Organisms: Amoebas can form symbiotic relationships with other organisms, such as certain bacteria. These relationships can be mutually beneficial and play a role in the health and functioning of various ecosystems.


Interesting Facts About Amoebas

  1. Single-Celled Organisms:
    • Amoebas are unicellular organisms, meaning they consist of only one cell. Despite their simplicity, they exhibit complex behaviors and processes.
  2. Movement:
    • Amoebas move using pseudopodia, which are temporary projections of their cytoplasm. This type of movement is called amoeboid movement. Pseudopodia also help in capturing food through a process called phagocytosis.
  3. Feeding:
    • Amoebas are heterotrophic and feed by engulfing food particles. They extend their pseudopodia around the food particle, encase it in a food vacuole, and digest it with enzymes.
  4. Reproduction:
    • Amoebas primarily reproduce asexually through binary fission. During this process, the cell divides into two genetically identical daughter cells.
  5. Variety of Species:
    • There are numerous species of amoebas, each with unique characteristics. Some well-known species include Amoeba proteus, Entamoeba histolytica, and Naegleria fowleri.
  6. Survival Mechanisms:
    • Amoebas can form cysts to survive unfavorable conditions. In the cyst form, they are dormant and can withstand extreme temperatures, desiccation, and lack of food.
  7. Habitat Diversity:
    • Amoebas inhabit a wide range of environments, including freshwater, marine, and moist terrestrial habitats. Some live as parasites in the bodies of animals and humans.
  8. Role in Ecosystems:
    • Amoebas play essential roles in ecosystems, particularly in nutrient cycling and controlling bacterial populations. They help decompose organic matter and recycle nutrients.
  9. Disease-Causing Species:
    • Some amoebas are pathogenic and can cause diseases in humans. For example, Entamoeba histolytica causes amoebic dysentery, and Naegleria fowleri, known as the “brain-eating amoeba,” causes primary amebic meningoencephalitis (PAM).
  10. Research Importance:
    • Amoebas, especially species like Dictyostelium discoideum, are used as model organisms in scientific research. They provide valuable insights into cellular processes, genetics, and the evolution of multicellularity.
  11. Microscopic Giants:
    • Some amoebas, like Amoeba proteus, can be quite large for single-celled organisms, measuring up to 600 micrometers in length. They are visible under a microscope and sometimes even to the naked eye.
  12. Unique Cell Structure:
    • Amoebas have a flexible and dynamic cell membrane that allows them to change shape constantly. They also have a contractile vacuole to regulate water content within the cell, preventing it from bursting in hypotonic environments.

What is an amoeba?

An amoeba is a single-celled, eukaryotic organism that moves and feeds using pseudopodia.

Where are amoebas found?

Amoebas inhabit freshwater, marine environments, soil, and inside host organisms.

How do amoebas move?

Amoebas move using pseudopodia, temporary cytoplasmic projections.

What do amoebas eat?

Amoebas feed on bacteria, algae, and organic matter through phagocytosis.

How do amoebas reproduce?

Amoebas reproduce asexually through binary fission, creating genetically identical daughter cells.

Are all amoebas harmful?

No, most amoebas are harmless, but some, like Entamoeba histolytica, can cause diseases.

What is binary fission?

Binary fission is a form of asexual reproduction where a single cell divides into two identical cells.

What diseases can amoebas cause?

Amoebas can cause diseases like amoebic dysentery and primary amebic meningoencephalitis.

Why are amoebas important?

Amoebas play crucial roles in nutrient cycling, soil health, and scientific research.

What are pseudopodia?

Pseudopodia are temporary, finger-like projections used by amoebas for movement and feeding.

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