Amoeba - Structure & Classification

Amoeba - Structure & Classification

Irshad AnwarUpdated on 19 Aug 2025, 05:00 PM IST

Amoeba is a unicellular, shapeless organism that moves with pseudopodia, feeds by phagocytosis, and reproduces by binary fission. It plays a role in microbiology and NEET exam preparation, with special focus on its structure, nutrition, and the brain-eating amoeba (Naegleria fowleri).

This Story also Contains

  1. Introduction of Amoeba
  2. Structure of Amoeba
  3. Classification of Amoeba
  4. Reproduction in Amoeba
  5. Nutrition in Amoeba
  6. Tips, Tricks, & Mnemonics for Amoeba
  7. Exam Relevance of Amoeba
  8. Amoeba NEET MCQs
  9. FAQs on Amoeba
Amoeba - Structure & Classification
Amoeba

Amoeba is a unicellular, belonging to phylum protista. It has no definite shape and moves with pseudopodia. They are found in both fresh- and salt-water and in moist soils. Some are planktonic, some prefer a substratum and a few are parasitic. Most amoebas reproduce by binary fission but sporulation and budding occur in some. Nutrition in amoebas is holozoic, they ingest and digest liquid or solid foods. Most amoebas are omnivorous, living on algae, bacteria, protozoa, rotifers, and other microscopic organisms. The shape of pseudopodia formed by each species of ameba has been used as a character for classification. Naegleria fowleri, a free-living organism that causes primary amebic meningoencephalitis (PAM) in humans. Organisms that cause human disease are frequently used as representatives in phylogenetic studies because they are more readily available than their wild counterparts.

Introduction of Amoeba

Amoeba highlights its role as a basic form of life that plays a crucial part in various ecosystems. Amoeba is defined as a unicellular organism that can only be seen by a microscope and not by the naked eye. It is eukaryotic, meaning that it is a true organism that possesses true organelles and a nuclear envelope. It moves with the help of false feet called pseudopodia. The amoeba's main characteristic is its ability to change shape.

The brain-eating amoeba, Naegleria fowleri, is known for its ability to cause a rare and often fatal infection in humans called primary amoebic meningoencephalitis (PAM).

Structure of Amoeba

Amoeba usually differs in both size and shape. When the earliest animals were identified, they were approximately 400 to 600 micrometres in size. Many sizes of amoeba are extremely small, 2-3 nanometers in size and as well as exceptionally large such as 20 cm amoeba. The body is generally transparent, and it looks normally like gelatin.

  • Movement in the amoeba occurs through false feet called pseudopodia, so it changes its shape.

  • The structure of amoebas is differentiated into three parts: cytoplasm, plasma membrane, and nucleus.

  • The cytoplasm of the amoeba is differentiated into two layers that are named as outer ectoplasm and inner endoplasm.

  • The plasma membrane of an amoeba is usually very thin, and it is a double-layered membrane that is made up of protein and lipid molecules.

  • When we talk about other cellular organelles, it contains contractile vacuoles, mitochondria, the Golgi apparatus, and some of the fat globules.

  • The contractile vacuole is used to maintain the osmotic equilibrium by separating out most of the water from the cell.

Classification of Amoeba

All the discussion of the amoeba and which domain and phylum it belongs to is defined here:

  • Domain: Eukaryota (true cells)

  • Kingdom: Amoebozoa

  • Phylum: Tubulinea

  • Order: Tubulinida

  • Family: Amoebidae

  • Genus: Amoeba

  • Species: many different species, such as Proteus and Animacule.

NEET Highest Scoring Chapters & Topics
Know Most Scoring Concepts in NEET 2024 Based on Previous Year Analysis.
Know More

Nowadays, studies have been focusing on classifying the amoeba based on the subunit present in its ribosomal RNA. Sarcodina is the most preferable and most accepted taxonomic group that belongs to the classification of the amoeba cell. It is classified based on its observable characteristics.

Reproduction in Amoeba

Binary fission:

  • Binary fission in amoeba is a type of asexual reproduction where a parent cell splits into two identical daughter cells.

  • In binary fission , the organism first duplicates its genetic content and then it elongates and splits into two equal halves.

  • In amoeba, a binary fission is relatively fast. In ideal conditions, a full cycle usually takes under half an hour.

  • After karyokinesis has divided the nucleus, cytokinesis separates the cytoplasm into two cells that eventually can develop into an independent living organism.

Spore formation:

When there are unfavourable conditions, the amoeba divides with the help of spore formation. It starts to break its nuclear membrane, and some of the chromatin blocks get released into the cytoplasm. Each chromatin block now gets inside a nuclear membrane and becomes a small daughter nucleus. Unlike multicellular organisms, amoeba does not involve gamete fusion or sexual reproduction.

Nutrition in Amoeba

Nutrition in amoebas is holozoic, they ingest and digest liquid or solid foods. Most amoebas are omnivorous. Amoeba engulfs the food particle and forms a food vacuole. The food is digested inside the vacuole. The are five stages in the mode of nutrition:

Process

Description

Ingestion

  • Amoeba uses its pseudopodia to surround and engulf food particles.

  • This is the most common method adopted by the amoeba to incorporate food into its body.

  • This process is known as ingestion.

  • The process of engulfing food around itself is done using pseudopodia, and this is termed phagocytosis.

Digestion

  • The next step is digestion, which is the process in which food in the food vacuole is broken down by enzymes into simpler molecules that can be absorbed.

  • Lysosomes help facilitate the breakdown of food material.

Absorption

  • Absorption occurs when the digested nutrients diffuse into the cytoplasm of the amoeba.

  • After digestion, food supplements are absorbed, while undigested particles are left behind.

Assimilation

  • Assimilation, in which absorbed nutrients are used to generate energy and support growth.

  • This is the process through which the amoeba derives energy from the food it consumes.

Egestion

  • Egestion is the removal of undigested food by rupturing the cell membrane to expel waste materials produced during digestion.

  • This is how the amoeba eliminates undigested food.

Brain-Eating Amoeba

  • Naegleria fowleri, a free-living organism that causes primary amebic meningoencephalitis (PAM), a serious health condition in humans.

  • Brain-eating amoeba thrives under unkept swimming pools and hot tubs. They enter the body through nostrils.

  • Research on brain-eating amoeba has opened the window to the survival capabilities under intense heat, providing a good explanation for the reason it is found frequently in warm waters.

  • While the name "brain-eating amoeba" certainly sounds ominous, it is worth noting that infections are incredibly rare and tend to occur under special conditions.

Tips, Tricks, & Mnemonics for Amoeba

Strategy

Description

Understand Key Concepts

Focus on the structure, nutrition, and reproduction of amoebas.

Practice with Diagrams

Draw and label diagrams of amoebas to reinforce learning.

Use Mnemonics

Create mnemonics to remember key terms:


- Pseudopodia: "Pseudopods Push Food"


- Nutrition Process: "I Eat Really Good Food"


- Binary Fission: "Bacteria Multiply Fast"


- Brain-Eating Amoeba: "Naegleria's Nasty Niche"

Additional Mnemonincs:

  • Essential Amino Acids: "Any Help In Learning These Little Molecules Proves Truly Valuable"

  • Arginine, Histidine, Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan, Valine.

  • Taxonomic Rank: "Do Kindly Put Cover On Fresh Green Spinach"

  • Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

  • Phases of Mitosis: "I Prefer Milk And Tea"

  • Interphase, Prophase, Metaphase.

  • Types of WBC (White Blood Cells): "Never Let Momma Eat Beans"

  • Neutrophils (60%), Lymphocytes (30%), Monocytes (6%), Eosinophils (3%), Basophils (1%).

Exam Relevance of Amoeba

The questions are asked on structre of amoeba, Binary Fission, Nutrition in Amoeba, and Brain-Eating Amoeba in different examinations such as, NEET, JEE, AIIMS Exams, UPSC, SSC CGL, NDA, ICAR AIEEA.

Amoeba NEET MCQs

Q1. Amoeba moves and engulfs food with the help of:

  1. Cilia

  2. Flagella

  3. Pseudopodia

  4. Contractile vacuole

Correct answer: 3) Pseudopodia

Explanation:

Amoeba is a unicellular organism that moves by forming temporary extensions of its cytoplasm known as pseudopodia (false feet). These structures help the organism in locomotion as well as in capturing food through phagocytosis. Unlike cilia or flagella (used by other protists), pseudopodia provide flexible movement and are essential for the amoeba's survival.

Hence, the correct answer is option 3) Pseudopodia.

Q2. The contractile vacuole in amoeba functions to:

  1. Store nutrients

  2. Maintain osmotic balance

  3. Aid locomotion

  4. Reproduction

Correct answer: 2) Maintain osmotic balance

Explanation:

The contractile vacuole of amoeba is a specialized structure that helps in osmoregulation, i.e., maintaining the water balance inside the cell. Since amoeba lives in freshwater, water constantly enters the cell by osmosis. The contractile vacuole collects excess water and periodically expels it out, preventing the cell from bursting. It does not play a role in nutrient storage, movement, or reproduction.

Hence, the correct answer is option 2) Maintain osmotic balance.

Q3. Which condition is caused by Naegleria fowleri?

  1. Amoebic dysentery

  2. Primary Amoebic Meningoencephalitis (PAM)

  3. Malaria

  4. Sleeping sickness

Correct answer: 2) Primary Amoebic Meningoencephalitis (PAM)

Explanation:

Naegleria fowleri is commonly called the “brain-eating amoeba.” It enters the human body when contaminated water enters through the nose. From there, it travels to the brain and causes Primary Amoebic Meningoencephalitis (PAM), a rare but usually fatal infection. Amoebic dysentery is caused by Entamoeba histolytica, malaria by Plasmodium, and sleeping sickness by Trypanosoma.

Hence the correct answer is option, 2) Primary Amoebic Meningoencephalitis (PAM).

Also read:

FAQs on Amoeba

What is an amoeba?

An amoeba is a unicellular eukaryotic organism that belongs to the kingdom Protista. It is characterized by its irregular shape and the presence of pseudopodia, which are finger-like extensions of the cytoplasm. Pseudopodia used for movement and capturing food. They have a cell membrane, cytoplasm, nucleus, contractile vacuole, and food vacuoles. Amoeba is widely studied as a model organism to understand cell biology, osmoregulation, and simple life processes in unicellular organisms.

How does an amoeba reproduce?

Amoeba mainly reproduces asexually by binary fission. In this process, the nucleus undergoes mitotic division, followed by division of the cytoplasm, resulting in two genetically identical daughter cells. Under unfavorable conditions, amoeba can form protective cysts and reproduce by spore formation, ensuring survival in harsh environments. Unlike multicellular organisms, amoeba does not involve gamete fusion or sexual reproduction.

How does amoeba obtain its food?

Amoeba feeds through a process called phagocytosis. It extends its pseudopodia around a food particle such as bacteria or algae, enclosing it within a food vacuole. Inside this vacuole, digestive enzymes break down complex food into simpler nutrients. These nutrients are absorbed into the cytoplasm and utilized for growth, energy, and repair. The undigested residue is expelled by egestion through the cell membrane. This simple but efficient process helps amoeba adapt to varied environments.

What is a brain-eating amoeba?

The term brain-eating amoeba refers to Naegleria fowleri, a free-living amoeba found in warm freshwater bodies such as lakes and ponds. It can infect humans if contaminated water enters the body through the nasal cavity. The amoeba then travels along the olfactory nerve to the brain, causing a rare but often fatal condition called Primary Amoebic Meningoencephalitis (PAM). Symptoms include headache, fever, stiff neck, and neurological damage. Although rare, the infection has a very high fatality rate and requires immediate medical intervention.

Frequently Asked Questions (FAQs)

Q: Is Amoeba Harmful to Humans?
A:

Yes, certain types of amoebas can be harmful to humans. The most notorious among them is Naegleria fowleri, commonly known as the brain-eating amoeba. This organism can cause a severe and often fatal infection called primary amoebic meningoencephalitis (PAM) when contaminated water enters the body through the nose. While infections are rare, they are frequently fatal, with only a few documented survivors. Other pathogenic amoebas, such as Entamoeba histolytica, can cause amoebic dysentery, leading to gastrointestinal issues.

Q: What Bacteria is Amoeba?
A:
Amoebas are not bacteria; they are single-celled eukaryotic organisms classified under the Kingdom Protista. However, some amoebas can interact with bacteria in various ways. For instance, certain amoebas feed on bacteria as part of their diet, while others may act as hosts for pathogenic bacteria. Notably, Acanthamoeba species can harbor bacteria like Legionella, which can be harmful to humans. Thus, while amoebas themselves are not bacteria, their interactions with them can have significant implications for health.
Q: What is the composition of the cytoplasm of an amoeba?
A:

The cytoplasm of an amoeba is usually differentiated as a thin and stiff plasma membrane, and inside the plasma membrane there is an ectoplasm. The inner one is the granular endoplasm.

Q: Amoeba considered a unicellular or multicellular organism?
A:

An amoeba is considered to be a unicellular organism as it is made up of one type of cell.

Q: What are some of the benefits of amoebas to the environment?
A:

Amoeba plays a great role in maintaining the environment.

It helps to regulate the amount of algae by feeding on them.

It helps in regulating the amount of bacteria, which causes the disbalance of algae.

It helps to clean the environment and balance the environment.

Q: Is an amoeba a mortal or immortal cell?
A:

Amoeba is considered to be an immortal cell, despite  having a lifespan of only 2 days. The reason behind this is that it produces two daughter cells that are exact replicas of the amoeba.

Q: Do amoeba possess the capacity for regeneration?
A:

Regeneration is defined as the process by which the organism develops its lost part again. Yes , amoeba has the capacity for regeneration. when amoeba is cut down into two pieces then it regenerates its lost part?

Questions related to

On Question asked by student community

Have a question related to ?

Correct Answer: Entamoeba


Solution : The correct answer is Entamoeba.

A parasitic intestinal infection known as amoebiasis or amoebic dysentery is brought on by any member of the Entamoeba genus of amoebas. It could show up with no symptoms at all or with dysentery, diarrhoea, or abdominal pain.