The Life of an Amoeba

Amoeba Anatomy, Digestion, and Reproduction

Amoeba Protozoan
Amoeba Protozoan Feeding. Credit: Science Photo Library-Eric Grave/Brand X Pictures/Getty Images

The Life of an Amoeba

Amoebas are unicellular eukaryotic organisms classified in the Kingdom Protista. Amoebas are amorphous and appear as jelly-like blobs as they move about. These microscopic protozoa move by changing their shape, exhibiting a unique type of crawling motion that has come to be known as amoeboid movement. Amoebas make their homes in salt water and freshwater aquatic environments, soils, and some parasitic amoebas inhabit animals and humans.

Amoeba Classification

Amoebas belong to the Domain Eukarya, Kingdom Protista, Phyllum Protozoa, Class Rhizopoda, Order Amoebida, and the Family Amoebidae.

Amoeba Anatomy

Amoebas are simple in form consisting of cytoplasm surrounded by a cell membrane. The outer portion of the cytoplasm (ectoplasm) is clear and gel-like, while the inner portion of the cytoplasm (endoplasm) is granular and contains organelles, such as a nuclei, mitochondria, and vacuoles. Some vacuoles digest food, while others expel excess water and waste from the cell through the plasma membrane. The most unique aspect of amoeba anatomy is the formation of temporary extensions of the cytoplasm known as pseudopodia. These "false feet" are used for locomotion, as well as to capture food (bacteria, algae, and other microscopic organisms).

Amoebas don't have lungs or any other type of respiratory organ. Respiration occurs as dissolved oxygen in the water diffuses across the cell membrane.

In turn, carbon dioxide is eliminated from the amoeba by diffusion across the membrane into the surrounding water. Water is also able to cross the amoeba plasma membrane by osmosis. Any excess accumulation of water is expelled by contractile vacuoles within the amoeba.

Nutrient Acquisition and Digestion

Amoebas obtain food by capturing their prey with their pseudopodia.

The food is internalized through the process of phagocytosis. In this process, the pseudopodia surround and engulf a bacterium or other food source. A food vacuole forms around the food particle as it is internalized by the amoeba. Organelles known as lysosomes fuse with the vacuole releasing digestive enzymes inside the vacuole. Nutrients are obtained as the enzymes digest the food inside the vacuole. Once the meal is complete, the food vacuole dissolves.


Amoebas reproduce by the asexual process of binary fission. In binary fission, a single cell divides forming two identical cells. This type of reproduction happens as a result of mitosis. In mitosis, replicated DNA and organelles are divided between two daughter cells. These cells are genetically identical. Some amoeba also reproduce by multiple fission. In multiple fission, the amoeba secretes a three-layered wall of cells that harden around its body. This layer, known as a cyst, protects the amoeba when conditions become harsh. Protected in the cyst, the nucleus divides several times. This nuclear division is followed by the division of the cytoplasm for the same number of times. The result of multiple fission is the production of several daughter cells that are released once conditions become favorable again and the cyst ruptures.

In some cases, amoebas also reproduce by producing spores.

Parasitic Amoebas

Some amoeba are parasitic and cause serious illness and even death in humans. Entamoeba histolytica cause amebiasis, a condition resulting in diarrhea and stomach pain. These microbes also cause amebic dysentery, a severe form of amebiasis. Entamoeba histolytica travel through the digestive system and inhabit the large intestines. In rare cases, they can enter the bloodstream and infect the liver or brain.

Another type of amoeba, Naegleria fowleri, causes the brain disease amoebic meningoencephalitis. Also known as brain-eating amoeba, these organisms typically inhabit warm lakes, ponds, soil, and untreated pools. If N. fowleri enter the body though the nose, they can travel to the frontal lobe of the brain and cause a serious infection.

The microbes feed on brain matter by releasing enzymes that dissolve brain tissue. N. fowleri infection in humans is rare but most often fatal.

Acanthamoeba cause the disease Acanthamoeba keratitis. This disease results from an infection of the cornea of the eye. Acanthamoeba keratitis can cause eye pain, vision problems, and may result in blindness if left untreated. Individuals who wear contact lenses most often experience this type of infection. Contact lenses can become contaminated with Acanthamoeba if they are not properly disinfected and stored, or if worn while showering or swimming. To reduce the risk of developing Acanthamoeba keratitis, the CDC recommends that you properly wash and dry your hands before handling contact lenses, clean or replace lenses when needed, and store lenses in a sterile solution.


  • "Naegleria fowleri — Primary Amebic Meningoencephalitis (PAM) — Amebic Encephalitis." Centers for Disease Control and Prevention. Updated September 24, 2015.
  • "Acanthamoeba Keratitis FAQs" Centers for Disease Control and Prevention. Updated August 21, 2012.
  • "Amoeba." UXL Encyclopedia of Science. 2002. Retrieved June 16, 2016 from