Amoeba (Protist)
by Alexander Soloviev
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Table of Contents

1. Classification/Diagnostic Characteristics
2. Relationship to Humans
3. Habitat and Niche
4. Predator Avoidance
5. Nutrient Acquisition
6. Reproduction and Life Cycle
7. Growth and Development
8. Integument
9. Movement
10. Sensing the Environment
11. Gas Exchange
12. Waste Removal
13. Environmental Physiology (Temperature, Water, Salt Regulation)
14. Internal Circulation
15. Chemical Control (Endocrine System)
16. Review Questions
17. References


Classification/Diagnostic Characteristics

The term amoeba covers an enormously diverse group of protists, a diverse category of eukaryotic microorganisms, that have adopted a crawling-like method of locomotion, one characterized by the use of pseudopods, or false feet. Because the amoeboid body form is one of the most widespread, it appears in several protist groups that are only distinctly related to one another. From comparisons of molecular data and from life history data, it is evident that the ameboid body form is adaptive and that the amoebae constitute a polyphyletic group. This means that the amoebae consist of multiple distantly related organisms that do not share a common ancestor and that are separated by other groups with very different lifestyles. The body forms of heteroloboseans, for example, closely resemble those of loboseans, an amoebozoan group that is not at all closely related to heteroloboseans. The different categories of amoebae can be distinguished by the appearance of their pseudopods.[1] In addition, the amoeba's most recognizable features include one or more nuclei and a vacuole that maintains osmotic equilibrium.[2]

A diagram of the common amoeba
A diagram of the common amoeba



















Relationship to Humans

The amoebae are of importance both medically and ecologically. Most people are familiar with the species Amoeba proteus and Chaos carolinense, as these have been studied since the early days of microscopy when attempts were made to gain information on the movement of cells. Even today, the amoeba continues to be a useful tool in cellular biology. From a medical standpoint, the amoebae are important because of the diseases and other detrimental symptoms that they can cause. For instance, fresh-water amoebas of the heteroloboseans genus Naegleria, which is a particular biological grouping, can enter the human body and cause a fatal neurodegenerative disease of the nervous system, destroying brain tissue and almost always killing the target in less than a week.[3] Furthermore, the amoeba Entamoeba histolytica is a parasitic organism that can infect the human gastrointestinal tract (GT), affecting about 10% of the world's population and acting as the third most common cause of death from parasitic infections.[4] Other species are more or less benign coresidents in the digestive systems of many animals, including humans.[5]


Habitat and Niche
Within each ecosystem, or biological community of interacting organisms, there exist various habitats in which populations of similar species reside. Although amoebae can be found in a variety of different habitats, or natural environments, they mainly exist in freshwater and saltwater ecosystems, soil ecosystems, and in the bodies of animals. A majority of loboseans are adapted for life on the bottoms of lakes, ponds, and other bodies of water, as their creeping locomotion and manner of engulfing food particles make them fit for a life close to a relatively rich supply of sedentary organisms. Consequently, many loboseans play the role of predators, which eat other organisms; parasites, which reside in host organisms and benefit from nutrients at the host’s expense; and scavengers, which feast on dead organic matter. Naked amoebae, on the other hand, are major consumers of bacteria in soil ecosystems and ensure nutrient regeneration and continued functioning of the ecosystem.[6] In many bodies of water, amoebas act as a control on algae and thus occupy their niche. Without the presence amoeba in some cases, algae would grow rampant and cause dead zones to develop.[7]


Predator Avoidance
Amoeba are able to ward off threatening predators and other amoeba competing for the same limited resources by secreting certain chemicals, including pyrrolnitrin, hydrogen cyanide, and pyoluteorin. Doing so, however, requires a lot of energy, so they judge the risk of the danger from a predator and secrete toxins as necessary.[8] These chemicals also prevent any occurrences of amoeba cannibalism.[9] Furthermore, amoeba rely on their pseudopods to avoid becoming prey for other organisms, such as certain nematodes, fleas, and mussels.[10]


Nutrient Acquisition

When an amoeba becomes aware of the presence of food in its surrounding medium, it is able to change its body shape and extend some parts of its membrane to form pseudopods, or false feet. In a process known as phagocytosis, a mechanism used by many protists to acquire nutrients, an amoeba engulfs its prey or particles of appropriate size with pseudopodia, forming cavities within the cytoplasm of its cell called food vacuoles. Digestive enzymes, the biological catalysts manufactured and secreted by the amoeba, are then subsequently poured into the vacuoles and the food particles are digested. Useful compounds are directly absorbed into the amoeba’s body, while useless residues remain in the vacuoles and are ultimately expelled as the vacuole comes into contact with the cell membrane. Loboseans generally employ the technique of phagocytosis to feed on small organisms and particles of organic matter. Amoebas can easily distinguish food from other material and are able to feed on algae, metazoa, protozoa, plant cells, bacteria, and other microorganisms.[11]

A close-up of how amoeba acquire nutrients
A close-up of how amoeba acquire nutrients
[12]


Reproduction and Life Cycle

Amoebae generally reproduce assexually by binary fission, the equal splitting of one cell into two by mitosis followed by cytokinesis. Mitosis is the name given for cell division in eukaryotes that leads to the formation of two daughter nuclei, each storing genetic information that is nearly identical to that of the original nucleus. Cytokinesis is the cytoplasmic division of a cell at the end of mitosis that brings about the separation of the two new cells. The asexual reproduction observed in amoeba results in offspring that are genetically nearly identical to their parents and differ only by mutations, or changes in DNA structure, that may arise during DNA replication. Amoebas of the heteroloboseans genus Naegleria usually have a two-stage life cycle, in which one stage has amoeboid cells and the other flagellated cells. In most cases, however, the life cycle of the amoeba cell is limited to simple mitosis and a typical pattern of growth and cell division.[13]

A step-by-step diagram of mitosis, showing how amoeba reproduce
A step-by-step diagram of mitosis, showing how amoeba reproduce
[14]


Growth and Development
The stereotypical growth of the amoeba encompasses a lack of any particular development that is not included in the process of mitosis. However, throughout the period of its regular cell growth, an amoeba cell may experience encysted states, which are comparable to suspended animation, flagellated forms, which involve the growth of a whip-like organelle called a flagellum, or transitions from sexual to asexual phases of growth. Amoebae consist of a single cell that has no definite shape and are capable of demonstrating pleomorphism, the occurrence of various distinct forms. Active amoeba do, however, possess a distinct front and a back end. The single cell of the amoeba is composed of a nucleus, which is a central compartment that contains the cell’s genetic information, and a cytoplasm, the jelly like fluid that surrounds the nucleus. The outer layer of the cytoplasm, referred to as the ectoplasm, is a clear gel, while the bulk of the amoeba consists of internal endoplasm, which is more fluid than the ectoplasm and contains minute structures.[15]

Diagram of cell division in an amoeba
Diagram of cell division in an amoeba


Integument
Both the amoeba cell’s organelles, the membrane-enclosed organized structures, and the cell’s cytoplasm are enclosed by a cellular membrane. Some amoebas have a more complex integument, or outer protective layer. Members of one group of loboseans, the testate amoebas, live inside shells that they produce by gluing sand grains together. For instance, the testate amoeba Nebela collaris produces a light bulb-shaped shell of sand grains called a test. Other amoebas have shells secreted by the organism itself.[16]


Movement
The lobe-shaped pseudopods, or false feet, that are present in amoebozoans characterize the amoeboid body form. Pseudopods constantly change shape as an amoeba moves and feeds or responds to different environmental stimuli. During movement, one point of the amoeba starts to spread out into a protuberance called a pseudopodium and the fluid endoplasm begins to flow into the extension, bringing the amoeba to a new position. The direction of an amoeba’s movement is determined by local differences in water, such as the water’s acidity or alkalinity as well as the chemicals diffusing from suitable food material. Either one of these factors can cause the cytoplasm to begin moving toward a particular location.[17]

An amoeba using pseudopods to move
An amoeba using pseudopods to move
[18]


Sensing the Environment
Amoeba are able to respond to a variety of distinct stimuli, including food, changes in surrounding temperature, the presence or absence of light, other organisms, and alterations in chemical concentrations. Food sources usually release some type of chemicals into the surrounding medium that the amoeba recognize and are attracted to, which triggers the mechanisms responsible for movement. Furthermore, if the ambient temperature becomes too cold, an amoeba may form a cyst and remain inactive until the temperature is warmer.[19]


Gas Exchange
In amoeba cells, gas exchange and cellular respiration, a metabolic process by which larger molecules are broken down into smaller ones and energy is released, occur by the diffusion of gases through the cellular membrane. Diffusion has remained an efficient method of gas exchange because amoeba have a relatively large surface area to volume ratio.[20] Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. The porous cellular membrane allows for the intake of oxygen gas and the release of carbon dioxide.


Waste Removal
Because the amoeba does not have a stomach, digestion of food takes place in a number of vacuoles inside the cell. Certain vacuoles also perform the function of excretion, the elimination of wastes from the cell, by expelling useless residues when the vacuole comes into contact with the cellular membrane.[21] The primary method by which amoebae remove waste is through diffusion of the cell membrane.[22]


Environmental Physiology (Temperature, Water, and Salt Regulation)
Water and salt regulation in amoebae is controlled by osmosis, the tendency of a fluid to pass through a semipermeable membrane into a solution where the solvent concentration is higher, and diffusion, the movement of particles from an area of higher concentration to an area of lower concentration. Excess water is collected in a spherical vacuole which gradually swells, and then contracts and bursts, liberating the accumulated water to the exterior. Freshwater amoebae take up water constantly through the process of osmosis, with water content being regulated by a pulsating contractile vacuole. Marine amoebae, on the other hand, lack a contractile vacuole. Since amoebae rely on their surroundings to fulfill a vital temperature range, these organisms have a limited method for regulating temperature. Encysted states are one way of combating significant drops in temperature during the winter.[23] In addition, amoeba can not live in highly acidic environments and become cysts quickly after being introduced to such conditions, if they are able to survive.[24] When the environmental conditions are favorable again, they will emerge from the cyst.[25]


Internal Circulation
Although the amoebae does not possess a complex system of internal circulation, the cell membrane, being partially permeable, allows for the direct intake of necessary nutrients into the endoplasm. Moreover, the contractile vacuole, as stated previously, is used for the excretion of wastes and for osmoregulation, or the maintenance of constant fluid pressure.[26]


Chemical Control (Endocrine System)
Because the amoeba is unicellular, it does not have an intricate endocrine system. As previously mentioned in the 'Environmental Physiology' section, most of the chemical control of the amoeba is the result of osmosis and simple diffusion.


Review Questions
1. How do amoebas regulate salt levels?

2. Amoebas have played a huge role in the development of modern medicine. What threats do amoebas pose to human health? Give two examples showing how amoeba invasion can be fatal.

3. Knowing that amoebas lack a stomach, explain the method by which amoebas are able to remove wastes.

4. How are amoebas able to recognize changes in the chemical concentration of food in their environment?

5. Which membrane enables amoebas to exchange gases with their environment?6. How are single-celled amoebas able to acquire nutrients? What specific structures facilitate this action?

7. In what kind of ecosystems is the amoeba most commonly found? What are two reasons why it is essential for the amoeba to occupy these kinds of ecosystems?
8. What are the potential downsides to the amoeba's primary reproduction method of asexual reproduction?
9. What chemicals are used to ward off predators? Despite their efficiency, why are they used rarely?
10. What are cysts and what are the benefits of this adaptation? How does this occur?


References
  1. ^
    Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  2. ^ http://www.scienceclarified.com/Al-As/Amoeba.html
  3. ^ http://www.cdc.gov/parasites/naegleria/
  4. ^ http://www.webhealthcentre.com/DiseaseConditions/ameb.aspx
  5. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  6. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  7. ^ http://www.fcps.edu/islandcreekes/ecology/amoeba.htm
  8. ^ http://www.ncbi.nlm.nih.gov/pubmed/20525866
  9. ^ http://www.fcps.edu/islandcreekes/ecology/amoeba.htm
  10. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  11. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  12. ^ http://faculty.ycp.edu/~kkleiner/fieldnaturalhistory/Protists.html
  13. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  14. ^ http://eweb.furman.edu/~wworthen/bio111/mitosis.htm
  15. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  16. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  17. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  18. ^ http://www.fcps.edu/islandcreekes/ecology/Miscellaneous/Amoeba/amoeba2.jpg
  19. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  20. ^ http://www.biology-resources.com/drawing-amoeba-breathing.html
  21. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  22. ^ http://www.buzzle.com/articles/amoeba-facts.html
  23. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
  24. ^ http://www.bms.ed.ac.uk/research/others/smaciver/Ecology%20of%20the%20Amoebae.htm
  25. ^ http://www.biologycorner.com/worksheets/ameba_color.html
  26. ^ Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.


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