Amoeba

**__Amoeba__**
By: Krithi Nathan





**Classification/Diagnostic Characteristics:**
Amoebozoans are unicellular eukaryotes that are part of the Kingdom Protista(Protists). They are classified as protozoa, or early protists that have animal- like properties, such as motility. They diverged from other eukaryotes about 1.5 billion years ago but it is unknown whether they are more closely related to opisthokonts, which include fungi and animals, or other eukaryotic groups.

Amoebae compose of cytoplasm, the fluid component of a cell which is divided into two parts: the thin, semi-permeable ectoplasm which acts as a plasma membrane; and the watery endoplasm, which supports the organelles inside the cell. Amoebas are known to contain one or more nuclei to regulate cellular activities and control reproduction as the nucleus holds all the genetic material of the cell. Several food vacuoles(cavities) allow for the breakdown and absorption of food using enzymes provided by lysosomes, a digestive cell organelle. Amoebazoans also contain structures called contractile vacuoles which expand and contract to regulate the flow of water into and out of the cell. Lobe-shaped pseudopods, or "false feet", are a major characteristic of amoeboid organisms, move in different directions to help them move and find food. Amoeba comes from the Greek word "amoibe", meaning change, since it constantly changes shape with the use of its pseudopods.

**Domain:** Eukaryota **Kingdom:** Protista **Phylum:** Protozoa **Class:** Rhizopoda **Order:** Tubulinida **Family:** Amoebidae **Genus:** Amoeba



**Relationship to Humans:** Some amoebae may be deadly for human contact. Amoebae of the genus Naegleria can enter the human body and cause the deterioration of the nervous system. Naegleria fowleri (commonly referred to as the "brain-eating amoeba" or "brain-eating ameba"), is a free-living microscopic ameba, (single-celled living organism) which only enters the brain for shelter and food. The brain provides heat, moisture, and food supply, the ideal conditions for reproduction. This amoeba is commonly found in warm freshwater and soil and usually infects people when contaminated water enters the body through the nose. Once the amoeba enters the nose, it travels to the brain and creates a hole in the cell membranes of neurons using its pseudopodia. It then consumes the cell matter, killing the cell. To prevent immune system attacks, the amoeba forms a cyst, or impervious layer around it, preventing white blood cells from devouring it. As a result, a rare and fatal infection of the brain called primary amebic meningoencephalitis (PAM) occurs as the immune system cannot stop the reproduction of the amoebas.

Amoebas can be a problem when they live in unhealthy areas, such as places with raw sewage, as they can sometimes cause illness when they get inside of bodies. An example is Amoebic dysentery(Amoebiasis) which is caused by Entamoeba histolytica, one of the most dangerous strains of free amoebae found in tropical, contaminated waters. They can burrow through the intestinal gut and infect many digestive organs via the bloodstream. By infecting organs like the stomach and intestines, humans can suffer from bloody diarrhea, fever, and severe dehydration. This disease can be treated with medicines but can create severe abdominal discomfort. Many amoebae are non- pathogenic(harmless) and are present in the human body for digestive functions. Entamoeba coli and many other forms of harmless amoeba line the gastrointestinal tract.

Few amoebae species are also used for scientific research as their cell functions are similar to that of many human cells, such as embryonic stem cells. Examples include Dictyostelium discoideum and Amoeba proteus.

**Habitat and Niche:**
Amoebozoans commonly live on the bottoms of freshwater and saltwater bodies of water where there is an abundant supply of sedentary(not very active) organisms and other organic matter for consumption. They play the roles of scavengers, predators, and parasites in such aquatic ecosystems and use pseudopods to catch their prey. Amoeba can also be found in caves, but are most abundant in nutrient-rich soil. Some non- pathogenic species can be found in the intestines of humans.

**Predator Avoidance:** Predators of amoeba include many different organisms such as nematodes, mussels, and water fleas. Some amoebas protect themselves against predators by secreting toxic substances such as DAPG, pyrrolnitrin, hydrogen cyanide, and pyoluteorin. This ability provides an adequate defense, making them undesirable and dangerous to their predators. When the risk of predation is high, the amoeba secretes more toxins, and when it is lower, less toxins are needed.

**Nutrient Acquisition:** The diet of amoebas' consists of algae, bacteria, dead plant and animal matter, and other protozoans. Amoebas use phagocytosis, a process of engulfing solid food particles with the use of pseudopods, in order to obtain nutrients in the forms of small organisms and particles of organic matter. Food is surrounded and trapped by the pseudopods and then taken into the food vacuole, which constantly moves along with the movement of the pseudopods. The food is then digested, using enzymes from lysosomes, in this organelles. The nutrients are then absorbed into the cell.

//Phagocytosis// media type="custom" key="24575356" align="center"

Free- living amoebas are present in two main developmental stages: the trophozoite stage and the cyst stage. Trophozoites are vegetative amoebas, meaning they are metabolically active and reproduce using binary fission( by splitting into two cells). They retire to the cyst stage when exposed to unfavorable changes in temperature, pH or low food supply by producing a protective cyst membrane around itself. This cyst contains 3 layers: the ectocyst(outermost layer), mesocyst(middle layer), and endocyst(innermost layer). The presence of these 3 layers allow the amoeba to be resistant to many different biocides and sterilization fluids. When present in ideal conditions, the amoeba undergoes exocystment, where the cyst ruptures.
 * Growth and Development: **

If an amoeba is cut in half, the part containing the nucleus can grow cytoplasm and regenerate into a full grown amoeba.



** Reproduction and Life Cycle: ** Amoebae mainly reproduce asexually using binary fission, which results in two daughter cells that are diploid(having a full set of chromosomes). Binary fission occurs when the nucleus of an amoeba cell undergoes cell mitosis(cell division), causing the cell endoplasm to pinch in half, evenly distributing each set of nuclei to each daughter amoeba cell. Amoebas, usually in the encysted state, use multiple fission, another form of asexual reproduction. When present in unfavorable conditions, the amoeba cell will form a 3 layer cyst around itself and inside, the nucleus will divide into several daughter nuclei followed by the division of several cytoplasmic( the fluid portions of the cell) parts. After exposed to moisture, the cyst will burst, releasing the numerous pseupodiaspores(immature amoeba) into the environment. These spores will later develop into individual amoeba cells.

Myxamoebae, the vegetative amoeboid cells of cellular slime molds use this process of multiple fission, also called sporulation characterized by the formation of spores. When the environment cannot be tolerated, slime molds produce spores(fruiting structures) and the individual myxamoebae combine to form a mass called a pseudoplasmodium or slug. Spores are then released from the slug releasing more myxamoebae. Recent studies now show that some species of amoeba are able to reproduce sexually. This is a scientific marvel as it is considered one of the "lower" eukaryotes on the evolutionary scale and is therefore not expected to have such developed properties.



**Integument:**
Many species of amoeba, such as Testate amoebae, live inside shell casings which they make by sticking sand grains together. Some amoebae secrete their own hardened, protective shell through which pseudopods extend from a mouthlike opening.

When present in unfavorable conditions, amoebae often undergo encystment, forming a protective cyst membrane around themselves until exposed to a supportive environment.

Amoebae use their lobe-shaped pseudopodia( false feet), which contain microfilaments in the cytoskeleton for support, in order for cell movement to occur. Pseudopods are soft extensions of the cell body, often temporary, which allow the organism to attach to surfaces and engulf particles through the process of endocytosis. When the amoeba decides which direction it wants to move, the temporary projection(pseudopod) extends in that direction, followed by the body moving to the location of the pseudopod. This movement is called cytoplasmic streaming.
 * Movement: **

This process occurs when the cytoplasm, the fluid component of the cell, stretches the plasmodium strands and causes it to move slowly with the help of a contractile protein called myxomyosin. As it moves, it uses the process of phagocytosis to engulf food particles. This gradual movement allows the amoeba to travel at a maximum speed of about 1 inch an hour. The broader, thicker pseudopods of amoeba can be contrasted with the longer, slender pseudopods of Rhizaria, a unicellular aquatic organism.

// Cytoplasmic Streaming //

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Amoeba use pseudopods to explore the environment around them and locate food sources. Scientists have recently found that when exposed to chemical stimuli, the cytoskeleton in the cell cytoplasm oscillates, or changes shape, allowing for reactive movements. They also use a mechanism called chemotaxis, which is directing movements according to the presence of chemicals in the surroundings. If amoebas are placed in a toxic solution, the ions present will determine their actions, either migration or encystment.
 * Sensing the Environment: **

**Gas Exchange:** Gas is exchanged through diffusion, a process where molecules are moved down their concentration gradient from a place of higher concentration to a place of lower concentration. Oxygen is taken in through the plasma membrane for respiration and carbon dioxide is diffused out the same way.

**Waste Removal:** Excess wastes( urea) and water are pumped out and removed by the contractile vacuole, an organelle that aids in excretion and water regulation.

**Environmental Physiology(temperature, water, salt regulation):**

__// Temperature Regulation: //__ Amoebae form cysts, a protective and impervious layer, around themselves in harsh temperatures in order to prevent any damage to the cell structure within. When the amoebae are exposed to an ideal environment with proper osmotic pressure, pH, and temperature, they will emerge from their cysts and reproduce asexually. __ //Water Regulation: // __ Water regulation, or osmoregulation, is controlled through the contractile vacuole, a membrane- bound organelle located inside the amoeba. Water is brought into the cell through the cell membrane via the process of osmosis. Osmosis is the diffusion of water molecules from a higher concentration to a lower concentration through a semipermeable (allowing certain things in) barrier. When the amoeba is present in an environment with a higher water concentration, the contractile vacuole expands to allow water in(diastole). In an environment that has a low concentration of water, the contractile vacuole will contract to pump water out(systole), preventing the cell from exploding with excess water. Contractile vacuoles are surrounded by mitochondria, which are the organelles that perform cellular respiration, providing ATP energy for the vacuoles to pump with force. These regulatory processes occur to maintain an osmotic equilibrium inside and outside the cell.

__ // Salt Regulation: // __ Amoebas that live in freshwater environments usually have a higher salt concentration inside the cell than the outside causing the contractile vacuole to pump out the excess salt, using energy provided by the mitochondria. Similarly, amoebas that thrive in saltwater environments have a lower concentration of salts inside the cell whereas the environment has a higher salt concentration. This gradient causes mineral salts to enter the cell via osmosis.

**Internal Circulation:**
Since amoebas do not have any circulatory system, the cell membrane plays the role in performing cellular respiration. The surface area to volume ratio is quite small, allowing oxygen to diffuse through the cell membrane into the mitochondria for respiration. Then, carbon dioxide diffuses out.

Amoebae are early eukaryotes and therefore don't have an endocrine system.
 * Chemical Control( Endocrine System): **

** Review Questions: **
1. What are pseudopodia? What are they made of and how do amoebae use them? 2. Explain how myxamoebae perform reproduction. Why are spores produced in some cases of reproduction but not others? 3. Why is it unnecessary for amoebae to have an endocrine system? What physical properties determine the need for an endocrine system? 4.Why is the contractile vacuole essential for amoebae survival? What are purposes does it serve? 5. How is Naegleria fowleri harmful to the human nervous system? Are all amoebae species harmful to humans? If not, how are they beneficial?

** References: **
1.[] 2. McGraw Hill Companies 3. "Ecology of the Amoebae." Ecology of the Amoebae. N.p., n.d. Web. 17 Nov. 2013. . 4. http://www.fcps.edu/islandcreekes/ecology/amoeba.htm 5. http://www.ncbi.nlm.nih.gov/pubmed/2052586 6. http://tolweb.org/notes/?note_id=51 7. [|http://www.microscopy-uk.org.uk/mag/imgsep01/amoebaproteus450.jp] 8. [] 9.[] 10.[] 11. [] 12. [] 13. [] 14. [|http://www.tutorvista.com/content/biology/biology-iv/reproduction-in-animals/asexual-reproduction-types.php#] 15. [] 16.[|http://www.fcps.edu/islandcreekes/ecology/amoeba.h]tp 17.[] 18.[] 19.[] 20.[]