The Physics
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Diameter of a Protozoan

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Curtis, Helen. Biology. N. Sue Barnes, 1989. "Cilia and flagella are long, thin (0.2 micrometer) structures extending from the surface of many types of eukaryotic cells." > 0.2 μm
Intestinal Protozoa: Amebas. William A. Sodeman, Jr., 1995. "The trophozoite is 10 to 60 μm in diameter, ameboid, actively motile, and often erythrophagocytic. In stained specimens, the nucleus has a central karyosome with finely beaded peripheral chromatin. The cyst form is rounded, 10 to 20 μm in diameter, with one to four nuclei showing the characteristic appearance." 10–60 μm
10–20 μm
(amoeba cyst)
The Amoebae. The World of Parasites. Jim Smith. "The motile trophic or feeding stage, the trophozoite measuring about 20-30 μm, is found in the large intestine and caecum, where it feeds on bacteria and other intestinal contents…. The cysts, 10-20 μm in size, pass out with the faeces and remain viable for about 10 days and up to 30 days if they are in water." 20–30 μm
10–20 μm
(amoeba cyst)
William R. Pendergras. Carolina protozoa and invertebrates manual: Carolina biological supply company, 1980. "Most protozoans are microscopic, but certain amoebae reach up to 4 to 5 mm in diameter, and the shells of their cousins the foraminifera may be 10 cm across." 4000–5000 μm
100,000 μm
(foraminifera) inc, Protozoans: 1999-2000 "Protozoa range in diameter from a few thousandths of a millimetre to several millimetres." 1 ~ 1000 μm

Protozoa are single celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are several times larger than bacteria typically ranging from 5 to 500 μm in diameter. Protozoa are classified into three groups based on their shape. Ciliates are the largest and move by means of hair like cilia. They eat the other two types of protozoa, as well as bacteria. Amoebae also can be quite large and move by temporary foot. Amoebae are further divided into testate amoebae that make a shell like covering and naked amoebae without a covering. Flagellates are the smallest of the protozoa and use a few whip like flagella to move.

In amoeba, actin and myosin are organized into a three dimensional network. Locomotion in amoeba is brought about by the continuous assembly and disassembly of actin and myosin containing complexes. As a result of this activity, pseudopodia are formed at the front end of the moving cell. Movement in the ciliates is accomplished through the action of cilia which cover the surface of the organism and function somewhat like oars. Flagellates movement is accomplished through a whip-like action of the flagella.

Protozoan diseases can cause serious health problems in people. Some of these include, amoebic dysentery which is caused by Entamoeba histolytica, an amoeboid protozoan. Transmission is oral- fecal such as from ingestion of contaminated lake water. Giardiasis is another disease and it is caused by Giardia lamblia, a protozoan spread by the oral- fecal route. Cysts of the organism, from contaminated water or food are ingested. This is a non-lethal, self-limiting disease. African sleeping sickness is caused by Trypanosoma brucei and is spread by the tsetse fly. In the blood, the protozoan multiplies and causes septicemia (blood infection) and fevers as the host immune system responds and attempts to kill the pathogen. Cattle and other livestock, as well as the tsetse fly, serve as reservoirs of the protozoan. Chagas disease is caused by Trypanosoma cruzi. Transmission is by the giant reduvid bug, or "kissing bug" (know to attack small birds) that defecates while biting a person on the face. Malaria is caused by Plasmodium vivax and other species of Plasmodium. Spread is by infected mosquitoes, where the protozoan resides in the salivary glands of the bug. Upon biting a host, the protozoan enters the circulation where it alternates between infecting and lysing red blood cells and liver hepatocytes. Fevers and chills are the rule, and infection is for life. Quinine is used for treatment, along with other drugs.

Gennadiy Zemel -- 2001