The Physics
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Speed of a Protozoan

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Bray, Dennis. Cell Movements. New York: Garland, 1992. [A table of velocities of cell movements described in the text indicates that paramecium can travel at 1000 μm/s.] 1000 μm/s
Haupt, W. & M.E. Feinleib. Encyclopedia of Plant Physiology — New Series v. 7. Berlin: Springer-Verlag, 1979. "Gliding rates are also generally slower than the rates of locomotion displayed by swimming forms. Reported rates of gliding range from 2 μm min−1 up to 11.1 μm s−1." 0.03–11.1 μm/s
Harris, Elizabeth H. The Chlamydomonas Sourcebook: A Comprehensive Guide to Biology and Laboratory Use. San Diego: Academic Press, 1989. "Flagellated Chlamydomonas cells are capable of creeping or gliding along the surface of solid media.-- Movement is bidirectional, at an average speed of 1.6 μm s−1." 1.6 μm/s
Alexander, R. McNeill. The Invertebrates. London: Cambridge University Press, 1979. "Ciliates swim faster than flagellates. Speeds of 0.4 to 2 mm s−1 are usual, while flagellates can only achieve 20-200 μm s−1." 400–2000 μm/s
20–200 μm/s
Alexander, R. McNeill. The Invertebrates.London: Cambridge University Press, 1979. "Amoebas crawl slowly, at speeds up to about 5 μm s−1." 5 μm/s

The speed of a single-celled animal is affected mostly by the cell's physiology and its environment. Unicellular organisms may swim or crawl through their environments, depending on what appendages the organism has for locomotion. Due to a cell's small size and fluid-filled environment, the viscosity of the organism's surroundings produces the greatest effect on its movement, rather than inertial forces that larger organisms would encounter.

The speeds of single-celled animals vary greatly along with the special locomotive structures that each cell has. Cells that glide over solid surfaces can move at speeds ranging from 0.3 to 11.1 μm/s. Flagellated cells can swim at speeds from 20 to 200 μm/s. Ciliated cells can push themselves to speeds as high as 400 to 2000 μm/s.
Water currents can easily carry along unicellular organisms because they are so tiny. The Reynolds number of an organism determines how easily viscous forces affect the organism's motion. The Reynolds number is equal to the inertial force acting upon the object divided by the viscous force acting on the object. Cells generally have a Reynolds number that is less than one. Swimming unicellular organisms must propel themselves with the use of viscous shear whereby viscous resistance is at its maximum during the cell's power stroke and at a minimum on the recovery stroke.

Single-celled animals, most of which are protozoan, have evolved to come equipped with special parts to allow them to move. Euglena use a long flagellum that spins around like a single bladed propeller. Other flagellated cells whip their flagella to create motion. Paramecium are equipped with cilia, which can act like small oars against the cell's fluid environment. Stylonychia can actually fuse its cilia together to produce thick leg-like appendages called cirri which can help the organism walk or jump off of surfaces.

Ciliated cells that are equal in size with other flagellated cells will travel faster because filiated organisms have so many more locomotive units than flagellated cells. While a Euglena has only one flagellum for use, a Paramecium may have five to six thousand cilia that beat in near synchrony to help it move along. Surprisingly, swimming cells only need to consume 2 × 10−18 watts of power to cause its locomotive parts to move. The hydrolysis of a single ATP molecule releases 10−19 joules of energy. For this reason, motion causes little energy expenditure in a swimming cell.

Amoebas crawl along solid surfaces by extending part of its body toward the direction of its desired movement and then pulling itself toward the extension (also known as a pseudopod). Cells can glide along surfaces using any type of mechanism including the flagellated Chlamydomonas, which propels themselves along a surface with its flagella.

On a related note, just as single-celled organisms have evolved from more primitive cells, humans have evolved from single-celled organisms that possess the ability to swim and glide. For this reason, the human cells may be flagellated, such as sperm cells, or ciliated, such as the cells that line the respiratory tract, or have the ability to glide such as white blood cells and certain embryonic cells. If certain human cells lack basic locomotive cellular parts, the body can be placed in danger. Certain embryonic neural cells would not crawl from their positions in the brain and spinal cord to become pigment cells and parts of the adrenal glands. White blood cells would not be able to more toward foreign bodies.

B.A. Afzelius discovered the symptoms and factors of immotile cilia syndrome, in which case a person would have hereditary bronchitis, sinusitis, chronic headaches, situs inversus (condition where the heart is located on the right instead of the left side of the body), and if male, sterility. These problems occurred due to the absence of locomotive organelles in certain cells. Sperm had no flagella to propel them. The absence of cilia in the body presumably prevented fluids from circulating properly through brain cavities and caused headaches. The lack of cilia in the respiratory tract prevented foreign particles from being properly expelled from the lungs and sinuses. The heart would end up on the wrong side of the body because ciliated embryonic cells were unable to migrate to properly form heart tissue.

Ross Krupnik -- 2000