The Physics Factbook™
Edited by Glenn Elert -- Written by his students
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|Bibliographic Entry||Result |
|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 |
|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 thecell's physiology and its environment. Unicellular organisms mayswim or crawl through their environments, depending on what appendagesthe organism has for locomotion. Due to a cell's small size andfluid-filled environment, the viscosity of the organism's surroundingsproduces the greatest effect on its movement, rather than inertialforces that larger organisms would encounter.
The speeds of single-celled animals vary greatly along withthe special locomotive structures that each cell has. Cells thatglide over solid surfaces can move at speeds ranging from 0.3to 11.1 µm/s. Flagellated cells can swim at speedsfrom 20 to 200 µm/s. Ciliated cells can push themselvesto speeds as high as 400 to 2000 µm/s.
Water currents can easily carry along unicellular organisms becausethey are so tiny. The Reynolds number of an organism determineshow easily viscous forces affect the organism's motion. The Reynoldsnumber is equal to the inertial force acting upon the object dividedby the viscous force acting on the object. Cells generally havea Reynolds number that is less than one. Swimming unicellularorganisms must propel themselves with the use of viscous shearwhereby viscous resistance is at its maximum during the cell'spower stroke and at a minimum on the recovery stroke.
Single-celled animals, most of which are protozoan, have evolvedto come equipped with special parts to allow them to move. Euglenause 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 likesmall oars against the cell's fluid environment. Stylonychiacan actually fuse its cilia together to produce thick leg-likeappendages called cirri which can help the organism walk or jumpoff of surfaces.
Ciliated cells that are equal in size with other flagellatedcells will travel faster because filiated organisms have so manymore locomotive units than flagellated cells. While a Euglenahas only one flagellum for use, a Paramecium may have fiveto six thousand cilia that beat in near synchrony to help it movealong. Surprisingly, swimming cells only need to consume 2 × 10-18watts of power to cause its locomotive parts to move. The hydrolysisof a single ATP molecule releases 10-19 joules of energy.For this reason, motion causes little energy expenditure in aswimming cell.
Amoebas crawl along solid surfaces by extending part of itsbody toward the direction of its desired movement and then pullingitself toward the extension (also known as a pseudopod). Cellscan glide along surfaces using any type of mechanism includingthe flagellated Chlamydomonas, which propels themselvesalong a surface with its flagella.
On a related note, just as single-celled organisms have evolvedfrom more primitive cells, humans have evolved from single-celledorganisms that possess the ability to swim and glide. For thisreason, 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 certainembryonic cells. If certain human cells lack basic locomotivecellular parts, the body can be placed in danger. Certain embryonicneural cells would not crawl from their positions in the brainand spinal cord to become pigment cells and parts of the adrenalglands. White blood cells would not be able to more toward foreignbodies.
B.A. Afzelius discovered the symptoms and factors of immotilecilia syndrome, in which case a person would have hereditary bronchitis,sinusitis, chronic headaches, situs inversus (condition wherethe heart is located on the right instead of the left side ofthe body), and if male, sterility. These problems occurred dueto the absence of locomotive organelles in certain cells. Spermhad no flagella to propel them. The absence of cilia in the bodypresumably prevented fluids from circulating properly throughbrain cavities and caused headaches. The lack of cilia in therespiratory tract prevented foreign particles from being properlyexpelled from the lungs and sinuses. The heart would end up onthe wrong side of the body because ciliated embryonic cells wereunable to migrate to properly form heart tissue.
Ross Krupnik -- 2000
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