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iLabs: Reader Question about Amoebic Movement

October 19, 2012

One of our regular readers, Maggie Ray, suggested in her response to our amoeba video, that we answer the question, what causes an amoeba to move in a certain direction vs another.

Posing this question on Google brought a whole range of responses:

From Biology Teaching Resources. Amoeba, by the site: Educational Resources for Biology by D G Mackean

Changes of direction are effected when a new pseudopodium begins to form at another point of the amoeba’s surface. The direction of movement is probably determined by local differences in the water. Slight acidity or alkalinity may cause the cytoplasm to start flowing or prevent its doing so altogether. The chemicals diffusing from suitable food material may cause the cytoplasm to flow in that direction.

From Yahoo! Answers

In just about every instance I know, it is chemotaxis, the movement toward or away from the concentration of a chemical that the surface of the amoeba cell can detect. Amoebas will generally move toward higher concentrations of things like glucose (a food source) that it can engulf and use for energy. There may be other chemicals that cause chemotaxis…haven’t done a whole lot of microbiology in a while. They’re movement is called amoeboid movement or motion, and basically just entails extending their pseudopods in the direction they want to move, and then the flowing of the cytoplasm toward the extended pseudopod.
  • The free living amoeba have receptors on the surface that can determine which direction has a higher concentration of nutrients. The amoeba has cytoskeletal actin (distantly related to the actin in vertebrate muscles) and can move internal contents along this cytoskeleton towards the activated receptors, shifting mass into the pseudopod and moving the amooeba closer to the food source.

Well from all of the above, (and what follows) the best answer for WHY an amoeba moves in a certain direction, is chemotaxis, as described here from Wikipedia:

Chemotaxis is the phenomenon whereby somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food (for example, glucose) by swimming towards the highest concentration of food molecules, or to flee from poisons (for example, phenol).

Specifics about amoebic chemotaxis can be found in a great article from the University of Edinburgh:

Amoeba feed on other protist, algae and  bacteria.  They must be able to adapt to the temporary absence of suitable prey, for example by entering resting stages such as cysts, but it is obviously an advantage to be able to “smell out” prey items and craw toward the source.  This ability is chemotaxis.  It is likely that all amoebae have this ability since it would confer such a huge advantage to these organisms, indeed chemotaxis has been demonstrated in Amoeba proteus, Acanthamoeba, Naegleria and Entamoeba.  The best studied chemotactic amoeboid organism however is Dictyostelium discoideumDictyostelium is chemotactic for bacteria, but has been extensively studied for its ability to climb gradients of cAMP, a signalling molecule involved in the development of the slug

For those of you who want to get the meat of the experiment involving cAMP and Dicotyostelium discoideum, here is a link where you can read the complete research paper documenting this phenomenon:

The Journal of Experimental Biology 202, 1–12 (1999)
Printed in Great Britain © The Company of Biologists Limited 1998 JEB1613

CHEMOTACTIC RESPONSES OF DICTYOSTELIUM DISCOIDEUM AMOEBAE TO A CYCLIC AMP CONCENTRATION GRADIENT: EVIDENCE TO SUPPORT A SPATIAL MECHANISM FOR SENSING CYCLIC AMP

TOMOMI TANI1,* AND YUTAKA NAITOH2

1Department of Biology, Graduate College of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan and 2Pacific Biomedical Research Center, University of Hawaii at Manoa, Snyder Hall 306, 2538 The Mall, Honolulu, HI 96822, USA

More recently, some research showed that apparently, a protein involved in this chemotactic movement, may also have a connection to dealing with cancer:

From an article in a 2002 issue of Modern Drug Discovery:

The same protein that determines an amoeba’s direction of motion also prevents the formation of cancerous tumors in animals.

Scientists from the Johns Hopkins University School of Medicine (Baltimore) recently reported that a protein called PTEN binds to the back of the amoeba’s cell membrane when a chemical attractant is sensed, allowing the cell to move purposely toward the attractant (Cell 2002, 109, 599–610). Because PTEN “brings up the rear”, the molecules crucial for allowing the cell to reach out and move for- ward are restricted to the front of the cell.

So there you have it, your introduction to amoebas, microscopic creatures, and the reason they move in the direction they do.

Thank you Dr. Ray for the question!!!!

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