Re: What is the control mechanism for an amoeba to extend a pseudopod?

John,

  Very good questions! First thing you need to understand is that movement 
of the pseudopod is not just dependant upon "sensing" food. Extension of 
pseudopdia allows the amoeba to get around. But, for the pseudopodia to 
move an amoeba it must be on a surface of some kind (submerged rocks, 
twigs, etc),not free floating. 

  Now to answer your questions.  An amoeba can sense its environment and 
upon the right stimulation move toward the signal.  Exactly what signals 
allow this to occur are not fully understood. You are correct in that 
chemicals are involved in the signaling process. Many signals are 
recognized at the cell membrane by specific receptors and are then 
transferred into the cell. Once the signal enters the cell it gets 
amplified by one of the "second messanger" cascades. This causes many 
things to happen very quickly depending upon what chemical binds to a  
specific receptor. In the case of pseudopodia, it is thought that a change 
in the intracellular level of calcium and hydrogen ions causes changes in 
the cytoplasim. Parts of the cytoplasim become gel-like (soft) and can 
flow into the pseudopod.  The "call to action" you mention would actually 
be the receptor recognizing a signal and the activation of the second 
messanger cascade.  Movement,in this case, would be directed toward a 
concentration gradient. 

  An amoeba is usually in an environment where food (other single cell 
organisms) is available. If the amoeba comes into contact with an 
organism, it extends its pseudopodia until it surrounds it and the 
pseudopodia fuse together. That's when it knows to stop.

   I hope this answers your questions. I have included a website that 
shows an amoeba surrounding a glass bead. Just the way it would real food.
  http//mac2031.fujimi.hosei.ac.jp/PDB/PCD3650/animation/A68_74.html

                             Jeff Stiefel