Physicists Explain Amoeboid Movement

[GRDixon]

Although some would argue that this is a Biology problem, I would like to read some views from physicists regarding the following problem: Amoebas have no muscles nor cilia (microscopic, "paddling" hairs). Yet they are highly mobile. They move about by pushing protuberances, called pseudopodia, out in the desired direction of movement, and then flowing the remainder of their innards into the protuberance. The riddle is, how do they pull this off without any internal muscles or known pumps, etc.? There is lots of info on the Internet; yet I haven't read any satisfactory PHYSICAL explanation of the phenomenon.

 

[Doc Al]

They move about by pushing protuberances

How do they 'push' without muscles? Or is 'push' not quite accurate?

Sounds like a biology question to me.

 

[arildno]

They ooze forth, and suck their body after them.

This can be done by building internal rods, one brick after another (i.e, the continual construction and decontruction of their own cyto-skeleton). Sort of like that.

Most definitely, they use energy to move about, but not through our way of doing it.

 

[Andy Resnick]

Although some would argue that this is a Biology problem, I would like to read some views from physicists regarding the following problem: Amoebas have no muscles nor cilia (microscopic, "paddling" hairs). Yet they are highly mobile. They move about by pushing protuberances, called pseudopodia, out in the desired direction of movement, and then flowing the remainder of their innards into the protuberance. The riddle is, how do they pull this off without any internal muscles or known pumps, etc.? There is lots of info on the Internet; yet I haven't read any satisfactory PHYSICAL explanation of the phenomenon.

Do they use actin polymerization? Pseudopodia are thought to form via remodeling of the actin cytoskeleton; and there's one (AFAIK) model: the "Brownian ratchet" model (Oster) which has several variants (elastic Brownian ratchet is one). Honestly, something about the model doesn't "smell' right to me- getting net work out of a stochastic process doesn't seem right- but I don't have a better idea.

http://www.ncbi.nlm.nih.gov/pubmed/11069185

 

[GRDixon]

How do they 'push' without muscles? Or is 'push' not quite accurate?

Sounds like a biology question to me.

Assuming a perfectly spherical shape would constitute the lowest energy state, it would appear to require work (via a "push") to deform, amoeba fashion. How they accomplish this without muscles is the big question. Ultimately, of course, amoebas are the business of biologists. But a survey of the Internet indicates that the biologists could use some competent input from physicists to explain amoeboid mobility. The next two posts seem to be more or less similar, and the mechanisms they suggest might just be the answer.

A similar question arises in embryology. Virtually all mammalian embryos are blastulas ... spheres with walls a single cell thick ... early in their development. But on cue these blastulas "invaginate" ... a dent appears in one end of the sphere, and the whole thing folds in, forming a sphere with walls that are 2 cells thick (with a hole at one end). No one understands the physics behind this invagination phenomenon since, at the time of its occurrence, no muscles have developed.

 

[DaveC426913]

Assuming a perfectly spherical shape would constitute the lowest energy state, it would appear to require work (via a "push") to deform, amoeba fashion.

This is not necessarily true. I give an example:

Fill a balloon with a viscous fluid. Cut a hole. The fluid will pour out because it is under pressure.

If the amoeba can selectively chemicelly render parts of its cell membrane permeable, the cytoplasm will ooze out of its own accord.

It should be noted, I do not profess to know how amoebae do extend their pseudopods; I'm simply poking holes in your somewhat sweeping claim that this is a big mystery.

But on cue these blastulas "invaginate" ... a dent appears in one end of the sphere, and the whole thing folds in, forming a sphere with walls that are 2 cells thick (with a hole at one end). No one understands the physics behind this invagination phenomenon since, at the time of its occurrence, no muscles have developed.

Well, I'll take your word that "no one understands it", I always thought I knew. When a sex cell is about to divide, its chromosomes are pulled to opposite sides of the cell. This process is facilitated by some sort of threads that form between the antipodal spots of the cells and the chromosomes, which then contract, pulling the chromosomes apart.

I saw this happen grade school health class in a film. Has this changed in 35 years?


If not, surely the same mechanism applies to divide the cell.

 

[Andy Resnick]

Ultimately, of course, amoebas are the business of biologists. But a survey of the Internet indicates that the biologists could use some competent input from physicists to explain amoeboid mobility.

No one understands the physics behind this invagination phenomenon since, at the time of its occurrence, no muscles have developed.

I think my problem with this post (other than the condescending attitude) is that the wrong questions are being asked. Biologists understand cell motility fairly well- that is, they can accurately *control* nearly every aspect of cell motility by a variety of methods: biochemical, genetic, physical (e.g. pH, T, %CO2).

You are implicitly asking about the *mechanism* by with cytoskeletal dynamics occurs, which is not known in detail. The Brownian ratchet model does provide a coherent framework to tie together much of the experimental results- and there are a *lot* of experimental results. If you don't like it, you are free to invent a different model. "Tensegrity" (Ingber) is another model.

I am simply cautioning you to at least survey the relevant literature before making claims.

 

[Andy Resnick]

If not, surely the same mechanism applies to divide the cell.

Again, a lot of experimental results have created a reasonably complete picture of cell division.

Some interesting unknowns regarding cell division still exist:

The axis of symmetry of the dividing cells (the line connecting the two centrosomes) is under some internal control, and the orientation changes in response to fluid flow, but how this is accomplished is not known.

At the end of cell division, the actin ring which contracts down the middle, creating the cleavage furrow must somehow pinch off to make two cells at telophase, and how this is accomplished is not known.

Also, the microscopic details of the process are not really known, either. For example, the structure of the kinetochore is not known.

What's needed are Physicists that can operate in a bio lab, to learn what experiments are needed to solve the outstanding problems- and then to do the experiments!

 

[peteratcam]

If you are interested in how they are able to move at all (as opposed to wobbling on the spot), a number of physicists work on that. In the physics literature, the problem is described as "swimmers/swimming at low reynolds number", but I sense you're more interested in the microscopic mechanism, rather than universal features of the necessary swimming strokes.

 

[GRDixon]

What's needed are Physicists that can operate in a bio lab, to learn what experiments are needed to solve the outstanding problems- and then to do the experiments!

I agree. There are probably a few fascinating jobs/careers to be had by freshly degreed physicists with minors in biology. Thanks for your own fascinating insights. I must confess my survey of available info, prior to posing the question, was limited to a few Internet sites.

 

[DaveC426913]

Annnnd someone's already made it into a robot...

http://www.youtube.com/watch?v=1P2FPA5Bwv0&feature=related

 

[Andy Resnick]

Annnnd someone's already made it into a robot...

http://www.youtube.com/watch?v=1P2FPA5Bwv0&feature=related

Not quite... more like this:

http://www.pnas.org/content/96/9/4908.full

or this

http://www.nature.com/nrm/journal/v1/n2/full/nrm1100_110a.html

 

[DaveC426913]

Sorry, you don't think that an amoeboid robot is six kinds of awesome?

 

[Andy Resnick]

Sorry, you don't think that an amoeboid robot is six kinds of awesome?

It did not move using anything like pesudopodia. That's irrelevant to whether or not the robot is interesting on its own merits.

 

[GRDixon]

Annnnd someone's already made it into a robot...

http://www.youtube.com/watch?v=1P2FPA5Bwv0&feature=related

Fascinating! Thanks for sharing. The concept of a multi-cell-walled "blastula" (without a stiff "nucleus") makes me think that an originally fluid-filled blastula might invaginate by pumping fluid from its internal cavity out to the environment. Then, like a leaking tire, it begins to dent inward owing to pressure from its environment. In any case it seems likely that amoeboid movement, blastula invagination, etc. are all orchestrated by DNA. WOW!