1. (28 pts) Amoeba proteus, a single-celled eukaryote, moves by means of psudopods attaching to and detaching from the substratum. Locomotion seems to be correlated with the forward flow of fluid cytoplasm (endoplasm) into an advancing pseudopod through a surrounding, gel-like ectoplasmic tube. The ectoplasm forms at the pseudopodial tip in a region called the Fountain Zone. As the amoeba advances the ectoplasmic tube “liquifies” at the posterior end to form endoplasm. These features are illustrated in the figure below.

Both locomotion and cytoplasmic streaming are inhibited by cytochalasin B. Consider everything you’ve learned so far and answer all the following questions.

A. (4 pts) When amoeba undergoes cell division, it stops streaming and rounds up into a spherical cell. Describe how this change in shape and behavior comes about and why it might be a necessary precondition for division.

(6 pts) Briefly describe how cytoplasmic streaming is most likely organized and generated at the cellular and molecular levels. Use a diagram to illustrate your description.

B. (5 pts) Briefly describe an additional experiment or observation that would test your hypothesis and indicate clearly what the results would show.

C. (8 pts) Describe clearly, with the aid of a well-labeled diagram, how streaming within a pseudopod could result in movement of the amoeba across the substratum.

D. (5 pts) Describe how your streaming mechanism might be regulated such that the amoeba might change its streaming pattern to form phagocytic pseudopods around a ciliate it had touched.

Now evaluate some past answers to these questions, in light of your own essays. Note that better answers contain more information that you are now able to synthesize, at this point in the course. On the other hand, you may now know more about the various mechanisms than the students who answered these questions in the mid ‘90’s did!

A. (4 pts) When amoeba undergoes cell division, it stops streaming and rounds up into a spherical cell. Describe how this change in shape and behavior comes about and why it might be a necessary precondition for division.

Answer Comment

Example 1. In order for the single cell to divide it must become a shape that is spherical enough for the spindle to form and an even distribution of cytoplasmic material to take place when cytokinesis happens. Also, cytokinesis cannot take place with a firm gel-like tube in the middle of the cell. In order for cell division to occur, the gel-like tube will dissolve into all endoplasm which is a liquid form. The cytoskeleton will form as a normal eukaryotic cell and cytokinesis will divide the cell. Once the cell has full divided, the endoplasm will form a new ectoplasm tube again and all will continue

A good start, but more mechanistic detail is required: what sorts of cytoskeletal elements are involved? Wordy! Simply restates information provided. “normal” means what?

Contrast the first answer with the following:

Example 2. Microfilaments are needed for cytokinesis. They form a “belt” perpendicular to the spindle fibers needed for mitosis. This belt contracts, pinching off the cytoplasm from the original cell into 2 daughter cells. Microfilaments are dynamic structures, and those which previously were involved in streaming or maintaining cell shape are disassembled and used for cell division. When this occurs, the cell assumes a natural round shape and all streaming stops due to lack of microfilaments which would turn the endoplasm into ectoplasm.

A focused, mechanistic answer from the start! Connection with streaming established; dynamic properties identified. A problem: what is “natural?”