Use the following information to answer the question.
The mechanism of cell crawling in protist species is not well defined. Pseudopodia extension involves interactions between actin and myosin (the same molecules that are involved in vertebrate muscle contraction). However, prior to the study described below, no one had provided convincing data that actin and myosin were actually involved in cell crawling in protists. Anatomical studies had identified the cytoskeletal protein actin just below the surface of the cell membrane in several species of protist, but physiological studies had failed to show a functional link between actin, myosin, and cell crawling.
In a study by N. Poulsen et al. (Diatom gliding is the result of an actin-myosin motility system, Cell Motility and the Cytoskeleton 44 (1999):23-22), researchers tested whether motility in a particular species of diatom involves interactions between actin and myosin.

Refer to the study by Poulsen et al. and the figure above. The data graphed in the figure could be an artifact if latrunculin A kills this species of diatoms (that is, that may be why the cells are not moving). Which of the following would be the best evidence that latrunculin A is not killing the cells?
A) When the toxin was washed off the culture, the cells began to move again.
B) There was still a small percentage of motile cells in the culture treated with the toxin.
C) Most of the cells in the control were moving, indicating that they were alive.
D) When the toxin was applied to another species of diatom, 25% of them continued to move.

Question

Use the following information to answer the question.
The mechanism of cell crawling in protist species is not well defined. Pseudopodia extension involves interactions between actin and myosin (the same molecules that are involved in vertebrate muscle contraction). However, prior to the study described below, no one had provided convincing data that actin and myosin were actually involved in cell crawling in protists. Anatomical studies had identified the cytoskeletal protein actin just below the surface of the cell membrane in several species of protist, but physiological studies had failed to show a functional link between actin, myosin, and cell crawling.
In a study by N. Poulsen et al. (Diatom gliding is the result of an actin-myosin motility system, Cell Motility and the Cytoskeleton 44 (1999):23-22), researchers tested whether motility in a particular species of diatom involves interactions between actin and myosin.

Refer to the study by Poulsen et al. and the figure above. The data graphed in the figure could be an artifact if latrunculin A kills this species of diatoms (that is, that may be why the cells are not moving). Which of the following would be the best evidence that latrunculin A is not killing the cells?
A) When the toxin was washed off the culture, the cells began to move again. 
B) There was still a small percentage of motile cells in the culture treated with the toxin. 
C) Most of the cells in the control were moving, indicating that they were alive. 
D) When the toxin was applied to another species of diatom, 25% of them continued to move.

Quizplus · Accepted Answer

If washing off the toxin allows the cells to move again, it suggests that the cells were not killed by the toxin and that the lack of motility was due to the toxin's effects on actin-myosin interactions. Choice B could also be a possible answer, as it suggests that some cells are still able to move despite the presence of the toxin, indicating that the toxin may not be killing all of the cells. However, choice A is the stronger answer, as it directly addresses whether the toxin is killing the cells or not. Choices C and D are not relevant to the question of whether the toxin is killing the cells, so they can be eliminated.

Use the Following Information to Answer the Question