Your research project involves sea cucumbers, a poorly-understood group of echinoderms. You are trying to determine whether they block polyspermy by changing membrane potential like sea urchins do. In the chart below, you have counted the number of embryos that complete normal development after fertilization in either normal or low-sodium artificial sea water (ASW), for each of three sets of experiments. Looking over your data, what do you conclude?

Question

A.  Sea cucumbers appear not to block polyspermy the same way as sea urchins, which use an influx of Na+ that changes the membrane potential.

B.  Sea cucumbers appear not to block polyspermy the same way as sea urchins, which use an outflow of Na+ that changes the membrane potential.

C.  Sea cucumbers appear to block polyspermy the same way as sea urchins, via an influx of Na+ that changes the membrane potential.

D.  Sea cucumbers appear to block polyspermy the same way as sea urchins, via an outflow of Na+ that changes the membrane potential.





Clarify Question

What is the key concept addressed by the question?



What type of thinking is required?





Gather Content

What do you already know about fertilization in sea urchins? What other information is related to the question?





Choose Answer

Given what you now know, what information is most likely to produce the correct answer?





Reflect on Process

Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

 

Answer 1

C.  Sea cucumbers appear to block polyspermy the same way as sea urchins, via an influx of Na+ that changes the membrane potential.

Clarify Question
What is the key concept addressed by the question?
     · This question is asking you to compare the blockage of polyspermy by sea urchins and sea cucumbers.

What type of thinking is required?
     · Evaluate level:
       o This question is asking you to weigh and judge, or evaluate, a data set that looks at the necessity of sodium to block polyspermy in echinoderms.


Gather Content
What do you already know about fertilization in sea urchins? What other information is related to the question?
     · In sea urchins, membrane contact by the first sperm results in a rapid, transient change in membrane potential of the egg, which prevents other sperm from fusing to the egg’s plasma membrane.
     · The importance of this event was shown by experiments where sea urchin eggs are fertilized in low sodium, artificial seawater.
     · The change in membrane potential is mostly due to an influx of Na+, so fertilization in low sodium water prevents this. Under these conditions polyspermy is much more frequent than in normal seawater.


Choose Answer
Given what you now know, what information is most likely to produce the correct answer?
     · The data for the sea cucumbers show a significant decrease in the number of embryos that complete normal development in low sodium ASW compared to normal seawater.
     · This is consistent with the idea that the sea cucumbers require an influx of sodium to block polyspermy, as sea urchins do.


Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
     · Evaluate level:
       o Answering this question correctly depended on your ability to weigh and judge, or evaluate, the incidence of polyspermy as it relates to sodium content of the environment. If you got an incorrect answer, did you remember that sodium is necessary for some echinoderms to block polyspermy, or that polyspermy results in lack of development or abnormal development? Did you have trouble weighing the merits of the sodium content of the environment to determine the correct answer?