Question 27 Answer and Explanation _ACT September 2017 Asia Form 75A Science Section

Passage V

When a sperm fertilizes a sea urchin egg, the egg's cortical granules (spherical structure, each bound by a membrane) fuse with the egg's plasma membrane. The contents of the granules, which include Ca²⁺ (unbound calcium ions), are released outward into the area between the plasma membrane and the vitelline layer (the egg's outer layer) in a process called the cortical reaction. The vitelline layer then expands to form the fertilization membrane. The fertilization membrane prevents other sperm from fusing with the egg (see figure 1).

Two experiments were done to study the hypothesis that the sperm causes the egg to release Ca²⁺ into the cytoplasm, which then triggers the cortical reaction.

Experiment 1

Three suspensions of unfertilized sea urchin eggs in seawater containing no Ca²⁺ were prepared. Aequorin, a substance that luminesces (shines) in the presence of Ca²⁺, was injected into the eggs' cytoplasm. Sea urchin sperm or an ionophore (a substance that can enter an egg and transport Ca²⁺ across membranes) was then added to 2 of the suspensions. Eggs were examined for luminescence in the cytoplasm and for the presence of a fertilization membrane (see Table 1).

Experiment 2

Experiment 1 was repeated except that both aequorin and a substance called EDTA (which binds with Ca²⁺, keeping it from reacting with other substances) were injected (see Table 1).

27. If, in Suspension 2, the eggs had NOT been injected with aequorin, which of the following results would most have likely have occurred?

A. Luminescence would have occurred in the cytoplasm, and the fertilization membrane would have formed.

B. Luminescence would have occurred in the cytoplasm, but the fertilization membrane would not have formed.

C. Luminescence would not have occurred in the cytoplasm, but the fertilization membrane would have formed.

D. Luminescence would not have occurred in the cytoplasm, and the fertilization membrane would not have formed.

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

Look at the description of aequorin in the second line of the text part of Experiment 1, aequorin can emit luminesce when it encounters Ca²⁺, so without aequorin it will not emit light, excluding A & B;

Secondly, the formation process of the fertilization membrane does not require the participation of aequorin, as long as there is sperm, the fertilization membrane will be formed, so you should choose C