After the fertilization of a sea urchin egg, the influx of which ion prevents polyspermy?Multiple choice question.PotassiumCalciumSodiumChloride

Step 1: Break Down the Problem

We need to determine which ion influx prevents polyspermy (the fertilization of an egg by multiple sperm) in sea urchins after fertilization.

Step 2: Relevant Concepts

Polyspermy prevention involves changes in the egg's membrane potential and biochemical processes that occur immediately after fertilization. One key event is the release of ions that create a blockage against additional sperm entry.

Step 3: Analysis and Explanation

1. Potassium (K⁺): While potassium ions are critical for many cellular processes, they do not play a significant role in preventing polyspermy.
2. Calcium (Ca²⁺): Upon fertilization, there is a significant influx of calcium ions. This influx triggers a series of cellular responses that lead to the cortical reaction, which serves to alter the vitelline layer, thereby preventing additional sperm from entering the egg.
3. Sodium (Na⁺): Sodium ions are important for generating action potentials in nerve cells, but they are not directly involved in the prevention of polyspermy.
4. Chloride (Cl⁻): Chloride ions help maintain osmotic balance and can be involved in cellular signaling, but they do not prevent polyspermy.

Step 4: Verify and Summarize

Based on the analysis, the preventive mechanism against polyspermy in sea urchins relies primarily on the influx of calcium ions (Ca²⁺) that triggers downstream processes leading to the block of additional sperm entry.

Final Answer

Calcium is the ion that prevents polyspermy after the fertilization of a sea urchin egg.