Chapter 47, Problem 1IQ

1. a. What assures that only a sperm of the correct species fertilizes a sea urchin egg?
2. b. What assures that only one sperm will fertilize an egg?

Summary Introduction

To determine: The way to assure that only sperm of correct species fertilizes a sea urchin egg.

Introduction: The process by which the male germ cells fuses with the female germ cell is termed as fertilization. The female germ cells are known as eggs, while the male germ cells are called sperms. The sperms are the mobile cells that have the capability to travel to the eggs.

Explanation of Solution

There are two types of fertilization. These are internal and external fertilization. Internal fertilization is the process in which the fusion of eggs and sperms takes place inside the females. However, if the fusion of egg and sperm takes place in the outer environment, it is called external fertilization.

Sea urchins represent external fertilization as the females release their eggs in the water. The male sea urchins also release the sperms in the water and the fertilization of egg and sperm occurs in the external environment.

There are high chances that the sperms of an incorrect species can mate with the eggs of sea urchin. The sea urchins have a technique by which they avoid the mating of eggs with sperms of incorrect species. The eggs have an outer layer called zonapellucida. The sperms of correct species release specified proteins that bind with the receptors present on zonapellucida of eggs. The complementary binding of proteins released by sperms with receptors of eggs is highly specific. This assures only sperm of correct species fertilizes a sea urchin egg.

Conclusion

Thus, the complementary binding of proteins released by sperms with receptors present on zonapellucida of eggs assures only sperm of correct species fertilizes a sea urchin egg.

Summary Introduction

To determine: The way to ensure that only one sperm fertilizes an egg.

Introduction: The process by which a single sperm fuses with the egg is termed as monospermy. However, the process by which multiple sperms are able to fertilize a single egg is called polyspermy.

Explanation of Solution

Polyspermy can lead to several abnormal conditions that can affect the offspring. All the organisms generally represent monospermy and not polyspermy. They have developed certain methods to ensure that only one sperm fertilizes with the egg.

The eggs have certain organelles called cortical granules. The function of these granules is to prevent polyspermy. They help in the regulation of monospermy. The complementary binding sperms and eggs activate cortical granules. Activated cortical granules release certain enzymes. These enzymes alter the extracellular matrix of zonapellucida. The enzymes secreted by cortical granules also initiate the formation of a hard outer layer around the zonapellucida. This layer prevents the fusion of other sperms with the eggs. The first sperm causes the membrane depolarization of zona pellucida. This leads to a fast blocking of the fertilized egg to prevent the fusion with other sperms. This process is used by the sea urchins to ensure that only one sperm fertilizes an egg.

Conclusion

Thus, the release of enzymes from cortical granules ensures that only one sperm fertilizes an egg. The first sperm that fuses with the egg causes membrane depolarization and fast blocking. This assures that only one sperm fertilizes an egg.