Question

Only one sperm enters an egg because a. sperm have an acrosome. b. the corona radiata gets larger. c. changes occur in the zona pellucida. d. the cytoplasm hardens. e. All of these are correct.

Short answer

The correct answer is option c - changes occur in the zona pellucida. These changes prevent other sperm cells from entering the egg and ensure that only one sperm cell fuses with the egg during fertilization.

Step-by-step solution

Understand the Fertilization Process

Fertilization is the process where a sperm cell fuses with an egg cell to form a zygote, the first cell of a new organism. Several sperm cells reach the vicinity of an egg cell, but only one is able to penetrate and combine its genetic material with that of the egg. The other sperm cells are prevented from entering due to certain changes that occur in the egg's structures. Now, let's analyze each given option:

Option a - Sperm have an acrosome

The acrosome is a structure present at the head of a sperm cell. It contains enzymes that help the sperm penetrate the egg's outer layers. While the acrosome is important for the sperm to reach the egg, it does not explain why only one sperm enters the egg.

Option b - The corona radiata gets larger

The corona radiata is a layer of cells surrounding the egg, providing nutrients and support. Changes in the size of the corona radiata do not play a role in preventing multiple sperm from entering an egg.

Option c - Changes occur in the zona pellucida

The zona pellucida is a glycoprotein layer that surrounds the egg's plasma membrane. When a sperm cell penetrates an egg, changes called the cortical reaction occur in the zona pellucida. These changes, also known as the zona reaction, involve the release of enzymes that modify the zona pellucida, making it impermeable to other sperm cells. This is the main reason why only one sperm enters an egg.

Option d - The cytoplasm hardens

The cytoplasm is the cell's internal material, and although it undergoes some changes during fertilization, it does not harden. Therefore, this option doesn't explain why only one sperm enters an egg.

Option e - All of these are correct

Since option c is the only correct answer, the option that states all of them are correct is not accurate.

Conclusion:

The correct answer is option c - changes occur in the zona pellucida. These changes in the zona pellucida prevent other sperm cells from entering the egg and ensure that only one sperm cell fuses with the egg during fertilization.

Key concepts

Zona Pellucida

The zona pellucida is a crucial component in the process of fertilization. It is a thick, transparent glycoprotein layer that envelops the plasma membrane of the egg. This layer plays a vital role as it not only protects the egg but also plays a key part in regulating the entry of sperm.
When a sperm successfully penetrates this layer, it triggers a reaction known as the cortical reaction. This reaction modifies the structure of the zona pellucida to prevent additional sperm from entering. This ensures that only one sperm cell fertilizes the egg, safeguarding the genetic integrity of the potential zygote.

Acrosome

The acrosome is a specialized region at the tip of the sperm cell. It plays an essential role in fertilization due to its composition and function. The acrosome contains powerful enzymes that are necessary for breaking through the outer layers of the egg.
When a sperm encounters the zona pellucida, the acrosome reaction occurs. This releases enzymes that dissolve this glycoprotein layer, allowing the sperm to push through and reach the egg beneath. Without the acrosome, the sperm would be unable to penetrate the egg's defenses and fertilization would not occur.
Even though the acrosome is crucial for entry, it doesn’t explain why only one sperm enters the egg.

Cortical Reaction

Upon successful entry of the first sperm, the cortical reaction triggers within the egg. This reaction is initiated by the fusion of the sperm with the egg's plasma membrane.
The cortical granules, which are present in the egg, release their contents into the space between the plasma membrane and the zona pellucida. These secretions modify the zona pellucida, making it impermeable to additional sperm.
Thus, this process ensures that only one sperm fertilizes the egg, which is critical for normal development of a zygote.

Sperm

Sperm cells are the male gametes involved in sexual reproduction. These cells are highly specialized for their role, featuring adaptations that make them effective in the fertilization process.
Key features of sperm include:

• A flagellum for motility, allowing them to swim towards the egg.
• An acrosome containing enzymes to penetrate the egg's external layers.
• A nucleus to deliver the paternal genetic material to the egg.

Each ejaculation contains millions of sperm, yet only one is needed to fertilize the egg, demonstrating the efficiency and competitiveness of these cells in seeking out the egg for successful reproduction.

Zygote

The formation of a zygote marks the beginning of a new organism. Once fertilization occurs and the genetic materials from the sperm and egg meet, a zygote is formed.
This single cell contains all the genetic information necessary to develop into a complex organism. It marks the first stage of embryonic development.
The zygote undergoes several rounds of cell division, eventually forming a blastocyst, which then implants into the uterine wall to begin the process of growth and development into a fetus.
The formation of a zygote is a result of the fusion of the egg and sperm, ensuring the combination of genetic material from both parents.