Egg, sperm, and fertilization | Behavior | MCAT | Khan Academy

Summary

This script delves into the intricate process of sexual reproduction, focusing on the specialized cells involved: the sperm and egg. The sperm is likened to a torpedo, equipped with a tail for propulsion and an acrosome containing enzymes crucial for fertilization. The egg, in contrast, is a large, immobile cell with a protective glycoprotein layer, the zona pellucida. The script outlines the steps of fertilization, from sperm binding to the acrosomal and cortical reactions, which prevent polyspermy and ensure the fusion of genetic material from both parents, highlighting the mother's predominant contribution to mitochondrial DNA.

Takeaways

• 🌟 Organisms that reproduce sexually package their genetic material into specialized cells called sex cells, with the sperm cell being the male's sex cell.
• 🚀 The sperm cell is designed like a torpedo with a pointed head for forward movement, a flagellum tail for propulsion, and mitochondria for energy.
• 🧬 The head of the sperm cell contains the genetic material (DNA) within the nucleus, which is crucial for fertilization.
• 🔬 The acrosome, located at the tip of the sperm, is a collection of enzymes that play a key role in the fertilization process.
• 🥚 The egg cell is significantly larger and rounder than the sperm cell, with a thick outer coating called the zona pellucida made of glycoproteins.
• 🔶 The zona pellucida serves as a protective layer for the egg and is the target for the sperm's acrosomal enzymes to digest during fertilization.
• 💥 The acrosomal reaction is triggered when the sperm binds to the zona pellucida, releasing enzymes that digest the glycoproteins and allow the sperm to penetrate deeper.
• 🚫 The cortical reaction is a process that occurs when the sperm binds to the egg's plasma membrane, causing the release of enzymes from the egg's cortical granules to prevent polyspermy.
• 🛡️ Polyspermy, the entry of multiple sperm into an egg, is prevented by the cortical reaction, ensuring that only one sperm's genetic material combines with the egg's.
• 🔄 Mitochondria in the egg cell are numerous, with 100 to 200 thousand present, providing energy for the developing embryo, unlike the sperm's limited mitochondria contribution.
• 🔬 The fusion of the sperm's genetic material with the egg's genetic material within the nucleus marks the completion of the fertilization process.

Q & A

What is the primary function of a sperm cell?

The primary function of a sperm cell is to transfer the male's genetic material into the female sex cell, or the egg, for the purpose of fertilization.

How is the sperm cell's shape adapted to its function?

The sperm cell is shaped like a torpedo, with a pointed head for forward movement, a flagellum at the back that acts like a propeller for propulsion, and a middle section containing mitochondria for energy.

What role do mitochondria play in a sperm cell?

Mitochondria in the sperm cell provide the energy required for the sperm's movement towards the egg, as they are packed at the base of the flagellum.

What is the acrosome and why is it important for fertilization?

The acrosome is a collection of enzymes at the tip of the sperm cell that is crucial for fertilization. It helps digest the zona pellucida, allowing the sperm to penetrate the egg.

How does the egg cell differ in shape and size from the sperm cell?

The egg cell is round and much larger than the sperm cell, being about 10,000 times more massive. It is not designed for active mobility like the sperm cell.

What is the zona pellucida and what is its significance?

The zona pellucida is a thick layer of glycoproteins on the outside of the egg cell that acts as a protective barrier. It must be penetrated by the sperm for fertilization to occur.

How does the egg cell prevent more than one sperm from fertilizing it?

The egg cell prevents polyspermy through the cortical reaction, where cortical granules are released upon the binding of a sperm, digesting the glycoproteins that other sperm need to bind, thus blocking additional sperm from entering.

What happens during the acrosomal reaction in fertilization?

During the acrosomal reaction, the enzymes contained within the acrosome of the sperm are released, which helps digest the zona pellucida and allows the sperm to penetrate deeper towards the egg's plasma membrane.

What is the significance of the number of mitochondria in the egg cell compared to the sperm cell?

The egg cell contains a much higher number of mitochondria (100,000 to 200,000) compared to the sperm cell (75 to 100), which suggests that nearly all of the mitochondrial DNA in the zygote comes from the mother.

What is the final step in the fertilization process described in the script?

The final step in the fertilization process is the fusion of the plasma membranes of the sperm and egg cells, allowing the genetic material from the sperm to be released into the egg, resulting in fertilization.

Why is the block to polyspermy important in the fertilization process?

The block to polyspermy is important to ensure that only one sperm contributes its genetic material to the egg, preventing complications that would arise from multiple sperm contributions, which could lead to a zygote with an abnormal number of chromosomes.