Explain the adaptive features of egg cells, limited to: energy stores and the jelly coat that changes at fertilisation

Egg cells (ova) have several adaptive features that enhance their role in reproduction. Two key features are energy stores and the jelly coat that changes at fertilization. Here’s a detailed explanation of each feature:

1. Energy Stores

• Description: Egg cells contain a substantial amount of cytoplasm, which is rich in nutrients and energy reserves. These reserves are primarily in the form of yolk, which consists of proteins, lipids, and carbohydrates.
• Function:
  • Nutritional Support: The energy stores provide essential nutrients for the developing embryo during the early stages of development before any placental connection is established (in mammals) or before the embryo can obtain nutrients from its environment (in other species).
  • Cell Division and Growth: The stored energy supports critical processes such as cell division and growth after fertilization. The presence of these nutrients ensures that the embryo has the necessary resources to develop properly until it can establish its own means of nourishment.

2. Jelly Coat

• Description: Surrounding the egg cell is a jelly-like layer known as the jelly coat (or zona pellucida in mammals). This layer is composed of glycoproteins and other materials that provide protection and facilitate fertilization.
• Function:
  • Protection: The jelly coat serves as a protective barrier that shields the egg from physical damage and environmental stressors.
  • Sperm Attraction and Activation: The jelly coat often releases chemical signals (chemoattractants) that attract sperm to the egg. This increases the likelihood of successful fertilization by guiding sperm toward the egg.
  • Prevention of Polyspermy: Upon fertilization, the jelly coat undergoes biochemical changes (cortical reaction) that harden it, preventing additional sperm from entering the egg. This ensures that only one sperm fertilizes the egg, maintaining the correct diploid chromosome number in the resulting zygote.

Summary Table