Model Answer
0 min readIntroduction
Cnidarians, including jellyfish, corals, and hydra, are characterized by specialized stinging cells called cnidocytes, containing organelles called cnidoblasts. These cnidoblasts are responsible for capturing prey, defense against predators, and attachment to substrates. The cnidoblast houses a complex intracellular structure called the nematocyst, a harpoon-like organelle capable of delivering venom. Understanding the structure and mechanism of action of cnidoblasts is fundamental to comprehending the biology and ecological role of Cnidarians. This answer will detail the structure of the cnidoblast and elucidate the intricate process of nematocyst discharge.
Structure of the Cnidoblast
The cnidoblast is a specialized cell containing the nematocyst. Its structure can be divided into several key components:
- Cnidocil: A sensory hair-like structure protruding from the cell. It acts as a trigger for nematocyst discharge.
- Operculum: A lid-like structure covering the nematocyst’s opening. It prevents premature discharge.
- Nematocyst: The capsule containing the coiled, thread-like tubule. This is the actual stinging apparatus.
- Cytoplasm: The general cellular contents surrounding the nematocyst.
- Nucleus: The control center of the cell.
The nematocyst itself is a complex structure. It consists of:
- Capsule: The outer covering of the nematocyst, containing the coiled tubule.
- Tubule: A hollow, thread-like structure that is rapidly everted during discharge. It can be barbed, sticky, or venomous.
- Volvent filament: A spirally coiled filament within the capsule, contributing to the force of eversion.
Mechanism of Action
The discharge of the nematocyst is one of the fastest biological processes known, occurring within microseconds. The mechanism involves the following steps:
- Stimulation: The cnidocil is stimulated by mechanical contact (e.g., prey touching the tentacle) or chemical signals.
- Ion Flux: Stimulation causes an influx of calcium ions (Ca2+) into the cnidoblast cytoplasm.
- Osmotic Pressure Change: The increased Ca2+ concentration leads to a rapid increase in osmotic pressure within the nematocyst capsule.
- Operculum Rupture: The increased pressure causes the operculum to rupture, exposing the nematocyst’s opening.
- Tubule Eversion: The coiled tubule is rapidly everted, propelled by the high osmotic pressure and the volvent filament. This eversion is incredibly forceful.
- Penetration & Injection: The everted tubule penetrates the prey’s skin or surface. Depending on the type of nematocyst, it may inject venom, entangle the prey, or adhere to the surface.
- Anchoring & Contraction: After penetration, the tubule anchors itself, and the capsule contracts, pulling the prey closer to the cnidarian’s mouth.
Types of Nematocysts
There are several types of nematocysts, each adapted for a specific function:
| Nematocyst Type | Function | Characteristics |
|---|---|---|
| Penetrants | Prey capture and defense | Barbed tubules that pierce the prey’s skin and inject venom. |
| Glutinants | Attachment | Small, sticky tubules that adhere to surfaces. |
| Volvents | Prey entanglement | Thread-like tubules that wrap around the prey. |
It’s important to note that once a nematocyst is discharged, it is not reusable. The cnidoblast must synthesize a new nematocyst to regain its stinging capability.
Conclusion
In conclusion, the cnidoblast is a remarkable example of cellular specialization, enabling Cnidarians to thrive in diverse marine environments. Its intricate structure, particularly the nematocyst, and the rapid, forceful mechanism of its discharge are crucial for prey capture, defense, and locomotion. Understanding these features provides valuable insight into the evolutionary success and ecological role of this fascinating phylum. Further research into the molecular mechanisms governing nematocyst discharge could have implications for biomedical applications, such as developing novel drug delivery systems.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.