Cellular Physiology — MCQs

Q: Emeiocytosis or reverse pinocytosis requires which ion?

Ca++. ### Explanation **Emeiocytosis** (also known as **exocytosis** or reverse pinocytosis) is the process by which a cell releases substances (like hormones, neurotransmitters, or enzymes) from intracellular vesicles into the extracellular space. **Why Ca++ is the Correct Answer:** Calcium ions ($Ca^{2+}$) act as the universal "coupling agent" in stimulus-secretion coupling. When a cell is stimulated, intracellular calcium levels rise—either through influx via voltage-gated channels or release from the endoplasmic reticulum. This increase in $Ca^{2+}$ triggers the fusion of secretory vesicle membranes with the plasma membrane. Specifically, calcium binds to proteins like **synaptotagmin**, which facilitates the **SNARE complex** to pull the membranes together, allowing the contents to be expelled. Without $Ca^{2+}$, the final fusion and release step cannot occur. **Why Other Options are Incorrect:** * **Na+ (Sodium):** Primarily involved in depolarization and maintaining osmotic balance, but does not directly trigger vesicle fusion. * **K+ (Potassium):** Essential for maintaining the resting membrane potential and repolarization; high extracellular $K+$ can cause depolarization, but it is the subsequent $Ca^{2+}$ influx that drives emeiocytosis. * **Mg++ (Magnesium):** Often acts as a calcium antagonist. High levels of $Mg^{2+}$ can actually inhibit exocytosis by blocking calcium channels (e.g., in the neuromuscular junction). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Example:** The release of **Insulin** from the Beta cells of the pancreas occurs via emeiocytosis in response to $Ca^{2+}$ influx. * **Neurotransmission:** At the synaptic cleft, the release of neurotransmitters (like Acetylcholine) is strictly $Ca^{2+}$-dependent. * **Botulinum Toxin:** Acts by cleaving SNARE proteins, thereby preventing $Ca^{2+}$-mediated emeiocytosis of Acetylcholine, leading to flaccid paralysis.

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