Q.33 Upon stimulation of a eukaryotic cell, the intracellular calcium (Ca2+) is released from
(A) Endoplasmic reticulum
(B) Nucleus
(C) Peroxisome
(D) Mitochondria
The correct answer to Q.33 is (A) Endoplasmic reticulum, the primary intracellular store for rapid Ca²⁺ release upon cell stimulation via IP3 receptors or ryanodine receptors.
Correct Answer
Upon stimulation, eukaryotic cells release Ca²⁺ from the endoplasmic reticulum (ER) through channels like IP3R (activated by phospholipase C-IP3 pathway) or RyR (calcium-induced), raising cytosolic Ca²⁺ for signaling in processes like contraction and secretion.
Option Analysis
(A) Endoplasmic reticulum: Correct
The ER (sarcoplasmic reticulum in muscle) stores Ca²⁺ at high levels (~1 mM) via SERCA pumps and releases it rapidly via IP3R or RyR channels in response to stimuli, acting as the main intracellular Ca²⁺ source.
(B) Nucleus: Incorrect
The nucleus maintains Ca²⁺ homeostasis via nuclear envelope channels linked to ER, but does not serve as a primary release source; Ca²⁺ fluxes here are secondary to cytosolic changes.
(C) Peroxisome: Incorrect
Peroxisomes handle lipid metabolism and ROS but lack significant Ca²⁺ storage or release mechanisms for signaling; they do not contribute to stimulus-induced Ca²⁺ transients.
(D) Mitochondria: Incorrect
Mitochondria buffer cytosolic Ca²⁺ via MCU uptake and can release it slowly under stress (e.g., permeability transition), but rapid stimulus-evoked release originates from ER, not mitochondria.
| Option | Organelle | Ca²⁺ Role | Stimulus-Induced Release? |
|---|---|---|---|
| (A) | ER | Primary store & release | Yes via IP3R/RyR |
| (B) | Nucleus | Secondary signaling | No |
| (C) | Peroxisome | Minor buffering | No |
| (D) | Mitochondria | Buffering & apoptosis | No (slow) |
