Final answer:
IP3 raises calcium concentration in the cytoplasm by causing calcium ions (Ca²⁺) stored in the endoplasmic reticulum to be released, not from mitochondria, outside of the cell, or through synthesis. Thus, the correct source of calcium ions in this context is the endoplasmic reticulum.
Step-by-step explanation:
When IP3, or inositol triphosphate, binds to receptors on the endoplasmic reticulum, it triggers the release of calcium ions (Ca²⁺) into the cytoplasm. These calcium ions don’t originate from the mitochondria, nor are they pulled in from outside of the cell or synthesized anew. They are instead released from storage sites within the endoplasmic reticulum, where they are held in readiness for just such signaling events.
The increase in cytoplasmic calcium concentration has several effects depending on the cell type. For instance, it can activate enzymes and influence various cellular activities either directly or through binding to calcium-binding proteins such as calmodulin, further propagating the cellular response to the initial signal. The correct option in the final answer is: 1) The Endoplasmic reticulum.