The organelle that stores calcium and is considered the membrane factory is the endoplasmic reticulum (ER).

Here's why:

* Calcium storage: The ER has specialized regions called sarcoplasmic reticulum in muscle cells and smooth ER in other cells, which act as reservoirs for calcium ions (Ca²⁺). This calcium plays a crucial role in various cellular processes, including muscle contraction, neurotransmitter release, and cell signaling.

* Membrane factory: The ER is a network of interconnected membranes that synthesize phospholipids and proteins, the building blocks of cell membranes. This process is crucial for creating new membranes for the cell itself and for other organelles.

Therefore, the ER plays a dual role, storing calcium and producing the building blocks for membranes, making it a critical organelle for cell function.