Power houses of the cell are...........

Explanation

**Step-by-Step Solution:**

1. **Identify the Powerhouse of the Cell**: The term "powerhouse of the cell" refers specifically to mitochondria.

2. **Structure of Mitochondria**: Mitochondria have two membranes:
- **Outer Membrane**: This membrane is selectively permeable, meaning it allows certain molecules to pass through while blocking others.
- **Inner Membrane**: This membrane has numerous infoldings known as cristae.

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