The ......... is the boundary between a cell and its environment.

Explanation

**Step-by-Step Solution:**

1. **Identify the Components of a Cell**: Understand that a cell consists of various parts, including the cell membrane, cytoplasm, and organelles.

2. **Define the Cell Membrane**: The cell membrane is a thin, flexible barrier that surrounds the cell. It is composed of a lipid bilayer with embedded proteins.

3. **Understand the Function of the Cell Membrane**: The primary function of the cell membrane is to act as a boundary between the cell and its external environment. It regulates what enters and exits the cell, maintaining homeostasis.

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