The cellular structure that separates the cytoplasm from the external environment is the plasma membrane, also known as the cell membrane.

Here's a breakdown of its function:

* Barrier: The plasma membrane acts as a selective barrier, controlling what enters and exits the cell. This is crucial for maintaining the cell's internal environment and allowing essential processes to occur.

* Communication: The membrane contains various receptors that allow the cell to communicate with its surroundings and respond to signals from other cells or the environment.

* Structure: The plasma membrane provides structural support and helps maintain the cell's shape.

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