The structure of an animal cell that keeps its cytoplasm from leaking out is the cell membrane.

Here's why:

* The cell membrane is a selectively permeable barrier: This means it controls what enters and exits the cell. It's made up of a phospholipid bilayer with embedded proteins, which act as gatekeepers.

* Protection and containment: The membrane forms a protective barrier around the cell's cytoplasm, preventing it from spilling out into the surrounding environment.

* Maintaining internal environment: The membrane also helps regulate the internal environment of the cell, ensuring the right balance of nutrients, water, and waste products.

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