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Membrane

The term "Membrane" in sub-cellular compartment context refers not to a single location, but to the fundamental structural and functional boundary defining cellular compartments. Membranes (primarily phospholipid bilayers embedded with proteins) form the selective barriers that:

1. Compartmentalize the cell (e.g., plasma membrane, nuclear envelope, organelle membranes),
2. Regulate transport of molecules (via channels, pumps),
3. Enable signaling (receptor-mediated processes), and
4. Maintain distinct biochemical environments essential for cellular function.

Thus, "Membrane" denotes the critical interface that organizes cellular space and governs interactions, rather than a specific sub-compartment itself. (Word count: 98)