Which cellular structure serves as a barrier controlling the entry and exit of substances?

Prepare for the National Clinical Assessment Test. Study with interactive quizzes, flashcards, and detailed explanations. Get ready to excel in your exam!

The cell membrane, also known as the plasma membrane, is the structure that functions as a barrier controlling the entry and exit of substances in and out of the cell. It is composed of a phospholipid bilayer with embedded proteins, which provide selective permeability—meaning it allows certain molecules to pass while restricting others. This selective permeability is essential for maintaining the internal environment of the cell, known as homeostasis, by regulating the concentrations of various substances critical for cellular function, such as ions, nutrients, and waste products.

The cell membrane also facilitates communication with other cells and can respond to external signals, further emphasizing its role in controlling substance movement. In contrast, the nucleus houses genetic material and is responsible for regulating gene expression; the cytoplasm is the jelly-like substance where cellular processes occur; the cell wall provides structural support and protection in plant cells but does not actively control substance movement as the cell membrane does.

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