Advanced Placement (AP) Biology Practice Exam

Which structure serves as a selective barrier for a cell, allowing certain substances to pass through?

1. Nuclear envelope

2. Plasma membrane

3. Cytoplasm

4. Cell wall

The correct answer is: Plasma membrane

The plasma membrane is the primary structure that serves as a selective barrier for a cell. It is composed of a phospholipid bilayer with embedded proteins, which together regulate the movement of substances in and out of the cell. This selective permeability is crucial for maintaining the cell's internal environment, allowing essential nutrients and ions to enter while keeping harmful substances and toxins out. The plasma membrane's structure enables it to differentiate between what can cross its boundary. Small, nonpolar molecules can pass freely, while larger or charged molecules require specific transport proteins to facilitate their movement. This selectivity plays a vital role in cellular processes, including communication, metabolism, and homeostasis. In contrast, other structures mentioned have different functions. The nuclear envelope surrounds the nucleus but is primarily involved in regulating what enters or exits the nucleus rather than the entire cell. The cytoplasm is the gel-like substance within the cell and does not function as a barrier. The cell wall, found in plants, fungi, and bacteria, provides structural support and protection but is not as selective as the plasma membrane regarding the entry and exit of substances.