A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

The primary role of the cell membrane is to serve as a selectively-permeable barrier. This function is essential for maintaining the internal environment of the cell. The cell membrane regulates the movement of substances into and out of the cell, allowing it to control which molecules can enter or exit, thereby maintaining homeostasis. It is composed of a phospholipid bilayer with embedded proteins that provide specificity and functionality, such as transport proteins that facilitate the passage of ions and nutrients.

This selective permeability is crucial for various cellular processes, including nutrient uptake, waste elimination, and signal transduction, ensuring that the cell’s internal conditions remain stable despite changes in the external environment. Other options suggest roles that are either not primarily associated with the cell membrane or pertain to different cellular structures or functions. For instance, the rigid support structure refers more to the cell wall in plant cells, while containing genetic information is the function of the nucleus. Cellular respiration occurs primarily in the mitochondria, not directly facilitated by the membrane itself. Thus, the role of the cell membrane as a selectively-permeable barrier is fundamental to the life of the cell.