Florida Biology EOC Practice Test

Which statement is true about the cell membrane?

1. It is made up entirely of proteins

2. It allows all substances to pass without restriction

3. It is selectively permeable

4. It surrounds the nucleus

The correct answer is: It is selectively permeable

The cell membrane is described as selectively permeable because it regulates what substances can enter and leave the cell. This characteristic is crucial for maintaining the internal environment of the cell, allowing it to control concentrations of ions, nutrients, and waste products while preventing the passage of harmful substances. The selective permeability of the membrane is primarily due to its lipid bilayer structure, which consists of phospholipids and embedded proteins that facilitate specific transport processes. In contrast, stating that the cell membrane is made up entirely of proteins is inaccurate, as it is primarily composed of a phospholipid bilayer with integrated proteins. The claim that it allows all substances to pass without restriction is also incorrect, as this would undermine the cell's ability to maintain homeostasis. Lastly, while the cell membrane does play a role in the overall structure of the cell, it does not surround the nucleus, as the nucleus is encased in its own nuclear membrane. Thus, claiming selective permeability accurately reflects the fundamental function of the cell membrane.