A Level Biology Practice Exam

Which term describes a plasma membrane that allows certain substances to cross while restricting others?

1. Freely permeable

2. Impermeable

3. Selective permeability

4. Non-permeable

The correct answer is: Selective permeability

The term that describes a plasma membrane allowing certain substances to cross while restricting others is selective permeability. This property is essential for cells, as it enables them to maintain homeostasis by controlling the internal environment. Selective permeability allows the cell to take in necessary nutrients like glucose and amino acids while keeping out harmful substances and regulating waste removal. Freely permeable membranes allow all substances to pass without restriction, which does not accurately describe the behavior of biological membranes. Impermeable membranes, on the other hand, do not allow any substances to cross at all, and non-permeable is synonymous with impermeable and also does not fit the description. Thus, selective permeability accurately reflects the nuanced functionality of the plasma membrane in cellular processes.