OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What characterizes a partially permeable membrane?

1. It allows all molecules to pass through freely

2. It has tiny holes that permit only small molecules

3. It blocks all water molecules from passing

4. It is rigid and does not allow any passage

The correct answer is: It has tiny holes that permit only small molecules

A partially permeable membrane is characterized by its ability to allow some molecules to pass through while restricting others based on size, charge, or other properties. This selective permeability is crucial for cellular processes, as it enables the regulation of the internal environment of cells. The correct understanding of a partially permeable membrane is that it typically has tiny holes or pores that permit only small molecules, such as water or certain solutes, to pass through, while larger molecules and ions are restricted. This selective process is essential for maintaining homeostasis within cells by controlling the movement of different substances in and out. In contrast, membranes that allow all molecules to pass freely would not exhibit selective permeability, and those that block all water molecules or do not permit any passage lack the necessary properties for regulating cellular processes. Thus, the correct identification of a partially permeable membrane is based on its ability to selectively allow the passage of specific small molecules while keeping other substances at bay.