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

What is a key characteristic of a partially-permeable membrane?

1. It allows all molecules to pass through

2. It restricts movement of large molecules only

3. It allows certain substances to diffuse through

4. It separates different types of cells

The correct answer is: It allows certain substances to diffuse through

A partially-permeable membrane is specifically designed to allow certain substances to pass through while restricting others, based on factors such as size, charge, and solubility. This selective permeability is crucial for maintaining homeostasis within cells, as it regulates the internal environment by controlling the entry and exit of ions, nutrients, and waste products. In contrast to the explanations for the other options, a membrane that allows all molecules to pass through is known as fully permeable, which is not the case here. Similarly, while a partially-permeable membrane may restrict movement of larger molecules, it doesn't solely do this and is not limited to just large molecules. Additionally, although membranes separate different types of cells and can provide structural support, this characteristic does not pertain specifically to their permeability. Thus, the key function of a partially-permeable membrane lies in its ability to allow certain substances to diffuse through while blocking others, making option C the correct choice.