A Level Biology Practice Exam

How is a partially permeable membrane defined?

1. A membrane that allows all substances to cross freely

2. A membrane that allows some substances to cross but not others

3. A membrane that is impermeable to all substances

4. A membrane that only allows gases to pass

The correct answer is: A membrane that allows some substances to cross but not others

A partially permeable membrane is defined as one that allows some substances to cross while restricting others. This selective permeability is crucial in biological systems, as it enables cells to maintain homeostasis by regulating the internal environment. For instance, water and small uncharged molecules can often pass through such membranes, while larger or charged molecules may be blocked. This characteristic is fundamental in processes like osmosis and nutrient uptake. The concept of selective permeability is key in understanding how cells interact with their surroundings, how they take in nutrients, expel wastes, and respond to various signals from the environment. By selectively allowing certain substances to enter or exit, these membranes help preserve the overall functionality of cells and maintain a stable internal condition despite fluctuations in the external environment.