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

What direction do water molecules pass during osmosis?

1. Only from high to low concentration

2. Only from low to high concentration

3. They move randomly both ways through the membrane

4. They cannot pass through the membrane at all

The correct answer is: They move randomly both ways through the membrane

Water molecules during osmosis indeed move in a manner that reflects their behavior in relation to concentration gradients. While it might seem plausible to think they only move in one direction or that they cannot pass through at all, that is not how osmosis works. In osmosis, water molecules tend to move from an area of lower solute concentration (which means a higher concentration of water) to an area of higher solute concentration (which means a lower concentration of water). However, it's important to recognize that the movement of water molecules is dynamic and occurs in both directions across a semipermeable membrane, resulting in random motion in all directions. This dual movement is significant because, although water will ultimately tend toward areas of higher solute concentration, there are constant movements back and forth. The net movement will favor the side with the higher solute concentration until equilibrium is achieved, where the concentration on both sides of the membrane becomes equal. Thus, the random movement of water molecules enhances the understanding that osmosis is not a simple one-way process but a balanced exchange that reflects the dynamic nature of cellular environments.