Advanced Placement (AP) Biology Practice Exam

What does osmosis specifically refer to in biological terms?

1. Movement of solutes across a membrane

2. Diffusion of water across a semi-permeable membrane

3. Hydration of cells in isotonic solutions

4. Loss of water from cells

The correct answer is: Diffusion of water across a semi-permeable membrane

Osmosis is defined specifically as the diffusion of water across a semi-permeable membrane. This process is crucial for maintaining cellular homeostasis. In biological contexts, a semi-permeable membrane allows certain substances (like water) to pass while restricting others (like solutes). Water will move from an area of lower solute concentration to an area of higher solute concentration until equilibrium is reached. This movement of water is vital for processes such as nutrient transport, waste removal, and the regulation of cell volume. The focus on the movement of water rather than solutes is what differentiates osmosis from other forms of transport. Understanding osmosis is critical in contexts like plant turgor pressure, kidney function, and the effects of different solutions (hypertonic, hypotonic, isotonic) on cells.