The concentration of solutes in a red blood cell is about 2%. Sucrose cannot pass through the membrane, but water and urea can. Osmosis would cause red blood cells to shrink the most when immersed in which of the following solution?

Question

Accepted Answer

The correct option is 3: Hypertonic sucrose solution

This question requires you to understand the concept of tonicity, which depends on the concentration of non-penetrating solutes in a solution relative to the concentration inside the cell. The red blood cell (RBC) has a solute concentration of approximately 2%. Shrinkage, or crenation, occurs due to osmosis when the cell is placed in a *hypertonic* solution, meaning the effective solute concentration outside the cell is greater than 2%, causing a net movement of water out of the cell. In the context of this specific problem, the cell membrane is impermeable to sucrose but permeable to urea. Therefore, only sucrose acts as a **non-penetrating solute** that establishes a lasting osmotic gradient. A **Hypertonic sucrose solution** contains a concentration of the non-penetrating solute (sucrose) that is significantly greater than 2%, creating the strongest outward osmotic force. Water rushes out of the cell in a large quantity to try and equalize the concentration gradient across the impermeable sucrose barrier, resulting in the most extreme cell shrinkage.

The incorrect option is 1: Pure water

Pure water represents the most extreme form of a *hypotonic* solution, as its solute concentration is 0%, which is significantly less than the 2% inside the red blood cell. In a hypotonic environment, the high concentration of solutes inside the cell (2%) relative to the outside creates a very strong osmotic gradient. As a result, there is a massive net movement of water *into* the red blood cell. This influx of water causes the cell to swell rapidly and eventually burst, a process known as hemolysis, which is the exact opposite of shrinkage.

The incorrect option is 2: Hypertonic urea solution

A hypertonic urea solution has a solute concentration greater than 2%. However, the key piece of information is that the red blood cell membrane is **permeable to urea**. Since urea is a *penetrating solute*, it will diffuse down its concentration gradient from the outside solution into the cell. As urea moves into the cell, it increases the total solute concentration inside, effectively reducing the osmotic gradient that would typically draw water out. Initially, there might be slight water loss, but as the urea equilibrates across the membrane, the cell will rapidly *swell* back towards or even beyond its original volume because the total final solute concentration inside will be higher than the initial 2%, and water follows the penetrating urea solute. Therefore, this solution will not cause significant or lasting shrinkage.

The incorrect option is 4: Hypotonic sucrose solution

A hypotonic sucrose solution has a solute concentration less than 2% sucrose. Since the membrane is impermeable to sucrose, sucrose acts as a non-penetrating solute, and the solution is truly *hypotonic* in terms of effective osmotic pressure. Because the external solute concentration is lower than the internal 2% concentration, there will be a net movement of water *into* the red blood cell. This influx of water causes the cell to swell, similar to the response in pure water, but typically to a lesser degree than in pure water, as the concentration difference is smaller. This outcome is swelling, not shrinking.