A red blood cell is placed in a solution containing 0.9% NaCl. The cell maintains its normal shape and size. Which statement best explains this observation?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The red blood cell (RBC) plasma membrane is studded with aquaporin-1 (AQP1) tetramers—integral channel proteins that form selective pores permitting rapid passive transport of H₂O molecules while excluding ions such as Na⁺ and Cl⁻. Water moves through these channels (and to a lesser extent across the phospholipid bilayer itself) according to the chemical potential gradient for water: molecules migrate from regions of higher effective water concentration toward regions of lower effective water concentration, a process termed osmosis. Inside a typical human erythrocyte, the cytoplasmic osmolarity is approximately 300 milliosmoles per liter (mOsm/L), established by dissolved hemoglobin, potassium ions (K⁺), and organic phosphates such as 2,3-bisphosphoglycerate (2,3-BPG). A 0.9% NaCl solution—known as physiological saline—dissociates into Na⁺ and Cl⁻ ions to produce an extracellular osmolarity of roughly 300 mOsm/L, nearly identical to the intracellular value. Because the osmotic pressures on either side of the selectively permeable membrane are equal, the thermodynamic driving force for net water flux collapses to zero. Water molecules continue to traverse the membrane in both directions through AQP1, but every molecule entering is matched, on average, by one exiting. This dynamic equilibrium preserves cytoplasmic volume and cell surface area, so the characteristic biconcave disc shape of the erythrocyte remains unchanged.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus explicitly states that the RBC "maintains its normal shape and size" after immersion in 0.9% NaCl. Shape preservation is a direct, observable readout of osmotic balance. If the external solution were hypotonic—possessing fewer dissolved solute particles than the cytoplasm—water would flow inward through AQP1 channels, raising internal hydrostatic pressure, swelling the cell beyond its biconcave morphology, and potentially causing hemolysis as the membrane ruptures. Conversely, if the solution were hypertonic—containing more solute particles than the cytoplasm—water would exit the cell, the membrane would buckle inward, and the erythrocyte would crenate into a shrunken, spiculated form. Because neither swelling nor shrinking is observed, the extracellular and intracellular compartments must exert identical osmotic pressures. By definition, this identifies 0.9% NaCl as an isotonic solution relative to the RBC interior. The correct explanation, therefore, must invoke equal solute concentrations on both sides of the selectively permeable membrane, yielding zero net water movement despite continued bidirectional molecular flux.

PILLAR 3 — DISTRACTOR ANALYSIS

Hypertonic and hypotonic answer choices are classic traps. A student selecting "hypertonic" may misinterpret the chloride concentration as inherently concentrated, forgetting that the relevant comparison is between extracellular and intracellular osmolarity—not the absolute percentage of NaCl alone. A student selecting "hypotonic" might reason backward from the observation that the cell did not shrink, erroneously concluding the solution must have had less solute, when in fact a hypotonic bath would cause the cell to swell and lyse. Both errors reflect confusion over directional water flow relative to solute gradients.

Another common distractor states that "water stops moving across the membrane." This conflates equilibrium with stasis. In reality, individual H₂O molecules continue to pass through AQP1 channels in both directions; the NET flux is zero, but bidirectional traffic persists. Selecting this option reveals a static mental model rather than an appreciation of dynamic equilibrium.

A fourth trap invokes active transport—suggesting the Na⁺/K⁺-ATPase or other pumps actively maintain cell volume in this scenario. While the erythrocyte does expend ATP to sustain transmembrane ion gradients over time, the immediate preservation of shape in isotonic saline requires no active mechanism; passive osmotic equilibrium alone suffices. This distractor exploits overgeneralization from the broader principle that cells use energy for homeostasis, misapplying it to a context where passive forces fully account for the observation.

Finally, some options may claim that NaCl's inability to cross the lipid bilayer explains the stable shape. Although Na⁺ and Cl⁻ are indeed largely impermeant without transport proteins, membrane impermeability to a solute cannot, by itself, prevent volume change when a concentration gradient exists. The decisive factor here is the absence of a gradient—matching osmolarities—rather than the membrane's selectivity properties.