Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologyeasymcq1 pt

Why is an approximately isotonic saline solution safer for red blood cells than distilled water?

A.The salt actively strengthens the red cell membrane against damage
B.Its solute concentration matches the cytosol, so net water movement in either direction is zero
C.Red cells pump the salt out faster than it flows in
D.Salt blocks aquaporins so that water cannot move at all

Explanation

Core Concept

An approximately isotonic saline solution has an effective osmotic concentration close enough to that of blood that it causes little net water movement across red blood cell membranes. Distilled water is strongly hypotonic and can drive water into erythrocytes until they swell and lyse. The useful concept is approximate isotonicity, not that saline literally matches the cytosol in every solute or concentration.

Correct Answer

BIts solute concentration matches the cytosol, so net water movement in either direction is zero

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