Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

Cooling a bacterial culture from warm growth conditions to refrigerator temperatures slows the diffusion of a small nonpolar gas across its membrane. What is the most direct physical explanation?

A.The gradient disappears in the cold because gas distributes evenly on both sides.
B.Lower temperature reduces molecular kinetic energy, so particles strike and cross the membrane less often.
C.Cold denatures the transport channels, cutting off the route the gas requires.
D.Lipids begin actively pumping heat instead of transporting dissolved gas.

Explanation

Core Concept

Cooling reduces molecular kinetic energy, so gas particles collide with and cross the membrane less frequently per unit time. Flux therefore drops even when the concentration gradient is unchanged, because flux depends on particle speed as well as gradient steepness. The process remains passive diffusion at every temperature.

Correct Answer

BLower temperature reduces molecular kinetic energy, so particles strike and cross the membrane less often.

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