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

A spherical cell doubles its radius, making its surface area four times larger and its volume eight times larger. Which consequence best explains why such growth eventually limits exchange with the environment?

A.Doubling the radius thickens the membrane proportionally, halving every molecule's crossing speed.
B.Extra membrane slack lets large cells transport faster per unit volume than small ones.
C.Diffusion converts into an energy-driven pump system once cells exceed a size threshold.
D.The surface area to volume ratio falls to half, so each unit of membrane must serve twice the cytoplasm.

Explanation

Core Concept

Nutrient inflow and waste removal scale with membrane area while metabolic demand scales with volume, so exchange capacity falls behind. Slower diffusion across longer internal distances compounds the shortage deep in the cytoplasm. Demand rising faster than exchange capacity is the core scaling problem cells face.

Correct Answer

DThe surface area to volume ratio falls to half, so each unit of membrane must serve twice the cytoplasm.

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