Membrane Permeability

AP Biology· difficulty 2/5

Agar cubes of three sizes (1, 2, 3 cm) containing pH indicator were submerged in vinegar. Time for acid to fully diffuse to the cube center was recorded.

Cube edge (cm) Time (s) 1 25 2 100 3 225

Which best explains the trend?

  • A

    Surface area-to-volume ratio increases with size.

  • B

    As edge length grows, volume increases faster than surface area, slowing diffusion to the center.

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  • C

    Larger cubes have less acid available.

  • D

    Acid diffuses faster in larger cubes.

Explanation

Time scales roughly as length^2 (25, 100, 225). The decreasing surface-area-to-volume ratio in larger cubes lengthens the diffusion path to the center.

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