AP Biology · Topic 2.5
Membrane Permeability Practice
Part of Cell Structure and Function.(ENE-1.E)
Practice questions
4
Sample questions
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Sample 1difficulty 1/5
The cell membrane is described as "selectively permeable" because
- A
It permits only water molecules to pass through aquaporins while completely excluding every other dissolved solute
- B
It allows any molecule to diffuse freely across the bilayer, ignoring size, polarity, and electrical charge
- Ccheck_circle
It lets small non-polar and tiny polar molecules cross while restricting most polar/charged solutes
- D
It actively blocks every molecule from crossing the bilayer regardless of size, polarity, or chemical character
Why
Lipid bilayer is permeable to small non-polar molecules and water; larger or charged solutes need protein channels/carriers.
- A
Sample 2difficulty 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.
Which best explains the trend?
- A
Surface area-to-volume ratio increases with size.
- Bcheck_circle
As edge length grows, volume increases faster than surface area, slowing diffusion to the center.
- C
Larger cubes have less acid available.
- D
Acid diffuses faster in larger cubes.
Why
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.
- A
Sample 3difficulty 3/5
A researcher measures unaided crossing rates of various molecules through an artificial phospholipid bilayer (no proteins). Small nonpolar gases pass freely; small polar uncharged molecules pass slowly; large polar and ionic species are nearly excluded.
Why does Na+ have such low permeability in the bar chart?
- Acheck_circle
Charged ions cannot easily cross the hydrophobic core of the bilayer
- B
Na+ ions are the largest molecules in the chart
- C
Na+ ions react with cholesterol in the membrane
- D
Na+ binds covalently to phospholipid heads
Why
The hydrophobic interior of the bilayer is incompatible with charged species; ions require channels or pumps to cross.
- A
Sample 4difficulty 3/5
Beet root tissue contains red pigment in vacuoles. Cores were exposed to 30, 50, and 70 deg C water for 5 minutes, then absorbance of the surrounding water at 530 nm was measured.
The trend is best explained by:
- A
Heat denatures the pigment, increasing absorbance.
- Bcheck_circle
High temperature disrupts the lipid bilayer and proteins, increasing membrane permeability.
- C
Beets actively transport pigment out at high temperatures.
- D
Pigment is synthesized faster at higher temperature.
Why
As temperature rises, phospholipid mobility increases and proteins denature, compromising membrane integrity. Pigment leaks out passively, increasing absorbance of the bath solution.
- A