AP Biology: Unit 2, Membrane Permeability (Drill 6)

Question 1. Based on the data in the table, which of the following best explains why Na+ has a negligible transport rate across the liposome membrane?

• A) Na+ is too large to pass through the hydrophobic core of the phospholipid bilayer.
• B) Na+ forms hydrogen bonds with phospholipid head groups, preventing it from entering the bilayer.
• C) Na+ carries a charge that makes it highly incompatible with the hydrophobic interior of the phospholipid bilayer, creating a large free energy barrier to passive diffusion. ✓
• D) Na+ is continuously and actively pumped back out of the liposomes by membrane-bound ATPase pumps, keeping its net transport rate negligible.

Explanation: The primary barrier to ion movement across a phospholipid bilayer is not size but charge. The hydrophobic core creates a large thermodynamic penalty for moving a charged species out of an aqueous environment. Na+ is actually quite small, so size is not the limiting factor (A). While ions interact with water molecules, B conflates hydrophilic interactions at the membrane surface with the mechanism of exclusion from the bilayer interior. Liposomes are artificial membranes containing no protein machinery, so ATPase-mediated pumping cannot occur (D).

Question 2. A student argues: "Because ethanol has a partially polar character, it should be completely excluded from crossing the liposome membrane, similar to glucose." Which of the following best identifies the flaw in this argument?

• A) The student is correct -- ethanol and glucose should have identical transport rates because both have polar character.
• B) The student incorrectly treats polarity as a binary property; ethanol is small and only partially polar, allowing it to partition into the hydrophobic bilayer at a moderate-high rate, unlike large, highly polar glucose. ✓
• C) The student fails to account for the fact that ethanol requires aquaporin channels in order to cross the liposome membrane at any measurable rate.
• D) The student confuses transport mechanisms; both ethanol and glucose can cross membranes passively, but glucose does so at an extremely low rate without transport proteins due to its size and polarity.

Explanation: Membrane permeability depends on both polarity and molecular size. Ethanol is small enough and sufficiently nonpolar in character to partition into the lipid bilayer at a meaningful rate. Glucose is both large and highly polar, creating two barriers to passive diffusion. The student's error is treating polarity as all-or-nothing rather than recognizing it as a continuum. A is directly contradicted by the data. Aquaporins transport water, not ethanol (C). D is a plausible distractor but does not identify the core flaw in the student's reasoning -- the binary treatment of polarity.

Question 3. The student proposes that if aquaporin channel proteins were inserted into the liposome membranes, the transport rate of water would increase but the transport rate of Na+ would remain negligible. Which of the following best evaluates this prediction?

• A) The prediction is incorrect because aquaporins increase the transport rate of all small solutes, including Na+.
• B) The prediction is incorrect because Na+ would diffuse through the inserted aquaporin channels at a low but clearly detectable rate alongside water, since the channel opening would allow small charged particles to pass whenever water is moving through the pore.
• C) The prediction is correct because aquaporins actively pump water while simultaneously blocking all solute movement.
• D) The prediction is correct because aquaporins are water-selective channels that exclude ions through size constraints and electrostatic interactions within the channel pore, so Na+ would still require a separate ion channel to cross the membrane. ✓

Explanation: Aquaporins are highly selective channel proteins that facilitate water movement while excluding ions -- a selectivity achieved through size constraints and electrostatic interactions within the channel pore. The student's prediction accurately reflects aquaporin function. A is incorrect -- aquaporins do not increase transport of Na+ or other ions. B is incorrect -- the aquaporin pore structure specifically excludes ions. C incorrectly describes aquaporins as active pumps; they facilitate passive osmosis.

Question 4. Which of the following changes to the experimental design would best allow the student to determine whether a transport protein is required for glucose to cross the membrane at a significant rate?

• A) Add extra sucrose to the solution outside the liposomes and observe whether glucose then begins crossing the membrane without any transport protein present.
• B) Increase the concentration gradient of glucose across the liposome membrane and measure whether the transport rate increases proportionally.
• C) Replace glucose with a smaller polar molecule and measure whether transport rate increases.
• D) Add a specific glucose transport protein (GLUT) to some liposome preparations and compare glucose transport rates between protein-containing and protein-free liposomes under identical conditions. ✓

Explanation: To determine whether a transport protein is required, the student must isolate the protein's contribution by comparing protein-containing liposomes to protein-free controls under identical conditions. An increase in transport rate in the GLUT-containing liposomes would directly support the conclusion that facilitated diffusion via a transport protein is required for glucose to cross at a significant rate. A changes bilayer composition, not the presence of transport proteins. B tests concentration dependence but does not address whether a protein is required. C changes the solute entirely, introducing a confounding variable.

Question 5. A researcher claims that the data in the table are sufficient to conclude that cell membranes in living organisms use only passive transport mechanisms. Which of the following best identifies the limitation of this claim?

• A) The claim is valid because some solutes can cross the liposome membrane without proteins, demonstrating that passive transport is sufficient.
• B) The data only measure transport over 30 minutes, which is too short a time period to draw conclusions about membrane transport mechanisms.
• C) The data are collected from protein-free artificial liposomes and therefore cannot account for the role of active transport proteins present in living cell membranes. ✓
• D) The claim is limited because the protein-free liposomes still contain active pumps that remove every solute from the membrane.

Explanation: The experiment uses artificial liposomes containing no membrane proteins. Living cell membranes contain pumps, carriers, and channels -- including active transporters that move solutes against concentration gradients using ATP. The liposome data describe passive permeability of the phospholipid bilayer alone and cannot speak to active transport in real cells. A misreads the implication of the data -- Na+ transport is negligible, demonstrating that passive diffusion alone is insufficient for some solutes, which directly contradicts the claim by showing passive diffusion alone is insufficient for some solutes. B critiques the time window, which is not the core limitation. D introduces an irrelevant variable.