(Q003) Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest).
alanine estrogen propanol sodium

Question

(Q003) Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest).
 alanine estrogen propanol sodium

BlackTom AI · Accepted Answer

We’re comparing diffusion rates across a phospholipid bilayer for four molecules with different chemical characteristics, keeping in mind that smoother, more hydrophobic molecules tend to diffuse faster through the lipid core, while charged ions diffuse slowly or not at all.
Option 1: 'sodium > propanol > alanine > estrogen' This ordering places sodium (an ion) as diffusing the fastest, which conflicts with fundamental principles: ions have difficulty crossing the hydrophobic core without transporters or channels, so sodium should be among the slowest, not the fastest.
Option 2: 'estrogen > propanol > sodium > alanine' Here estrogen is placed first, which makes sense due to its hydrophobic nature, but placing sodium before alanine implies sodium diffuses faster than alanine. In reality, sodium (a small cation) is typically slower than neutral organic molecules like alanine is zwitterionic and not significantly charged in all contexts; however, in general, ions (like sodium) diffuse poorly through the lipid bilayer without channels, so this ordering is questionable and not the strongest rationale.
Option 3: 'alanine > propanol > sodium > estrogen' This sequence starts with alanine as the fastest, but alanine is a small polar/charged amino acid; its diffusion across a pure lipid bilayer is limited compared to neutral, nonpolar molecules. Therefore, placing alanine ahead of propanol (a small, less polar molecule) and especially ahead of estrogen (a hydrophobic molecule) is inconsistent with expected permeability trends.
Option 4: 'estrogen > propanol > alanine > sodium' This ordering follows the general trend: estrogen is a hydrophobic, nonpolar steroid that diffuses relatively well through lipid, propanol is a small polar molecule that can cross the bilayer but more slowly than estrogen, alanine is a small polar/charged amino acid that diffuses more slowly than neutral organics, and sodium is an ion with very limited crossing of the hydrophobic core. Overall, this sequence aligns with widely taught principles of diffusion through lipid membranes, balancing polarity, charge, and size across the bilayer.

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