Which of the following molecules would most likely pass unaided through a plasma membrane?

• A. Na\(^+\) (sodium ion)
• B. O\(_2\) (oxygen gas)
• C. Cl\(^-\) (chloride ion)
• D. Glucose

Answer: (B)

Verified step by step guidance

1. Understand the structure and function of the plasma membrane: The plasma membrane is primarily composed of a phospholipid bilayer, which has hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads. This structure allows small, nonpolar molecules to pass through easily, while larger or charged molecules require assistance.
2. Analyze the properties of each molecule: Sodium ion (Na\(^+\)) and chloride ion (Cl\(^-\)) are charged particles, making them polar and unable to pass through the hydrophobic core of the membrane unaided. Glucose is a large polar molecule, which also cannot pass through without assistance. Oxygen gas (O\(_2\)), however, is small and nonpolar, making it capable of diffusing through the membrane unaided.
3. Recall the principle of selective permeability: The plasma membrane is selectively permeable, meaning it allows certain molecules to pass through based on size, polarity, and charge. Nonpolar molecules like O\(_2\) can diffuse freely, while polar or charged molecules require transport proteins or channels.
4. Consider the role of diffusion: Small, nonpolar molecules like O\(_2\) move across the membrane via simple diffusion, which does not require energy or transport proteins. This is because they can easily interact with the hydrophobic interior of the membrane.
5. Conclude the reasoning: Based on the molecular properties and the structure of the plasma membrane, O\(_2\) is the molecule most likely to pass unaided through the plasma membrane, while Na\(^+\), Cl\(^-\), and glucose require assistance due to their size, charge, or polarity.