Which of the following will pass through a cell membrane most easily?

• A. Ions such as Na\(^+\)
• B. Large polar molecules such as glucose
• C. Proteins
• D. Small nonpolar molecules such as O\(_2\)

Answer: (D)

Verified step by step guidance

1. Understand the structure of the cell membrane: The cell membrane is primarily composed of a phospholipid bilayer, which has hydrophilic (water-attracting) heads on the outside and hydrophobic (water-repelling) tails on the inside. This structure makes the membrane selectively permeable.
2. Identify the types of molecules and their properties: Small nonpolar molecules like O\(_2\) are hydrophobic and can easily dissolve in the hydrophobic core of the phospholipid bilayer. In contrast, ions like Na\(^+\) are charged and require specific transport proteins, large polar molecules like glucose are too big and hydrophilic, and proteins are even larger and require specialized mechanisms to cross the membrane.
3. Recall the principle of passive diffusion: Small nonpolar molecules such as O\(_2\) can pass through the cell membrane via passive diffusion, which does not require energy or transport proteins. This is because they can move directly through the hydrophobic core of the membrane.
4. Compare the ease of transport for each molecule: Ions like Na\(^+\) are repelled by the hydrophobic core due to their charge, large polar molecules like glucose cannot pass due to their size and polarity, and proteins are too large and complex. Small nonpolar molecules like O\(_2\) face no such barriers and pass through most easily.
5. Conclude based on the properties of the molecules and the cell membrane: Small nonpolar molecules such as O\(_2\) are the most likely to pass through the cell membrane easily due to their compatibility with the hydrophobic core of the phospholipid bilayer.