Why do charged molecules have difficulty crossing a phospholipid bilayer?

Charged molecules have difficulty crossing a phospholipid bilayer because the phospholipid bilayer is composed of hydrophobic tails that repel charged particles. The phospholipid bilayer is made up of two layers of phospholipids, with their hydrophilic (water-loving) heads facing outward and their hydrophobic (water-repelling) tails facing inward. This creates a barrier that prevents charged molecules from easily passing through.

Option A, "They are too big," is not the correct answer because the size of the molecule does not directly affect its ability to cross the phospholipid bilayer. Option B, "They are too nonpolar," is also incorrect because nonpolar molecules can easily pass through the hydrophobic region of the bilayer. Option D, "They are hydrophobic," is also incorrect because hydrophobic molecules can easily pass through the hydrophobic region of the bilayer.

The correct answer is option C, "They are too polar." Charged molecules are polar, meaning they have a separation of positive and negative charges. The hydrophobic interior of the phospholipid bilayer repels charged molecules, making it difficult for them to cross. This is why charged molecules, such as ions or polar molecules, require specialized transport proteins or channels to facilitate their movement across the phospholipid bilayer.

In summary, charged molecules have difficulty crossing a phospholipid bilayer because the bilayer is composed of hydrophobic tails that repel charged particles. The correct answer is C.