Which of the following would be least likely to diffuse through a plasma membrane without the help of a transport protein?

• A. A large, nonpolar molecule
• B. Dissolved gases such as oxygen or carbon dioxide
• C. A small, nonpolar molecule
• D. Any of the listed molecules would easily diffuse through the membrane.
• E. A large, polar molecule

Answer: (E)

Verified step by step guidance

1. Understand the structure of the plasma membrane: It is primarily composed of a phospholipid bilayer with hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads.
2. Recognize that the plasma membrane is selectively permeable, allowing certain molecules to pass through more easily than others based on size, polarity, and charge.
3. Identify that nonpolar molecules can diffuse through the lipid bilayer more easily than polar molecules because the interior of the membrane is hydrophobic.
4. Consider the size of the molecule: Smaller molecules can pass through the membrane more easily than larger ones due to less steric hindrance.
5. Conclude that a large, polar molecule would be least likely to diffuse through the plasma membrane without the help of a transport protein, as it is both large and polar, making it difficult to pass through the hydrophobic core of the membrane.