Which of the following is NOT an example of facilitated diffusion?

• A. Chloride ions moving through a channel protein
• B. Glucose transport into cells via GLUT transporters
• C. Water movement through aquaporins
• D. Oxygen crossing the plasma membrane by simple diffusion

Answer: (D)

Verified step by step guidance

1. Understand the concept of facilitated diffusion: Facilitated diffusion is a type of passive transport where molecules move across the plasma membrane with the help of transport proteins, such as channel or carrier proteins, without requiring energy (ATP). It is used for molecules that cannot easily pass through the lipid bilayer due to their size, polarity, or charge.
2. Analyze the examples provided: Chloride ions moving through a channel protein, glucose transport via GLUT transporters, and water movement through aquaporins all involve transport proteins, which are characteristic of facilitated diffusion.
3. Contrast facilitated diffusion with simple diffusion: Simple diffusion is another type of passive transport, but it does not involve transport proteins. Molecules move directly through the lipid bilayer, typically small, nonpolar molecules like oxygen and carbon dioxide.
4. Identify the exception: Oxygen crossing the plasma membrane by simple diffusion does not involve transport proteins. Instead, oxygen is a small, nonpolar molecule that can easily diffuse through the lipid bilayer without assistance, making it an example of simple diffusion rather than facilitated diffusion.
5. Conclude the reasoning: The correct answer is oxygen crossing the plasma membrane by simple diffusion because it does not fit the definition of facilitated diffusion, which requires the involvement of transport proteins.