Which solute is most likely to cross a membrane by facilitated diffusion?

• A. Oxygen (\(O_2\))
• B. Sodium ion (\(Na^+\)) via the sodium-potassium pump
• C. Carbon dioxide (\(CO_2\))
• D. Glucose

Answer: (D)

Verified step by step guidance

1. Facilitated diffusion is a type of passive transport where molecules move across a membrane with the help of transport proteins, without requiring energy input.
2. Identify the characteristics of the solutes listed: Oxygen (\(O_2\)) and Carbon dioxide (\(CO_2\)) are small, nonpolar molecules that can diffuse directly through the lipid bilayer without the need for transport proteins. Sodium ion (\(Na^+\)) requires active transport via the sodium-potassium pump, which uses energy (ATP). Glucose, being a larger polar molecule, cannot diffuse directly through the lipid bilayer and requires transport proteins for facilitated diffusion.
3. Understand why glucose is the correct answer: Glucose is transported across the membrane by specific carrier proteins through facilitated diffusion, which allows it to move down its concentration gradient without energy expenditure.
4. Review the role of transport proteins in facilitated diffusion: These proteins provide a pathway for polar or larger molecules like glucose to cross the hydrophobic lipid bilayer of the membrane.
5. Conclude that facilitated diffusion is the mechanism used by glucose to cross the membrane, as it matches the criteria of requiring transport proteins and moving passively along its concentration gradient.