Question

which solute is least likely to cross a membrane by simple diffusion?
○ dissolved oxygen
○ dissolved gases, ions, and steroid molecules are equally likely to cross a membrane by simple diffusion.
○ dissolved sodium ions
○ a very small, hydrophobic steroid molecule
○ dissolved carbon dioxide

Explanation:

Simple diffusion across membranes favors small, nonpolar/hydrophobic substances. Dissolved oxygen (gas, nonpolar) and carbon dioxide (gas, nonpolar) diffuse easily. Small hydrophobic steroid molecules are nonpolar and cross readily. Sodium ions are charged (polar), so they cannot pass through the hydrophobic lipid bilayer of membranes via simple diffusion, making them the least likely.

Answer:

dissolved sodium ions