Question

Which one of the following substances does not pass across membranes by simple diffusion?

• A. $O_2$
• B. $C{O}_2$
• C. $H^{+}$
• D. $H_{2}O$

Answer 1

Answer: (C)

$H^{+}$

Water diffusion is called osmosis. Oxygen is a small molecule and it is nonpolar, so it easily passes through a cell membrane. Carbon dioxide, the byproduct of cell respiration, is small enough to readily diffuse out of a cell. Hydrogen ion cannot pass through a plain phospholipid bilayer, it only passes by binding with another molecule.

So, the correct option is 'H+'.