# Question #984b2

Nov 25, 2017

$C {O}_{2}$ and ${O}_{2}$ are both small, nonpolar, non-charged molecules.

#### Explanation:

Substances that can cross the cell membrane by passive diffusion need to be small, nonpolar, and non-charged substances.

$C {O}_{2}$ and ${O}_{2}$ fit this description perfectly and thus can cross the cell membrane by passive diffusion.

This is very efficient because ${O}_{2}$ constantly has to get into the cell and $C {O}_{2}$ has to get out.

If the cell had to expend energy in transporting these molecules, it would have to use a TON of energy that it wouldn't have.