Question

Question #984b2

Answer 1

`CO_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.

`CO_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 `CO_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.