CO2 is a relatively small non-polar molecule that can easily fit through the phospholipids in the plasma membrane without getting repelled by the hydrophobic core of the bilayer. Hence CO2 diffuses across the membrane without the aid of any intrinsic proteins (simple diffusion).
Amino acids however are large molecules that cannot fit through phospholipids in the bilayer. Hence, amino acids are most likely to diffuse across the cell plasma membrane with the help of channel and carrier proteins via facilitated diffusion.
You'll be able to make contrast and compare by reading both of them.
