Carbon dioxide and oxygen diffuse across the semipermeable membrane due to the variation of concentration across the membrane (known as concentration gradient). Oxygen diffuses into the cell from outside because its concentration is high on the outer side of the cell wall. While carbon dioxide diffuses out of the cell, because its concentration is high on the inner side of cell membrane.
Also, both carbon dioxide and oxygen are non polar and small sized molecules due to which they can be easily transported across the lipid membrane.
