Oxygen and carbon dioxide are both gases that diffuse directly across the point membrane. What determines WHICH DIRECTION they move?*

Concentration of the gases inside the membrane.

Explanation:

Diffusion is the movement of gases from a region of high concentration to a region of low concentration until equilibrium is reached. So if there's less concentration behind the membrane, the gases will diffuse and move towards it since they want to obey the rule of equilibrium.