They will also move out of the cell at the same rate, and a dynamic equilibrium will exist. However, if the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell. Likewise, as the chemical reactions in the cell use up oxygen they produce carbon dioxide. The concentration of carbon dioxide inside the cell increases so that more CO 2 molecules strike the inside of the cell and move out than strike the outside of the cell and move in. So the overall effect is that the CO 2 moves out of the cell.
Osmosis is a special case of diffusion.
In this case, a large molecule like starch is dissolved in water. The starch molecule is too large to pass through the pores in the cell membrane, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semipermeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the starch, the starch molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter.