Only small, uncharged molecules (like 02 for example) can go in or out of the cell by simple diffusion.
Since glucose is a relatively large molecule and ions (like Na+, Cl-) have an electric charge, they are unable to diffuse into or out of the cell.
However, since ions, glucose and other big or charged molecules are necessary for the functioning of the cell, there are special means of transport for them.
Integrated into the cell wall there are membrane proteins which serve as canals for transporting specific molecules like ions, glucose, etc.