An obstruction between the interior of the cell and its external environment is the cell membrane. The tendency of a molecule to pass through a cell membrane relies on its charge, concentration, and size.
Certain of the polarized molecules are so small that it can pass through the cell membrane, for example, let us consider a polarized molecule like water, whose small size permits it to diffuse freely through the cell membrane.
Other molecules, like that of oxygen, which exhibits no polarity and is small enough to readily pass through the cell. The molecules of sugar that comprise five or more atoms of carbon, are too large to pass via the cell membrane and need to be mediated with the help of transmembrane proteins.
