Question

Water molecules can pass into a cell, while larger molecules cannot. Which feature of a cell allows this to be true?

Answer 1

The selective permeability of a cells membrane determine what size and type of molecules can enter or not.  If a molecule has a charge and the membrane has a neutral charge the molecule has a harder time diffusing through.  The differences in charge affect permeability through a cell membrane. 

Answer 2

The cell has a semi-permeable membrane that allows water molecules to pass in a cell while larger molecules cannot. Lipid bilayer is an example of a biological semi-permeable membrane. The diffusion of water is called osmosis. Solutes such as salt and other contaminants are filtered.