The plasma membrane is made out of structures called phospholipids that form a phospholipid bilayer. The tails of the phospholipids are hydrophobic (water-hating) while their heads are hydrophilic (water-loving.
Because they are so tightly packed, large molecules such as glucose can’t get into the cell without the help of protein channels. However, non-polar molecules such as carbon dioxide and oxygen can easily pass through.