Cells are selectively permeable due to the Phospolipid Bilayer. It allows only certain molecules to pass through. The cell membrane is made of two sheets of phosolipid bilayers
The hydrophobic center to a cell membrane (also known as a phospholipid bilayer) gives the membrane selective permeability. If a substance is permeable , something such as water or gas can pass through it or soak.
Some solutes cross the membrane freely, some cross with assistance, and others do not cross at all.
Large molecules do not cross intact cell membranes, except in certain special cases.
