Membranes of cells and organelles allow certain types of molecules to pass through but not others. This selectivity is due to the construction of cellular membranes. Cell membranes are made of a lipid bilayer of hydrophobic molecules. The easiest way to visualize this is to imagine pouring vegetable oil into a glass partly filled with water. The vegetable oil which is hydrophobic will float on top of the water. In this model, hydrophobic molecules of cell membranes resemble vegetable oil and repel water.
Which type of molecule will pass MOST easily through the membrane of a cell or organelle?