If you've ever taken an organic chemistry class, this question is just a bunch of gibberish to you. It's amazing how much I hear this myth. All matter is made up of molecules, and molecules are made of atoms (like hydrogen, carbon, boron, plutonium, nitrogen, etc.). Even one atom can make a tremendous difference in the qualities of a material. For example, we breathe oxygen everyday, it is essential for life. The oxygen we breathe is composed of 2 oxygen atoms bonded together (O2). Ozone, which resides in the upper atmosphere and protects us from solar radiation, is made up of 3 oxygen atoms (O3). Ozone is toxic to us, and is considered a pollutant lower in the atmosphere. That's just one atom difference between essential to breathe and toxic!
Another example: water is made up of 2 hydrogen atoms and 1 oxygen atom (H2O). We need to drink water to live. Hydrogen peroxide is made up of 2 hydrogen atoms and 2 oxygen atoms (H2O2). Hydrogen peroxide at low concentrations (3%) is used to sterilize wounds and clean things. At higher concentrations it can remove layers of your skin, and if you drank it it would burn away the lining of your esophagus and stomach. Definitely different from water! So saying margarine is "one molecule different" from plastic is like saying "margarine is completely different from plastic."