Why does plastic decompose so slowly compared to biomolecules?

What happens if you throw an apple and a plastic bag into the forest? The apple will decompose after some time, but the bag will be blown around by the wind for a couple of hundred years. Novaator reader Jüri was curious about why nature works this way. The answer comes from Darja Lavõgina, Associate Professor of Bioorganic Chemistry at the University of Tartu. The article is from Novaator.

Indeed, both natural molecules and many plastics consist of the same chemical elements – carbon, hydrogen, and often oxygen and nitrogen, among others. At the level of individual bonds, such as C-H and C-O, the chemical bonds in these substances are not different in any way.

However, it must be understood that biodegradation mostly means decomposition by the enzymes of living organisms, such as bacteria or fungi. Enzymes are proteins that accelerate chemical reactions. Each enzyme has evolved to catalyze a specific type of reaction. For example, breaking peptide bonds in other proteins or glycosidic bonds in saccharides.

At the molecular level, this means that an enzyme figuratively recognizes the molecules in which it breaks bonds. This means that in the degradable molecule, whether it is a peptide or a saccharide, there is a spatial pattern of several atoms and the bonds between them.

Historically, this phenomenon has even been described using the so-called lock-and-key analogy – the enzyme as the lock and the substance it modifies as the key, which must uniquely fit the lock. Although this lock-and-key model has several shortcomings from the perspective of modern biochemistry, it is probably the best way to provide an intuitive and simple answer to this question.

However, with plastics, the story is different. They have not evolved in nature alongside enzymes. On the contrary, for many plastics, it is important that they withstand environmental conditions, which include not only moisture or UV radiation but also microorganisms.

In a complex environment, plastics must withstand external conditions for some time. Therefore, they can be used in coatings, various constructions, as well as toys, medical devices, etc.

Thus, most plastics lack the patterns that are easily recognizable by the enzymes of living organisms.