Plastics You Could Eat - Recyling

Beautiful crystals

Plastic bags may not look or feel very crystalline, but if you zoom into a plastic bag by a factor of a million, you can actually see lots of tiny crystals. These little crystals form regular structures themselves: they spiral out from a central point, forming little spheres called 'spherulites'. The crystals spiral completely regularly, and the resulting spherulite structures can be incredibly beautiful.

For years, polymer physicists used polythene, or more correctly polyethylene, as a model to represent the behaviour of other polymers. But over the last 20 years some scientists have opted to use PHB instead. It crystallises more slowly than polyethylene, and so is easier to study. It's been a remarkably useful material, and many new discoveries have been made thanks to its big, slowly-forming crystals.

An unanswered question

PHB is everywhere. Trace amounts in short chains of only about 150 units have been found in the cells of yeast, carrots, spinach, sheep, pigs, cattle and even in humans. It exists in the cells of a staggering variety of different organisms. In fact, it seems that you can find PHB in any cell that you care to choose, if you look hard enough. And nobody knows what it's there for. Surely, for something to be so ubiquitous, it must have some function. It's inconceivable that it's just an accident that PHB is present in so many places. Some scientists have even claimed that PHB could be as important as proteins and that HB units (hydroxy butyrate) might have been present in the primordial soup on earth, before amino acids and proteins. These claims may be extravagant, but whatever the real story is, watch this space; PHB must do something significant in cells.

It grows on trees...almost

A recent development in the PHB story has perhaps changed its destiny forever. A team at the DoE Plant research lab at Michigan State University considered that there might be a faster way of making PHB. In 1992, they took two genes from PHB-making bacteria and inserted them directly into two cress plants and then crossed them. Some of the offspring plants incorporated both the new genes and produced PHB in their leaves. They had managed to create a plant that could grow plastic.

Chris Somerville White PHB blobs inside a cressleaf

There were problems -the new cress plants were pathetic: their growth was severely stunted. The leading geneticist Chris Sommerville (by now at the Carnegie Institute, Washington), wanted to try out an unusual idea. He decided to add more DNA to the cress plants, but this time from a pea plant. It worked. Now instead of PHB being manufactured randomly over the plant's leaves, it was only made in one place, the chloroplasts, and the plant's growth was completely normal. The PHB formed 14% of the dry weight of the leaves.