Most common plastics biodegrade very slowly, sometimes to the extent that they are considered non-biodegradable. As polymers are ordinarily too large to be absorbed by microbes biodegradation initially relies on secreted extracellular enzymes to reduce the polymers to manageable chain-lengths. This requires that the polymers bare functional groups that the enzymes are able to 'recognise', such as ester or amide groups. Long-chain polymers with all-carbon backbones such as polyolefins, polystyrene and PVC will not degrade by biological action alone and must fist be oxidised to create chemical groups which the enzymes can attack.