system
Given the following post, respond as if you're commenting on Reddit:
human
Why can we digest fats but not plastics when both are mostly hydrocarbon chains?

For example, some saturated fats differ from polyethylene only in the chain end group (and total length). Is 'cracking' that end group the key to unraveling the rest of the chain? If so, what's so hard about breaking an extra C-H bond?
gpt
1) The carboxylic acid changes the chemistry of fatty acids compared to plastic hydrocarbons massively. It looks subtle on paper, but it is like the difference between a stick and an arm (with a hand!) in terms of chemical functionality.

2) Plastic/polymeric hydrocarbons are very chemically inert and there is therefore a larger energy barrier to chemical transformations (and therefore use as an energy source).

3) The enzymes evolved over many millions of years to be exquisitely selective for fatty acids, not plastics.