In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Mitochondrial abnormalities have been associated with several aspects of epileptogenesis , such as energy generation , control of cell death , neurotransmitter synthesis , and free radical ( FR ) production .
### Output Text:
Mitochondrial abnormalities have been associated with several aspects of epileptogenesis , such as energy generation , control of cell death , neurotransmitter synthesis , and free radical ( FR ) production . 