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 analysis , respiration intensity , levels of reactive oxygen species , permeability transition , and cytochrome c release were assessed 3 and 6 days after gentamicin administration .
### Output Text:
Mitochondrial analysis , respiration intensity , levels of reactive <span class="chemical">oxygen</span> species , permeability transition , and cytochrome c release were assessed 3 and 6 days after <span class="chemical">gentamicin</span> administration . 