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: These injuries , partly mediated by a rise in reactive oxygen species from the electron transfer chain , were significantly decreased by metformin .
### Output Text:
These injuries , partly mediated by a rise in reactive <span class="chemical">oxygen</span> species from the electron transfer chain , were significantly decreased by <span class="chemical">metformin</span> . 