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: We then examined the morphology , expression of beclin 1 gene , mitochondrial permeability transition ( MPT ) , and Na + - K + ATPase activity in vivo .
### Output Text:
We then examined the morphology , expression of beclin 1 gene , mitochondrial permeability transition ( MPT ) , and Na + - <span class="chemical">K</span> + ATPase activity in vivo . 