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: Alternatively , anemia could result from accelerated suicidal erythrocyte death or eryptosis , which is characterized by exposure of phosphatidylserine ( PS ) at the erythrocyte surface and by cell shrinkage .
### Output Text:
Alternatively , anemia could result from accelerated suicidal erythrocyte death or eryptosis , which is characterized by exposure of <span class="chemical">phosphatidylserine</span> ( <span class="chemical">PS</span> ) at the erythrocyte surface and by cell shrinkage . 