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: An alternative source of energy is d - galactose ( the C - 4 - epimer of d - glucose ) which is transported into the brain by insulin - independent GLUT3 transporter where it might be metabolized to glucose via the Leloir pathway .
### Output Text:
An alternative source of energy is <span class="chemical">d - galactose</span> ( the C - 4 - epimer of <span class="chemical">d - glucose</span> ) which is transported into the brain by insulin - independent GLUT3 transporter where it might be metabolized to <span class="chemical">glucose</span> via the Leloir pathway . 