Question: where do the electrons come from in the electron transport chain
[{'answer': 'electron donors', 'provenance': [{'wikipedia_id': '92236', 'title': 'Electron transport chain', 'start_paragraph_id': 1, 'start_character': 88, 'end_paragraph_id': 1, 'end_character': 103, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '92236', 'title': 'Electron transport chain', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 796, 'bleu_score': 0.9829771687435411, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4148173462205054324}, 'section': 'Section::::Abstract.'}]}]