Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…do not donate electrons to the respiratory chain, yet in bacteria they are membrane-bound enzymes transferring their electrons directly to ubiquinol (Barnes and Kaback 1970; Lascelles 1978; Doig et al. 1999; Modun and Williams 1999; Dym et al. 2000; Delgado et al. 2001; Fuller et al. 2011).
Category: