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:…acids, are quantitatively taken up by intact cells or isolated mitochondria, the only relevant way of expressing their effective “concentration” is per mg protein of the biological material used rather than per volume
of the incubation mixture (Dymkowska et al., 2006; Schönfeld & Wojtczak, 2007).
Category: