. "Any molecular entity that serves as an electron acceptor and electron donor in an electron transport system." . . "adrenodoxin-type ferredoxin" . "azurin" . "electron transporter activity" . "chloroplast-type ferredoxin" . "rubredoxin" . "electron transfer flavoprotein" . "polyferredoxin" . "amicyanin" . "iron-sulfur electron transfer carrier" . "electron carrier activity" . "2Fe-2S electron transfer carrier" . "3Fe-4S/4Fe-4S electron transfer carrier" . "bacterial-type ferredoxin" . "copper electron carrier" . "dicluster bacterial-type ferredoxin" . "electron acceptor activity" . "electron carrier, chlorophyll electron transport system" . "electron donor activity" . "electron transfer flavoprotein, group I" . "electron transfer flavoprotein, group II" . "high-potential iron-sulfur carrier" . "monocluster bacterial-type ferredoxin" . "mononuclear iron electron carrier" . "redox-active disulfide bond electron carrier" . "small blue copper electron carrier" . "thioredoxin-like 2Fe-2S ferredoxin" . . . . . . . . . . . .