Suggested schematic drawing showing the possible fit of inner part of c-rings from different organisms with corresponding quinone molecules inside. (A,D) – c-ring from Bacillus Pseudofirmus with menaquinone-7 (MQ-7) inside. (B,E) – c-ring from yeast mitochondria with ubiquinone-10 (UQ-10). (C,F) – c-ring from Arthrospira platensis with plastoquinone-9 (PQ-9). MQ-7, UQ-10 and PQ-9 are electron carriers in Bacillus Pseudofirmus, yeast and plants, respectively. The molecules are oriented in different directions and shown in sticks. C-rings are shown in surface representation. The surface is colored according to the hydrophobicity of amino acids (see legend on the right).