Figure 21.9 Proton motion across the membrane drives the rotation of the c ring. A proton enters from the intermembrane space into the cytoplasmic half-channel to neutralize the charge on a glutamate or aspartate residue in a c subunit. With this charge neutralized, the c ring can rotate clockwise by one c subunit, moving the amino acid residue out of the membrane into the matrix half-channel. This proton can move into the matrix, resetting the system to its initial state.