Answer:
from the intermembrane space to the matrix
Explanation:
In the electron transport chain (ETC), electrons flow from one protein complex to another. However, as this electrons are transfered, protons (H+) is built up from the intermembrane space of the mitochondria to the mitochondrial matrix.
Hence, according to this question, a proton gradient is formed when hydrogen ions (H+) are moving from the intermembrane space to the matrix of the mitochondrial.
Carbon is the answer. Interestingly your question sounded like a riddle!!!
