As the electron transport system carriers move electrons by redox reactions, they actively pump across the membrane maintaining a concentration gradient.

Answer;
As the electron transport system carriers move electrons by redox reactions, they actively pump hydrogen ions across the membrane maintaining a concentration gradient. 

Explanation;
Electron transport is a phase of cellular respiration in which majority of ATP is formed. During this phase electron carriers that are reduced during glycolysis and citric acid cycle to NADH+ H+ and FADH2. The carriers then donate electrons and protons to the electron carrier proteins of the ETC, with oxygen being the final acceptor. 

1) noble gases2) by increasing atomic number3) noble gases