Answer:
The correct word to be fill in the blank is 'energy.'
Explanation:
Secondary active transport is a type of active transport, which involves transport of molecules or ions across the membrane against their concentration gradient by using energy.
In secondary active transport, energy is provided by electrochemical gradient. It involves transport of two molecules or ions (either in same or opposite direction) and required energy is generated due to difference in electrochemical gradient formed by pumping or ions across the cells.
An example of secondary transport is transport of potassium ions by K+/H+ exchanger that transports hydrogen ions and potassium ions in opposite direction. It uses energy from proton motive force (generated by transport of hydrogen ions or protons) to transport potassium ions.
Thus, the correct answer is 'energy.'
