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.
The answer is A, 'a mammal' because it goes with the copulative verb 'to be', i. e., 'is'.
A CELL WALL because the other three appear in different types of cells
