Why proteins that cause active transport only transport the substance carried in one direction across the cell?
Active transport requires energy, which suggests that it is working against the normal osmotic equilibrium. In order for the protein to be useful at all, it has to be pumping in only one direction. An example of this would be oxidative phosphorylation (which occurs in mitochondria): complexes I, III, and IV are all pumping protons (H+) into the cytoplasm, creating the pH/electrostatic charge difference needed to power complex V. Complex V harnesses the energy difference to produce ATP out of ADP and Pi. Without these proteins actively facilitating protons out of the matrix and into the mitochondrial cytoplasm, this pH/charge difference in the osmotic equilibrium would never have been formed, and ATP (the cell’s energetic currency) would never be produced.
