Energy moves through the cell as electrons, which carry energy, are passed from one substance to another. The substance with a stronger “pull” receives the electron as the substance with less of a pull loses the electron. In chloroplasts, electrons originate in the water-splitting photosystem from the splitting of water molecules: oxygen is a by-product, and hydrogen is used by the chloroplasts. Electrons are energized by light and pass down an electron transport chain, where ATP is produced, and enter the NADPH-producing photosystem, where they are again energized by light—but this time they are picked up by the electron carrier NADPH. The NADPH shuttles the electrons to the Calvin cycle, where they are used in the synthesis of glucose. In mitochondria, glucose enters the cellular respiration pathway, where electrons are removed and ultimately pass down the electron transport chain in the inner membrane of the mitochondria. The energy is used to concentrate hydrogen ions (protons) in the intermembrane space, and this proton gradient is used to make ATP. The electrons that pass down the electron transport chain are accepted by oxygen, which combines with hydrogen to form water.