A redox reaction is a chemical reaction that involves the transfer of one or more electrons from one reactant to another. It is also known as an oxidation-reduction reaction. Oxidation is the loss of electrons from substance and reduction is the addition of electrons to another substance. In cellular respiration, glucose loses electron in hydrogen atoms (oxidation), while O2 gains electrons in hydrogen atoms (reduction). In plants, such as the one pictured here, the food-producing redox reactions of photosynthesis involve an uphill climb. In photosynthesis, when water molecules are split apart to yield O2, they are oxidized, losing electrons along with hydrogen ions. Meanwhile CO2 is reduced to sugar as hydrogen ions and electrons are added to it. Plants perform both these redox reactions, cellular respiration and photosynthesis.