Cellular respiration is the process by which living cells extract energy from food molecules, primarily glucose. This chemical reaction converts the energy stored in nutrients into adenosine triphosphate (ATP). The process requires oxygen and generates two main byproducts: carbon dioxide and water. Understanding the amount of water produced offers a precise view of energy metabolism’s chemical efficiency.
The Stoichiometric Answer
The overall process of aerobic cellular respiration is summarized by a single, balanced chemical equation. For one molecule of glucose, the reaction is C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy. This stoichiometry shows that the net production from the complete oxidation of one glucose molecule is six molecules of water. This count accounts for any water molecules consumed in intermediate steps, such as those within the Krebs cycle. The six molecules of water are considered a byproduct, alongside the six molecules of carbon dioxide.
Overview of Cellular Respiration Stages
Cellular respiration is a sequence of three main metabolic stages occurring in different parts of the cell. The first stage, glycolysis, takes place in the cytoplasm, splitting glucose into two pyruvate molecules. Following this, pyruvate enters the mitochondria to begin the second stage, the Krebs cycle (citric acid cycle). The Krebs cycle breaks down carbon compounds, releasing carbon dioxide and generating high-energy electron carrier molecules.
Neither glycolysis nor the Krebs cycle directly produces the net water molecules shown in the overall equation. Water production occurs during the final stage, oxidative phosphorylation, which is situated in the inner membrane of the mitochondria.
The Mechanism of Water Formation in the Electron Transport Chain
Water production is confined to the electron transport chain (ETC), the final component of oxidative phosphorylation. This chain is a series of protein complexes embedded in the inner mitochondrial membrane. High-energy carrier molecules (NADH and FADH2) generated in earlier stages deliver electrons to the chain.
As electrons pass from one complex to the next, energy is released and used to pump protons (H+ ions) across the inner membrane. This creates a high concentration gradient of protons, which drives the synthesis of ATP. At the end of the transport chain, the spent electrons must be removed to keep the process running.
Molecular oxygen (O2) plays its role here, acting as the final electron acceptor. Oxygen collects the electrons that have traveled down the chain. One atom of oxygen picks up two electrons and two protons from the mitochondrial matrix to form one molecule of water. This reaction is represented as 1/2 O2 + 2H+ + 2e- → H2O, and it is the exclusive source of the water molecules produced by the overall reaction.
The Biological Utility of Metabolic Water
The water generated by cellular respiration is referred to as metabolic water, distinguishing it from water consumed in food or drink. Although often viewed as a byproduct, this water serves a biological function in maintaining an organism’s fluid balance. In humans, metabolic water contributes a small portion, roughly 8 to 10 percent, of the total daily water requirement.
This contribution becomes more important in species living in arid environments, such as the kangaroo rat. These organisms rely heavily on the water produced from the oxidation of fats and carbohydrates as their main source of hydration. Metabolic water allows these animals to survive without needing to drink free water.