The Krebs cycle, commonly known as the citric acid cycle or the tricarboxylic acid (TCA) cycle, is the second stage of cellular respiration. Nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH_{2}FADH
2
​
), which act as a kind of catalyst and/or accelerator in producing energy, are two of the critical components in the Krebs cycle. Cellular respiration ends with the electron transport chain – the third stage in which ATP and water are produced which are essential for life. A sequence of events converts adenosine diphosphate (ADP) into ATP, by passing the protons across the mitochondria, which are driven by NADH and FADH_{2}FADH
2
​
dependent protons pumps. Finally, in the electron transport chain, hydrogen molecules combine the cell’s oxygen to produce water, and ATP is synthesized in the process by ATP synthase. The water that is produced as the byproduct or metabolic product of all these chemical reactions is known as metabolic water.
Uses of metabolic water
The free drinking water, plus the water included in meals, has a greater total quantity than metabolic water, which exists in a tiny quantity. Even though it is negligible and irrelevant in creatures such as fish and humans, it is critical for animals that live in dry environments and ecosystems without a water supply. Animals that live in the desert (in particular, xerocoles – any animal that adopted to reside in dry conditions) depend on metabolic water entirely. While migrating, migratory birds entirely depend on metabolic water production for their water needs, which means that they can fly non-stop.
On the other hand, humans get just around 8-10\%8−10% of their water through their metabolism. Kangaroo rats use metabolic water to get their water, and it has been used in larger quantities by animals in the desert. Drinking water is never necessary since water is made within their cells.
Up to 15\%15% of total water consumption in cats and dogs comes from metabolic water, which may range between 10-15\%10−15% depending on the food source. Different nutrients have different water yields for metabolite production. It is believed that the water produced daily in people and pets is very little in comparison to their whole daily water consumption.
Metabolic water in microorganisms
Research suggests that the daily metabolic water generated by microbial biomass ranges from 4\%4% to 70\%70% of the total biomass. In boreal, Mediterranean, and tropical forests, research has shown that around 50\%50% of soil microbial biomass is turned over each day. Water in metabolic processes has been overlooked in the ecophysiology of microorganisms.
Researchers at the Northern Arizona University conducted experiments to determine whether metabolic water was necessary for soil microorganisms’ water balance. They quantified the amount of metabolic water created by microorganisms in various terrestrial habitats, and the researchers utilized published values for heterotrophic soil respiration and microbial biomass. Estimates show that aerobic respiration generates a daily mass of water equal to 10-30\%10−30% of bacteria’s body mass. In soil incubations, direct measurements of oxygen consumption showed that daily microbial respiration produced an average of 5% water (in the absence of glucose) to 15% water (in the presence of glucose) of microbial cells’ water mass. In addition, the researchers identified the presence of metabolic water’s oxygen in microbial biomass using an isotopic tracer method. This shows that metabolic water is essential in microorganisms’ water balance, and it is especially crucial when soil is drying.
