The final stage of the breakdown of a food molecule is responsible for the release of the majority of the chemical energy. NADH and FADH2 are responsible for the final phase, which consists of transferring the electrons that were obtained through the oxidation of food-derived organic molecules to the electron transport chain that is encased within the inner membrane of the mitochondrion. As electrons move further down this long chain of specialized electron acceptor and donor molecules, they experience a gradual decrease in the level of energy they possess. The energy that is generated by the electrons during this process propels H+ ions (protons) across the membrane, so producing a gradient of H+ ions from the mitochondrial compartment that is the most interior to the mitochondrial portion that is intermembranous (and eventually to the cytosol). This gradient serves as a battery to power a variety of reactions that need a significant amount of energy and is the principal source of energy for cells. The phosphorylation of ADP, which results in the production of ATP, is the one of these events that is most readily seen.

After that, the electrons are moved onto oxygen gas molecules (O2) that have already made their way into the mitochondrion. At this point, the oxygen gas molecules combine with the protons (H+) that are present in the solution that is all around them to form water. The oxidized food molecule has lost all of its energy because the electrons have dropped to a lower energy level. This process, which is known as oxidative phosphorylation, also takes place in the plasma membrane of bacteria.

A single molecule of glucose is utilized by the cell in order to accomplish the full oxidation of glucose, which results in the production of around 30 individual molecules of ATP. In contrast, glycolysis occurs by itself and only results in the production of two molecules of ATP for each molecule of glucose.

The metabolism of carbohydrates has taken up the majority of our attention up until this point, and we have not yet begun to consider the metabolism of nitrogen or sulfur. These two components play an important part in the functioning of macromolecules in living organisms. Nitrogen and sulfur atoms move back and forth between compounds, organisms, and the environment through a series of cycles that can be reversed.

Even though there is a significant amount of molecular nitrogen in the atmosphere of the Earth, this gas does not react with other substances in any way. A small number of living species are responsible for the process that is known as nitrogen fixation. This involves the incorporation of nitrogen into organic molecules. Lightning discharge is one example of a geophysical process that can fix nitrogen, along with other microorganisms and processes. It is essential to the biosphere as a whole because, without it, there would be no possibility of life existing on this planet. However, a very insignificant proportion of the nitrogenous molecules that make up modern organisms come from the fresh byproducts of nitrogen fixation that come from the surrounding environment. Since quite some time ago, the vast majority of the organic nitrogen that exists has been moving from one living thing to another. As a result, one could argue that modern nitrogen-fixing reactions act as a source of "top-up" nitrogen for the entire nitrogen supply.

Proteins and nucleic acids, which are the primary sources of nutrition for vertebrates, satisfy the vast majority of their requirements for nitrogen. These large molecules are broken down into their constituent amino acids and nucleotide building blocks while still inside the body. The nitrogen that these large molecules contain is then used in the synthesis of new proteins, nucleic acids, and other compounds. Nearly half of the 20 amino acids that are found in proteins are considered to be essential for vertebrates. This means that the body is unable to synthesize them from other foods. It is feasible to synthesize the other amino acids by making use of a wide variety of raw materials, such as the intermediates of the citric acid cycle. The creation of essential amino acids takes place in plants and other living things via metabolic pathways that are often time-consuming and energy-intensive and which have been extinct in vertebrates during the course of evolution. The nucleotides that are necessary for the production of RNA and DNA can be obtained through the utilization of specialized biosynthetic pathways. The plentiful amino acids glutamine, aspartic acid, and glycine are the sources of all of the nitrogens in the purine and pyrimidine bases (as well as some of the carbons). On the other hand, glucose is the source of the ribose and deoxyribose sugars. The term "essential nucleotides" does not have to be included in the diet at all.

It is possible that amino acids that are not used in the process of biosynthesis will be oxidized in order to provide metabolic energy. The vast majority of their carbon and hydrogen atoms will eventually combine to form CO2 or H2O, but their nitrogen atoms will go through a number of different configurations before finally appearing as the substance urea that will be excreted. Each amino acid is broken down in its own unique way, and this process, known as catabolism, involves an intricate network of enzyme reactions. On Earth, sulfate (SO4 2-), which is the most oxidized form of sulfur, can be found in a variety of settings. In order for sulfate to be beneficial to living things, it must first be transformed to sulfide (S2). Sulfide is the oxidation state of sulfur that is required for the production of essential biological components such as coenzyme A, the amino acids methionine and cysteine, and the iron-sulfur centers that are required for electron transport. Sulfide can only be produced when sulfur is first oxidized to sulfate. In bacteria, fungi, and plants, there is a specific class of enzymes that makes use of ATP and reducing power to construct a sulfate absorption pathway. This pathway is the first stage in the process of sulfur reduction. Due to the fact that humans and other animals are unable to decrease sulfate, they must receive the sulfur that is necessary for their metabolism from the food that they ingest.

Each of these processes must have its own unique enzyme in order to occur, and they are all carried out within a cell that has a diameter of less than 0.1 mm. Pyruvate, for example, is a substrate for at least a half-dozen and probably more than a dozen enzymes, all of which bring about various chemical changes in it. Pyruvate is converted into acetyl CoA by one enzyme, oxaloacetate by another, the amino acid alanine by a third, lactate by a fourth, and so on, using a series of enzymes. All of these different pathways compete with one another for the same pyruvate molecule, along with thousands upon thousands of other minute molecules. When considering a creature composed of multiple cells, the complexity of the issue increases. In general, the different types of cells have slightly varying requirements for the enzymes they need. In addition, distinct tissues each make their own unique contribution to the overall chemistry of the body. In addition to differences in specialized products such as hormones or antibodies, there are significant changes in the "common" metabolic pathways that exist between different types of cells that exist within the same organism. These pathways are found in the same organism.

Enzymes that are necessary for glycolysis, the citric acid cycle, lipid synthesis and degradation, and amino acid metabolism are present in practically all cells; however, the degrees of activity required by these processes varies greatly depending on the type of tissue. For instance, nerve cells, which are perhaps the most sensitive cells in the body, do not store much glycogen or fatty acid and instead rely almost entirely on a continuous supply of glucose derived from the bloodstream. This is because nerve cells have almost no glycogen reserves. On the other hand, liver cells are responsible for providing glucose to muscle cells that are actively contracting and for recycling the lactic acid that is produced by muscle cells back into glucose. All of the different cell types each have their own unique metabolic features, and they work closely together both in the normal state and in the responses to things like stress and hunger. It is possible to envision that the entirety of the system must be in such impeccable harmony that even the smallest disruption, like as a momentary change in the amount of nutrients taken in, would be fatal.