The energy stored in the chemical bonds of food molecules supplies the cells with the consistent supply of energy that they need to develop and preserve the biological order that allows them to remain alive.

In order for our cells to make use of the proteins, lipids, and polysaccharides that make up the majority of the food that we consume, whether as a source of energy or as building blocks for other molecules, the larger molecules that make up these macronutrients must first be broken down into their component parts. Enzymes are responsible for the breakdown of large polymeric food molecules into their respective monomer sub-units. These monomer sub-units include amino acids for proteins, sugars for polysaccharides, and fatty acids and glycerol for lipids. Following digestion, small organic molecules derived from food make their way into the cytosols of cells, where they initiate a more leisurely process of oxidation.

Sugars are particularly important fuel molecules that, when subjected to oxidation in a regulated manner via a series of tiny steps, result in the production of carbon dioxide (CO2) and water. In this section, we will discuss the fundamental processes involved in the catabolism, also known as the breaking down of carbohydrates, and show how animal cells generate ATP, NADH, and other activated carrier molecules as a consequence of these processes. In addition, plants, numerous types of bacteria, and fungi all use a mechanism that is quite similar to one another. As we're about to see, the process of oxidizing fatty acids is very important for the health of the cell. Other molecules, such as proteins, are also capable of functioning as sources of energy if they are guided through the appropriate enzyme pathways.

The primary technique for oxidizing sugars is a chain of processes called glycolysis, which derives from the Greek words glukus, which means "sweet," and lusis, which means "rupture." Glycolysis, which does not require the presence of molecular oxygen, results in the production of ATP (O2 gas). The cytoplasm of the majority of cells, including many anaerobic bacteria, is where it is found. Glycolysis most likely emerged early on in the history of life, far before the appearance of organisms capable of producing oxygen through photosynthesis. During the glycolytic process, a molecule of glucose that has six carbon atoms is broken down into two molecules of pyruvate, each of which contains three. During the initial phases of the process, two molecules of ATP are hydrolyzed for every molecule of glucose in order to provide the necessary energy. However, during the subsequent phases, four molecules of ATP are generated. Because of this, when glycolysis is nearing its conclusion, there is a gain of two molecules of ATP for every molecule of glucose that was broken down in the process. Two molecules of the active carrier NADH are also produced during this process.

Glycolysis is comprised of a sequence of ten separate steps, each of which generates a different sugar intermediate and is mediated by a different enzyme. Glycolysis occurs in the presence of oxygen. These enzymes, like with the vast majority of other enzymes, have names that end in ase to indicate the type of chemical reaction that they catalyze. For example, isomerase and dehydrogenase are both examples of such enzyme names. During glycolysis, there is no consumption of molecular oxygen; nonetheless, oxidation does take place as a result of part of the carbons derived from the glucose molecule being subjected to electron removal by NAD+, which results in the synthesis of NADH. This process causes oxidation. As a result of the sequential nature of the process, the energy that is generated by the oxidation is only released in small amounts. This, in turn, makes it possible to store some of this energy in activated carrier molecules as opposed to letting it all be converted into heat. Because of this, some of the energy that is produced during oxidation is used to directly produce ATP molecules from ADP and Pi, while the remaining portion of the energy is stored as electrons in the electron carrier NADH.

During the glycolysis process, two molecules of NADH are produced for every single molecule of glucose that is broken down. These NADH molecules donate their electrons to the electron transport chain in aerobic organisms, which subsequently makes use of the resultant NAD+ (NADH plus NAD+ plus NAD+) to restart the glycolysis process.

In the majority of animal and plant cells, glycolysis is just the initial step in the last stage of the process that breaks down the molecules of the food that they eat. These cells' mitochondria are responsible for turning the pyruvate that is created during glycolysis into carbon dioxide and acetyl CoA. The acetyl group of the acetyl CoA is subsequently completely oxidized to produce carbon dioxide and water.

In contrast, glycolysis is the primary source of ATP in the cells of many anaerobic species. Anaerobic organisms do not require the presence of molecular oxygen in order to sustain life and are capable of expanding and reproducing in its absence. Skeletal muscle is one of the animal tissues that can continue to function even when there is a shortage of molecular oxygen. Under these anaerobic conditions, both the NADH electrons and the pyruvate will be retained in the cytosol. Pyruvate is converted into molecules that are released from the cell, such as lactate in muscle or ethanol and CO2 in yeasts used in brewing and breadmaking. Examples of these substances are lactate in muscle and pyruvate. During this phase, the NADH molecule gives up its electrons, transforming itself back into NAD+. It is necessary to supply NAD+ in order to keep the glycolysis processes continuing.

Fermentations are processes that produce energy. Fermentations often take place in anaerobic settings and involve organic substances that can both contribute and absorb electrons. Studies of fermentations carried out by yeasts that were significant on a commercial scale were a major driving force in the development of early biochemistry. As a result of the study done in the nineteenth century, an important breakthrough occurred in 1896 when it was found that these processes could be studied in cell extracts rather than in living beings. This was a significant development. This ground-breaking discovery eventually made it possible to deconstruct and analyze each individual fermentation reaction, which was a huge step forward in the field. An important step forward in biochemical research occurred in the 1930s with the discovery of the whole glycolytic pathway. This was quickly followed by the realization that ATP plays an essential role in the processes that take place within cells.

The production of ATP during glycolysis is a particularly useful example of how enzymes combine energetically unfavorable processes with favorable ones to drive the numerous chemical reactions that are necessary for life. These enzymes are responsible for the numerous chemical reactions that are necessary for life. The three-carbon sugar intermediate glyceraldehyde 3-phosphate, which is an aldehyde, undergoes two important reactions during glycolysis (steps 6 and 7), which oxidize an aldehyde group to a carboxylic acid group. This results in the formation of the carboxylic acid 3-phosphoglycerate. The overall reaction still gives off enough heat to the environment to make it an energetically favorable process (the G° for the overall reaction is -12.5 kJ/mole), but it also gives off enough free energy to convert a molecule of ADP into ATP and to transfer two electrons (and a proton) from the aldehyde to NAD+ to form NADH. All of this occurs while the overall reaction still gives off enough heat to the environment to make it an energetically favorable process.

While the two molecules are connected, the first enzyme, glyceraldehyde 3-phosphate dehydrogenase, links to the aldehyde through a reactive -SH group and catalyzes the oxidation of the aldehyde by NAD+. Following this, an inorganic phosphate ion will displace the reactive enzyme-substrate connection. This will result in the creation of a high-energy phosphate intermediate, which will then be released from the enzyme. ATP is produced, and the process of oxidizing an aldehyde to a carboxylic acid is finished. The second enzyme, phosphoglycerate kinase, which binds to this intermediate and catalyzes the energetically advantageous transfer of the recently produced high-energy phosphate to ADP, is responsible for this. It is important to note that at step 6, the energy released from the oxidation of the C-H bond serves as the driving force for the synthesis of NADH as well as a highly energetic phosphate bond. After that, the breaking of the high-energy bond will start the process of producing ATP.