For the cells of all living things to remain in a coherent biological state, there must be a high ratio of ATP to ADP. Even though plants have to go through the night without sunlight in order to keep themselves alive when they are unable to produce sugar through photosynthesis, animals only have access to food on an occasional basis. Because of this, both plants and mammals are capable of transforming carbohydrates and lipids into certain forms for the sake of storage.

In order to survive extended periods without food, animals have the ability to store fatty acids in fat droplets, which are made up of water-insoluble triacylglycerols (also called triglycerides). Triacylglycerols are most commonly seen collected in the cytoplasm of adipocytes, which are specialized fat cells in animal bodies. Sugar is temporarily stored in the body as glucose subunits in the vast branching polysaccharide glycogen. Glycogen is found as small granules in the cytoplasm of many cells, including those in the liver and the muscle. In reaction to changes in the level of demand, glycogen synthesis and breakdown are swiftly regulated. When cells require more ATP than they can produce from the food molecules in the bloodstream, they degrade glycogen in a mechanism that produces glucose 1-phosphate. This glucose 1-phosphate is then quickly transformed into glucose 6-phosphate for glycolysis. When this happens, the cells are able to meet their ATP needs.

When it comes to the quantity of energy that can be stored, fat is by far superior to glycogen in terms of importance for animals. This is likely because fat can be stored in a more effective manner. The oxidation of one gram of fat produces roughly twice as much usable energy as the oxidation of one gram of glycogen. In addition, glycogen differs from fat in its capacity to bind a significant quantity of water. As a consequence, the actual mass of glycogen that must be stored in order to achieve the same level of energy storage capacity as fat is six times greater. The average adult stores just enough glycogen for around a day's worth of routine activity, while they have enough fat for about an entire month's worth of energy. If glycogen rather than fat had to be carried as our major fuel storage, this would result in an increase of approximately 60 pounds in body weight.

Mitochondria are responsible for the production of ATP, while mitochondria are responsible for the production of sugar in plant cells. Both sugar and ATP are necessary for the proper functioning of plant cells. In spite of the fact that chloroplasts in plants generate significant quantities of both ATP and NADPH, this organelle is partitioned off from the rest of the plant cell by a membrane that is impermeable to both types of activated carrier molecules. This allows chloroplasts to maintain their ability to generate large amounts of both types of activated carrier molecules. Additionally, the plant contains a significant number of cells that do not contain chloroplasts, such as those found in the roots. These cells are unable to produce their own sugars on their own. Following this step, sugars are moved from the chloroplasts to the mitochondria that are present in every plant cell. These mitochondria produce the majority of the ATP necessary for normal plant cell metabolism by utilizing the exact same mechanisms for the oxidative breakdown of carbohydrates as are found in non-photosynthetic organisms. This ATP is then transferred to the remaining portion of the cell to continue the process of normal plant cell metabolism.

A percentage of the sugars that chloroplasts create during times of high photosynthetic capability during the day are transformed into lipids and starch, a polymer of glucose that is comparable to the glycogen that is found in animals. This process takes place during the day. Plant lipids are triacyl-glycerols (triglycerides), just like animal fats, with the primary difference being the types of fatty acids that are predominately present in plant fats. Starch and fat are both stored in the chloroplast until the point at which they are required for the oxidation process that produces energy during the night.

The embryos that are contained within plant seeds have to rely on stored energy sources for a significant amount of time in order to survive until they germinate and grow leaves that are capable of harvesting the energy from sunlight. As a consequence of this, the fat and carbohydrate content of plant seeds is often exceptionally high, making them an important source of food for mammals like humans.

Sugars that are consumed as part of an animal's diet are responsible for fulfilling the vast majority of its post-meal energy requirements. In the event that there are any surplus sugars, they are put to use in the production of lipids, which serve as a kind of food storage, or in the replenishment of glycogen stores that have become depleted. However, after a fast for the previous night, the fat that is stored in adipose tissue begins to be utilized, and by morning, fatty acid oxidation has produced the majority of the ATP that is necessary for our bodies.

The digestion of triacylglycerols, which are found in fat droplets that are held in adipocytes, results in the release of fatty acids and glycerol, both of which are then delivered to the cells of the body by the circulation. Sugars can be converted into fats quite quickly in animals, whereas fats cannot be converted into sugars. The fatty acids, on the other hand, are subjected to direct oxidation.

During the process of aerobic metabolism, the pyruvate that was produced as a byproduct of the glycolysis of cytosolic carbohydrates is transported into the mitochondria of eukaryotic cells. The pyruvate dehydrogenase complex, which is an enormous complex consisting of three enzymes, decarboxylates it very quickly there. The process of pyruvate decarboxylation results in the production of a molecule of CO2 as a waste product, as well as a molecule of NADH and acetyl CoA as byproducts. The fatty acids that are taken in from the circulation are delivered to the mitochondria, which are the sites of all of the oxidation that occurs in response to fatty acids. As the activated molecule fatty acyl CoA, a cycle of reactions that removes two carbons from the carboxyl end of each fatty acid molecule, resulting in the synthesis of one molecule of acetyl CoA for each turn of the cycle, results in the formation of acetyl CoA. During this process, molecules of FADH2 and NADH are also produced alongside the other products.

Sugars and fats are the primary sources of fuel for the vast majority of organisms that cannot produce their own food through photosynthesis, including humans. The majority of the useful energy that may be taken from the oxidation of either type of food is still contained within the molecules of acetyl CoA that are formed by the two types of reactions that were just explained. During the series of processes known as the citric acid cycle, which is an essential part of the energy metabolism of aerobic organisms, the acetyl group (-COCH3) in acetyl CoA is oxidized to produce carbon dioxide and water. In eukaryotic cells, all of these actions take place in the mitochondria. Because of this, it shouldn't come as much of a surprise that the mitochondrion is the location in animal cells where the majority of ATP is produced. On the other hand, aerobic microorganisms only use one compartment of their cells, which is called the cytosol, to carry out all of their processes, including the citric acid cycle.

In the nineteenth century, biologists made the discovery that when cells are not exposed to oxygen, they make lactic acid (for example, in muscle) or ethanol (for example, in yeast). On the other hand, when cells are exposed to oxygen, they consume oxygen and produce carbon dioxide and water. Pyruvate oxidation became the focus of efforts to describe the pathways of aerobic metabolism, which ultimately led to the discovery of the citric acid cycle in 1937. This cycle is also known as the tricarboxylic acid cycle or the Krebs cycle. Other names for this cycle include the citric acid cycle and the Krebs cycle. The citric acid cycle generates carbon dioxide and high-energy electrons in the form of NADH as its principal end products. This cycle is responsible for approximately two-thirds of the overall oxidation of carbon compounds in most cells. During this process, CO2 is released as a byproduct, and high-energy electrons from NADH are moved to a membrane-bound electron transport chain. It is along this chain that the electrons will eventually mix with oxygen to make water. The citric acid cycle does not require oxygen in the gaseous state to function (it uses oxygen atoms from H2O). However, in order for the cycle to continue, future reactions must involve oxygen. This is because there is no other efficient path for NADH to get rid of its electrons and, as a result, replenish the requisite amount of NAD+. This is the reason why this is the case.

The citric acid cycle is a metabolic process that only occurs in the mitochondria of eukaryotic cells. It is responsible for the complete oxidation of the carbon atoms that are found in the acetyl groups of acetyl CoA, which results in the formation of carbon dioxide. However, there is no acetyl group that is directly oxidized in this process.

Instead, this group is transferred from the smaller molecule of acetyl CoA to the larger molecule of four-carbon oxaloacetate, where it is then transferred. This results in the formation of citric acid, a six-carbon tricarboxylic acid that gives its name to the subsequent cycle of events. After that, the energy released from this oxidation is put to use in the process of gradually oxidizing the citric acid molecule in order to produce energy-rich activated carrier molecules.