Coupling processes, which are important to all of the energy exchanges that take place in the cell, require enzymes in order to function properly. In a typical scenario, the heat generated by the collision of falling boulders with the ground would entirely burn the energy that the rocks possess. On the other hand, if one were to plan things out carefully, a portion of this energy may be used to power a paddle wheel that elevates a pail of water. Because the rocks can no longer reach the ground without first turning the paddle wheel, we believe there is a direct relationship between the energetically beneficial reaction of rock falling and the energetically unfavorable reaction of lifting the bucket of water. This is because the rocks can no longer reach the ground.

Similar chemical reactions take place inside of cells, where enzymes serve as a transitional medium between an event that is favorable to the cell's energy level, such as the oxidation of food, and an event that is unfavorable to the cell's energy level, such as the production of an active carrier molecule. Similar chemical reactions also take place outside of cells. The quantity of heat that is generated by the oxidation reaction is precisely equivalent to the quantity of energy that is stored in the energy-dense covalent bonds of the activated carrier molecule in this particular instance. In addition, an energy packet that is big enough to power a chemical reaction somewhere else in the cell is grabbed by the active carrier molecule and transported to that location.

Within cells, ATP is the most important and versatile active carrier that there is (adenosine triphosphate). In order to generate ATP, the phosphate group of ADP must first undergo an unfavorable phase of the phosphorylation process (adenosine diphosphate).

In situations where it is necessary, ATP will release its energy by hydrolyzing ADP and inorganic phosphate in a way that is advantageous from an energetic standpoint.

The synthesis of additional molecules, which would normally result in unfavorable reactions, is coupled to the energetically favorable reaction of ATP hydrolysis. This results in the synthesis of the molecules. As part of these linked events, the phosphate group that is attached to the terminal of ATP is commonly moved to another molecule.

Because it is the active carrier that is found in the greatest quantity in cells, ATP serves as the primary medium of exchange for energy. ATP is the source of energy for many of the pumps that transport substances into and out of the cell, as well as the molecular motors that enable muscle cells to contract and nerve cells to carry materials from one end of their long axons to the other. To name just a few examples, ATP provides energy for many of the pumps that transport substances into and out of the cell.

Other significant active carrier molecules typically take part in coupled processes within cells as well as oxidation-reduction reactions. These processes are both examples of linked processes. These activated carriers were developed with the express purpose of transporting hydrogen atoms in addition to electrons possessing high energies (sometimes referred to as "high-energy" electrons). The molecules nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide (NADP+), which are tightly related to one another, are the most important of these electron carriers (nicotinamide adenine dinucleotide phosphate). The molecules NADH (reduced nicotinamide adenine dinucleotide) and NADPH (reduced nicotinamide adenine dinucleotide phosphate) each take in a "packet of energy" consisting of two electrons and a proton (H+) in order to convert into their respective reduced forms of nicotinamide adenine dinucleotide.

NADPH is an active carrier that, in a manner analogous to that of ATP, takes part in a multitude of significant biosynthetic activities that, in the absence of NADPH, would not be to the metabolic advantage. During a particular chain of catabolic reactions that result in the production of energy, two hydrogen atoms are removed from the substrate molecule. In order to produce NADPH, the nicotinamide ring of NADP+ must first accept both electrons, but only one proton (in the form of a hydride ion, H-). The other proton, H+, must then be discharged into solution. In this time-honored oxidation-reduction process, the substrate undergoes oxidation while NADP+ undergoes reduction.

During a later stage of an oxidation-reduction reaction, NADPH willingly gives up the hydride ion that it is transporting since it is possible that the nicotinamide ring will develop a more stable electron configuration if it is not there. In the subsequent reaction, which regenerates NADP+, NADPH is oxidized while the substrate is reduced. This results in NADP+ being regenerated. NADPH is able to donate its hydride ion to other molecules in an effective manner for the same reason that ATP can easily transfer phosphates to other molecules. In either case, the transfer is accompanied by a large reduction in the amount of available free energy.

During a later stage of an oxidation-reduction reaction, NADPH willingly gives up the hydride ion that it is transporting since it is possible that the nicotinamide ring will develop a more stable electron configuration if it is not there. In the subsequent reaction, which regenerates NADP+, NADPH is oxidized while the substrate is reduced. This results in NADP+ being regenerated. NADPH is able to donate its hydride ion to other molecules in an effective manner for the same reason that ATP can easily transfer phosphates to other molecules. In either case, the transfer is accompanied by a large reduction in the amount of available free energy.

In contrast to NADH, the electron-transfer properties of NADPH are unaffected by the presence of an extra phosphate group on NADPH because this group is located a significant distance away from the region where electrons are being transferred. Since it does offer a molecule of NADPH with a slightly different shape than that of a molecule of NADH, it is possible for NADPH and NADH to bind as substrates to wholly separate sets of enzymes. Therefore, electrons (or hydride ions) are moved from one group of molecules to another utilizing the two distinct types of carriers. This occurs between two different groups of molecules.

NADPH serves its primary purpose in cooperation with enzymes that are responsible for catalyzing anabolic reactions. It does this by supplying the high-energy electrons that are necessary for the synthesis of biological molecules that are high in energy. In contrast, as we shall talk about in just a moment, NADH plays a one-of-a-kind role in the catabolic system of reactions that make ATP through the oxidation of food molecules. These reactions are referred to as intermediates.

Both the production of NADH from NAD+ and NADPH from NADP+ follow separate biochemical routes and are independently regulated in the cell. This is necessary for the cell to be able to control the availability of electrons for these two distinct functions. Inside the cell, the ratio of NADP+ to NADPH is maintained at a low level, while the ratio of NAD+ to NADH is maintained at a high level.

In addition, other activated carriers are able to pick up and carry a chemical group while maintaining a high-energy link that is easy to move around. For example, coenzyme A, which is often referred to as acetyl CoA, possesses an acetyl group that may be transferred across molecules thanks to its thioester link (acetyl coenzyme A). Acetyl CoA is used in the biosynthesis of larger molecules to include two more carbon atoms into the resulting product.

As is the case with other carrier molecules, the part of the molecule that is occupied by the transferable group in acetyl CoA is rather minor. The remainder is composed of a sizeable organic segment that plays the role of a practical "handle," making it simpler for specific enzymes to identify the carrier molecule. This unusual characteristic, that the handle component commonly contains a nucleotide (typically adenosine), just as acetyl CoA, may be a leftover from an earlier stage in the process of evolution.

Therefore, ATP is responsible for the transfer of phosphate, NADPH is responsible for the transfer of electrons and hydrogen, and acetyl CoA is responsible for the transfer of two-carbon acetyl groups. In the same way that NADH is utilized, reduced flavin adenine dinucleotide, also known as FADH2, is put to use in electron and proton exchanges. For the purposes of biosynthesis, several other active carrier molecules go through procedures in which a methyl, carboxyl, or glucose group is moved to a different location. Therefore, the catabolic processes that result in the production of ATP supply the energy that is necessary for the utilization of their groups throughout the process of biosynthesis.

Macromolecules account for the vast majority of the dry mass that makes up a cell. In a condensation reaction, the two reactants lose the components that make up a water molecule (OH and H), and the resulting molecules are constructed from smaller subunits (also known as monomers) that are bonded together. As a consequence of this, the addition of water, along with the assistance of enzymes, causes the reaction to go in the opposite direction, which ultimately leads to the decomposition of all three kinds of polymers. In contrast to this hydrolysis reaction, which provides an energetic benefit, the biosynthetic reactions require an external supply of energy.