What is the type of chemical reaction used to rebuild ADP into ATP?

A. Hydrolysis.

B. Rehydration synthesis.

C. Hydration synthesis.

D. Dehydration synthesis.

Diagram showing the structure of ATP
Diagram showing the structure of ATP

The correct answer is D. Dehydration synthesis.

The energy molecule of the cell is adenosine triphosphate (ATP), which is used to power many different reactions such as cell transport mechanisms and metabolic processes.

This molecule is similar to RNA and DNA molecules in that it is a nucleic acid consisting of a sugar, phosphates and a nitrogen base. ATP is constantly being used in the cell and therefore has to be constantly regenerated from ADP.

The process by which a large molecule, a polymer is formed from two or more monomers is known as dehydration synthesis. In many molecules, the process involves the removal of water and action of enzymes.

To break down a polymer to smaller molecules, monomers involves the breaking of bonds, and often, the addition of water.

ATP is formed in the cell during cellular respiration reactions. The most ATP is generated during the electron transport chain of aerobic respiration, which occurs when oxygen is present in the cell.

ATP synthesis occurs by the movement of protons through special integral proteins that are lined with the enzyme ATP synthase. The ATP is made when inorganic phosphate combines with ADP. This is a dehydration synthesis reaction that occurs and in aerobic respiration, large quantities of the molecule are formed.

When needed, the ATP molecule is broken down and energy associated with a phosphate group is released. This reaction is known as hydrolysis and results in the formation of ADP (which has 2 phosphates) and an inorganic phosphate molecule.

ATP: energy molecule of the cell

ATP is adenosine triphosphate, the main energy molecule that is found in all living cells. The structure of this molecule is similar to that of RNA and DNA.

It is a nucleic acid which consists of a nucleotide made of a nitrogen base known as adenine that is bonded to a ribose sugar and three phosphate groups.

The energy of the molecule is found in the phosphate bonds, thus the more phosphate groups that are attached, the more energy the molecule is carrying. Hence ATP has more energy stored than does ADP (adenosine diphosphate) or AMP (adenosine monophosphate).

ADP has only two phosphate groups and AMP has only one phosphate group present. ATP can be broken down to produce ADP, which can then be broken down further to form AMP.

The cell needs to constantly regenerate ATP in order to ensure that there is adequate energy to power several cellular reactions and transport processes.

Metabolic reactions such as photosynthesis and respiration both depend on ATP as a source of energy. Respiration is the cellular process that produces large quantities of ATP from the catabolism of glucose.

Furthermore, active transport processes also use ATP for energy to move substances across cell membranes against their natural gradients.

Monomers and polymers

The monomer is the simplest unit of a biological macromolecule and the smallest entity that a large molecule can be broken down to.

Monomers are units that bond together to form polymers, which can be large and complex molecules. The processes involved in breaking down or building molecules usually involves water and often enzymes.

A large molecule can be broken down into smaller monomers by the addition of one or more molecules of water. This process is therefore called hydrolysis, in that the molecule is usually being split with water. This is a catabolic process since molecules are being broken down.

To form a larger molecule, a polymer from two or more monomers, water is normally removed. Hence, the name of this process is dehydration synthesis. This process is the reverse of hydrolysis and is an anabolic process in which larger molecules are actually being synthesized from several small subunits.

There are many catabolic and anabolic reactions that occur in living cells, which are necessary for the survival of the organism.

Dehydration synthesis: an anabolic process

The cell needs a constant supply of energy in the form of ATP, which is why adenosine triphosphate is continually being formed in the cell.

The process involves the action of the enzyme ATP synthase that is present in special membrane proteins. ATP synthesis occurs in all living cells through the reactions of cellular respiration.

ATP is formed from a reaction in which an inorganic phosphate is joined to a molecule of ADP. This dehydration synthesis occurs in all cells, but the greatest number of ATP is actually produced in the presence of oxygen.

ATP formation

A limited amount of ATP is formed during glycolysis of cellular respiration. This process occurs in both anaerobic respiration (when no oxygen is present) and in aerobic respiration (when oxygen is present).

It is the last reactions of aerobic cell respiration that produce the most ATP. These reactions occur in the mitochondrion across the inner membrane, the cristae.

The electron transport chain of aerobic respiration results in the formation of 32 ATP for every one glucose molecule that is broken down. Since the process involves oxygen, it is called oxidative phosphorylation and occurs in cells of eukaryotic organisms.

Oxygen is a strongly electronegative molecule which acts as a strong force to move electrons along a chain of transport molecules. Electrons enter the transport chain from carrier cofactors such as NADH and FADH, which were formed in earlier reactions.

The movement of electrons from molecule to molecule in the chain provides energy to establish a proton gradient. This gradient is established by the pumping of protons across the cristae of the mitochondrion.

Since the protons are pumped against their natural concentration gradient, this means that they have the tendency to diffuse back across the membrane. However, they can only pass back through the membrane passively through special membrane proteins.

These proteins contain the enzyme ATP synthase which is needed for the reaction in which ATP formation occurs. As the protons move through this protein channel it then catalyzes the dehydration synthesis reaction in which ATP is formed from ADP and an inorganic phosphate.

Hydrolysis: a catabolic process

Hydrolysis is the reverse process of dehydration synthesis and it is a catabolic process since a molecule is being broken down.

When the ATP molecule is broken down, energy is released by the breaking of a phosphate bond. The result of this reaction is that an inorganic phosphate ion is liberated and ADP is formed which has only two phosphate groups present.

The ADP can be broken down further and energy released. The result of this process is that AMP is formed in which only one phosphate group remains.


  1. Editors of Encyclopedia Britannica (2019). Cellular respiration. Retrieved from Encyclopedia Britannica.
  2. K Rogers (2019). Mitochondrion. Retrieved from Encyclopedia Britannica.
  3. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing
  4. Editors of Encyclopedia Britannica (2019). ATP. Retrieved from Encyclopedia Britannica.
  5. RJ Roberst (2019). Nucleic acid. Retrieved from Encyclopedia Britannica.


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