What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?

a) Entropic.

b) Hydrolysis.

c) Dehydration decomposition.

d) Anabolism.

e) Dehydration synthesis.

Diagram showing the hydrolysis of ATP
Diagram showing the hydrolysis of ATP

The correct answer is b) hydrolysis.

The ATP molecule is a nucleic acid which has a similar structure to those of other nucleic acid molecules. It consists of a sugar (ribose), a nitrogen base (adenine) and three phosphate groups.

It is the main source of energy for cells that is used in several types of reactions, including cell signaling processes and in the active transport of substances across the plasma membrane.

Energy is stored in the phosphate bonds of the ATP molecule. To release energy the bonds have to be broken. This is achieved by the addition of a water molecule in the process known as hydrolysis.

ATP can be broken down in this way to produce ADP and an inorganic phosphate (Pi). The ADP can be broken down further to produce AMP and Pi.

The reverse process to hydrolysis is dehydration synthesis in which water is removed. However, the enzyme ATP synthase is also needed to form ATP.

ATP is formed during cellular respiration, particularly aerobic respiration in which oxygen is present. ATP synthesis occurs during the electron transport chain during which protons move through protein channels lined with the synthase enzyme.

This catalyzes the reaction in which ATP is formed from ADP and Pi, and water is removed in this process.

Forming and breaking bonds

The simplest unit of all biological molecules is known as a monomer. Several of these monomers link together by bonds to form a larger molecule called a polymer. Making and breaking bonds involves water molecules.

If a molecule of water is removed from two or more monomers, then they are able to link together and bond. This process is called dehydration synthesis since a polymer is being formed by the removal of water.

The reverse of dehydration synthesis is hydrolysis in which water is used to split a polymer into two or more monomers.

Dehydration synthesis is an anabolic process in which larger molecules are being formed, while hydrolysis is a catabolic process in which large molecules are being broken down into smaller subunits or monomers.

These two processes occur in many of the chemical reactions of the cell, such as photosynthesis, cellular respiration and in the digestion and manufacturing of substances.


Adenosine triphosphate (ATP) is one of the types of nucleic acid that occur in living cells. It has a very similar structure to that found in RNA and DNA. In fact, ATP has the same type of sugar present in the molecular structure as RNA.

The nucleotide of ATP is comprised of a ribose sugar, a nitrogenous base called adenine and three phosphate groups. These phosphates bond to the ribose molecule and the sugar bonds in turn to the adenine.

ATP is viewed as the main energy currency of the living cell since it is frequently used to power cellular reactions such as respiration and photosynthesis.

It is also often used to provide the energy needed for active transport of substances into and out of cells. ATP is also used in cell signaling processes such as signal transduction methods.


The source of energy in ATP is actually the phosphate bonds. This is where the energy is being stored. Therefore whenever energy is needed the phosphate bond is broken in a reaction that involves the addition of a water molecule; this is the process of hydrolysis.

When such a bond is broken by hydrolysis, and energy is released, the reaction is called oxidation. The result of this reaction is that ATP is converted to ADP, adenosine diphosphate, which has two phosphate groups and one inorganic phosphate group.

The adenosine diphosphate can be broken down further if needed to form adenosine monophosphate (AMP), which has one phosphate group, and an inorganic phosphate. Once again this is a hydrolysis reaction in which a molecule of water is added in order to separate the ADP into AMP and Pi.

Dehydration synthesis

To form an ATP molecule requires a special enzyme known as ATP synthase and it involves the reverse process to hydrolysis.

Forming a larger molecule from two or more smaller molecules entails removing a molecule of water in a process called dehydration synthesis. This process involves a reduction reaction which is the opposite of oxidation.

The most ATP is synthesized in the process of cellular respiration which occurs in many living cells. There are both aerobic and anaerobic methods of cell respiration that occur, but of the two methods, the most ATP is generated by the aerobic method of respiration.

ATP synthesis in respiration

Aerobic cellular respiration uses oxygen to make ATP from the breakdown of a sugar molecule. A series of complex reactions occur in respiration during which many intermediates are formed and several enzymes are involved.

The most ATP is formed in aerobic respiration during the last stage of the reactions, known as the electron transport chain and oxidative phosphorylation.

In aerobic respiration, there is oxygen present at the end of a chain of molecules. This oxygen molecule is very electronegative which acts as a strong force to drive electrons down along a chain of molecules.

The electrons are carried into the chain by coenzymes such as NADH formed during earlier stages of respiration. These electrons provide the energy to pump protons against their concentration gradient across the inner membrane (crista) of the mitochondrion.

Protons then diffuse back across the membrane through special integral proteins that are embedded in the membrane. These protein channels are lined with the enzyme ATP synthase.

When the protons move through this protein channel it activates the enzyme which then catalyzes the reaction in which an ADP is combined with Pi to form ATP.

Dehydration synthesis occurs in which water is removed and the two molecules are combined. Large quantities of ATP are generated in the mitochondrion in this process.


  1. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing
  2. Editors of Encyclopedia Britannica (2018). ATP. Retrieved from Encyclopedia Britannica.
  3. RJ Roberst (2018). Nucleic acid. Retrieved from Encyclopedia Britannica.
  4. Editors of Encyclopedia Britannica (2018). Cellular respiration. Retrieved from Encyclopedia Britannica.
  5. K Rogers (2018). Mitochondrion. Retrieved from Encyclopedia Britannica.


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