Definition, Reaction & Equation I StudySmarter (2023)

Have you ever had too much sugar and suddenly felt like climbing a wall? Most people equate sugar with more energy. What is really going on inside our bodies that provide us that extra pep after we eat? How can solid food become broken down and turned into stimulation, motivation, and inspiration?

You are likely aware of glucose as being an important nutritional component of your food. On the same sub-microscopic scale, another molecule is equally indispensable to energy production: ATP, or adenosine triphosphate. When ATP breaks down through hydrolysis, it produces energy!

Now, grab a snack to supply energy to your brain cells, and let’s explore ATP hydrolysis!

  • First, we will look at the structure of an ATP molecule.
  • Then, we will learn the definition and mechanism of ATP hydrolysis.
  • After, we will look at the reaction involved in ATP hydrolysis.
  • Lastly, we will explore free energy from ATP hydrolysis and also talk about ATP hydrolase.

ATP molecule

Let's begin our journey by defining ATP.

Adenosine triphosphate, or ATP, is a molecule whose central role is energy delivery.

The structure of ATP consists of one adenosine and three phosphates (figure 1).

  • Adenosine is a nucleoside, which are molecules containing an organic ring with nitrogen, and sugar.

  • Phosphate is a functional group composed of a phosphate atom surrounded by four oxygen atoms.

Definition, Reaction & Equation I StudySmarter (1)Fig. 1. Molecular structure of Adenosine Triphosphate (ATP), and its functional groups, licensed by CC BY 3.0.

The main source of ATP synthesis in cells and living organisms is respiration.

  • In plants, ATP is also synthesized during photosynthesis.

  • In environments with little to no oxygen, ATP can alternatively be created by anaerobic respiration, such as fermentation by bacteria.

    (Video) Study Smarter, Not Harder | Kiki Fan | TEDxKerrisdaleLive

Does the term adenosine sound familiar? You may have encountered a similar term during your studies about RNA or DNA.

That is because ATP is a nucleotide, defined by having a nitrogen-containing base (in this case, adenine), a phosphate group and a sugar group.

If you recall, adenine is one of the four building blocks for RNA and DNA. The other three are cytosine, guanine, and uracil (for RNA) or thymine (for DNA). Yet, functionally, RNA and ATP are much different. Nucleotides have earned a reputation as building blocks for RNA and DNA, while ATP instead is a nucleotide whose function is that of an energy synthesizing molecule.

ATP Hydrolysis Definition

Just like it takes effort to hold hands, chemical bonds require a certain amount of energy to be maintained. When a bond is broken, the energy needed to hold the bond is now “freed”. In other words, the reaction is exergonic.

  • An exergonic reaction is a chemical reaction where energy is released.

  • An endergonic reaction is a chemical reaction where energy is absorbed.

Chemical reactions are interactions between molecules, and the release of energy from ATP is no exception. It needs a reaction partner: water.

Hydrolysis is a type of chemical reaction where a molecular bond is broken by water.

Now, let's look at the definition of ATP hydrolysis.

ATP Hydrolysis is a chemical reaction where a phosphate bond on ATP is broken by water, thereby releasing energy.

ATP Hydrolysis Mechanism

To continue our journey of ATP hydrolysis, let's look at its mechanism. ATP stores and, more importantly, supplies energy in its phosphate bonds.

(Video) Marty Lobdell - Study Less Study Smart

During ATP hydrolysis, dephosphorylation occurs.

Dephosphorylation describes the breaking of a phosphate bond from ATP to release energy, and the loss of a phosphate group.

Specifically, it loses an orthophosphate, which is a single, unbound phosphate group. The resulting molecule is called adenosine diphosphate, or ADP.

The prefix di- means two, as in two phosphate. The prefix tri- in ATP means three, as in three phosphate.

It should be noted that ADP can be further de-phosphorylated by hydrolysis, into a molecule called AMP, or adenosine monophosphate (mono- means one, as in one phosphate).

Interestingly, ADP hydrolysis actually releases even more energy! So, why bother with ATP then?

There doesn’t seem to be a known explanation, but one theory suggests cells have simple co-evolved with ATP, and therefore cells have the proper mechanisms (molecules, enzymes, receptors, etc.) to use ATP for energy. AMP nonetheless occasionally supplies energy in specific situations for some organisms!

ATP Hydrolysis Equation

The equation for ATP hydrolysis is as follows:

ATP+H2OADP+PO43-+H++30.5 kJ
Adenosine triphosphate WaterAdenosine diphosphate Orthophosphate Hydrogen Energy

ATP Hydrolysis Reaction

The ATP hydrolysis reaction is exergonic, which means it releases energy. This exergonic reaction releases 30.5 kJ per mole of ATP under standard conditions.

  • A standard reaction (under standard condition) presumes an equal amount of ATP and water. Of course, in a cell, there is plenty of water and much less ATP. Correcting for a non-standard reaction, the ATP hydrolysis reaction has the potential to release 45 to 75 kJ/mol.

The reversal of ATP hydrolysis is called condensation. Since ATP hydrolysis is an exergonic reaction, then the reverse is clearly an endergonic reaction. This means energy must be added to the reaction to bind the orthophosphate on ADP. During condensation, the hydroxyl group on orthophosphate unbinds and bonds with a free hydrogen proton to form water.

(Video) StudySmarter - The all-in-one study app

Free Energy from ATP Hydrolysis

Now, let's talk about free energy.

Free energy is a term used in chemistry to describe the amount of energy that is available to perform work.

At 30.5 kJ per mole, the phosphate bond is considered a high-energy bond because it releases a lot of free energy! The bond itself isn’t special, though. ATP contains phosphoanyhdride bonds, which are chemical bonds between two phosphate groups.

So, why is it labeled “high-energy”? Let's find out!

  1. The unique structure of ATP contributes to its efficacy as an energy delivery molecule. The chain of phosphate groups on ATP, all with -3 charge, act like magnets with the same polarity. They exert repulsive forces against each other, so that when a reaction occurs that releases a phosphate group, it releases it strongly and willingly!

  2. Also, ATP hydrolysis increases entropy. Recall the second law of thermodynamics, which say the natural state of a closed system favors entropy. Thus, ATP hydrolysis is spontaneous.

  3. Orthophosphate is highly stable, more so than ATP. This implies the forward movement of the chemical reaction (i.e. ATP hydrolysis, not condensation) is favored.

Orthophosphate has four oxygen bonded to its central phosphorus atom. One of those bonds is a double bond that is mobile and can jump between the oxygen atoms (Fig. 2). The moving double bond rearranges the charge distribution and makes orthophosphate less prone to form or reform phosphoanhydride bonds.

Besides energy distribution, ATP hydrolysis also yields a phosphate group. This detached phosphate group doesn't go to waste, it is recycled during ATP synthesis!

During the glycolysis step, a free phosphate group attaches to glucose to become phosphorylated glucose. The phosphate group act as a way to label the glucose molecule so that it moves forward during ATP synthesis.

ATP hydrolase (ATPase)

If ATP hydrolysis is a spontaneous reaction, you may be imagining a torrent of ATP being produced by hydrolysis. Cells are full of water, after all! However, this is not the case. ATP hydrolysis in cells often requires a catalyst, such as an enzyme.

ATP hydrolase, or ATPase, are a group of enzymes that catalyze ATP hydrolysis.

(Video) Study Smarter not Harder

The use of ATP hydrolase allows for some control on when and where ATP hydrolysis. Energy coupling is the combination of two reactions, in which the energy producing reaction powers a second reaction. ATP hydrolysis, the exergonic reaction, is frequently coupled with an endergonic reaction which performs a vital cellular function.

Without energy coupling, ATP hydrolysis would occur aimlessly! Nearly all the energy produced would be converted to thermal energy.

Thermal energy is important because it allows cells and organisms to regulate their own temperature. Yet, energy regularly needs to be directed and converted to perform a specific function. Instead of heat, the energy can be used to perform movement, to create molecules, or for storage.

Here are some examples of energy coupling that use ATP hydrolysis:

  • Muscle Contraction: In muscles, ATP binds to the contracting protein myosin. This triggers myosin to shift, which contracts the muscle.

  • Anabolism: Sometimes, a cell needs to assemble molecules. To do so, it must form bonds between molecules, which requires the energy provided by ATP hydrolysis.

  • Ion transport: The typical example is the sodium-potassium pump, a protein in the cell membrane. ATP provides energy to this protein to move sodium or potassium actively, against its concentration gradient.

ATP Hydrolysis - Key takeaways

  • Adenosine triphosphate, or ATP, is a molecule whose central role is energy delivery. The structure of ATP consists of one adenosine and three phosphates.

  • Hydrolysis is a type of chemical reaction where a molecular bond is broken by water.

  • Hydrolysis causes ATP to dephosphorylate, or lose a phosphate, which releases energy.

  • ATP Hydrolase, or ATPase, are a group of enzymes that catalyze ATP hydrolysis.

  • Energy coupling is the combination of two reactions, one exergonic and one endergonic. ATP hydrolysis couples with vital cellular functions to supply them with energy.

    (Video) 5 BEST Ways to Study Effectively | Scientifically Proven


  1. Fig 1. 230 Structure of Adenosine Triphosphate (ATP)-01 ( by OpenStax College is licensed by CC BY 3.0 (


1. Clinician's Corner: Tips on how to study smarter
(Osmosis from Elsevier)
2. how i study SMART, not HARD (high school students must watch)
(Annie Long)
3. 9 Study Techniques that got me through Cambridge Medical School *science-backed*
(Izzy Sealey)
4. Study Planning for Exams - The Ultimate Guide
(Dr Hugo Lau)
5. The 80/20 Rule in Studying - how you can study smarter and faster
(Mind Maps Unleashed)
6. 45: Electrolysis of molten ionic compounds
(Roxi Hulet)
Top Articles
Latest Posts
Article information

Author: Melvina Ondricka

Last Updated: 12/13/2022

Views: 6465

Rating: 4.8 / 5 (68 voted)

Reviews: 83% of readers found this page helpful

Author information

Name: Melvina Ondricka

Birthday: 2000-12-23

Address: Suite 382 139 Shaniqua Locks, Paulaborough, UT 90498

Phone: +636383657021

Job: Dynamic Government Specialist

Hobby: Kite flying, Watching movies, Knitting, Model building, Reading, Wood carving, Paintball

Introduction: My name is Melvina Ondricka, I am a helpful, fancy, friendly, innocent, outstanding, courageous, thoughtful person who loves writing and wants to share my knowledge and understanding with you.