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?
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Generate flashcards Summarize page Generate flashcards from highlight Review generated flashcards Sign up for free to start learning or create your own AI flashcards You have reached the daily AI limit Start learning or create your own AI flashcardsYou 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!
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).
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.
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.
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.
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.
To continue our journey of ATP hydrolysis, let's look at its mechanism. ATP stores and, more importantly, supplies energy in its phosphate bonds.
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!
The equation for ATP hydrolysis is as follows:
| ATP | + | H2O | ⇾ | ADP | + | PO4 3- | + | H + | + | 30.5 kJ |
| Adenosine triphosphate | Water | Adenosine diphosphate | Orthophosphate | Hydrogen | Energy |
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.
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.
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!
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.
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.
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:
