Lesson Explainer: Energy and ATP Biology • Second Year of Secondary School

In this explainer, we will learn how to describe the structure of ATP, how it is synthesized and hydrolyzed, and the properties of ATP that make it an essential component of cellular processes.

All living things require a continual supply of energy in order to function. Adenosine triphosphate, better known by its initials, ATP, is the primary molecule responsible for short-term storage and energy transfer in cells. No matter what goes into an organism as a fuel source, whether it is carbohydrates, fats, or proteins, it is ultimately used to generate ATP in order to supply all of the immediate power needs of the living cell. Our bodies make and break down our body weight in ATP, every day. So, if you weigh about 50 kilograms, in one day, you would use about 50 kg of ATP.

Although ATP is a small, relatively simple molecule, within its bonds, there is enough energy to perform all types of cellular work. This is why ATP is called the primary energy currency of cells, because in much the same way as money is the currency that people exchange for the things they need, ATP is used to store the energy for reactions in the cell. Even though ATP can be used to store energy in the cell, because it is constantly being broken down and reformed, it is more of an immediate energy source rather than a long-term one.

ATP is a nucleotide, which may seem surprising. The word nucleotide is a term that we hear most often when discussing genetics and molecules like DNA. However, this is a bit limiting, as nucleotides are more than just DNA; they are the building blocks of nucleic acids. Nucleotides are small, basic, molecular units that can be joined together to form larger, more complex molecules. So, ATP, DNA, and RNA are all nucleotides.

All nucleotides have a specific structure composed of three subunit molecules: a nitrogen-containing base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. Given the full name of ATP, adenosine triphosphate, and the typical structure of a nucleotide, we can deduce a few things about the structure of ATP. You can see the structure of a typical nucleotide molecule in Figure 1.

The word adenosine tells us that the nitrogenous base found in ATP is adenine, and the word triphosphate indicates the number of phosphate groups; here, the prefix tri- means “three.” So, in Figure 2, in the diagrammatic structure of ATP, we see a nitrogenous adenine base, a five-carbon ribose sugar, and three phosphate groups.

The three phosphate groups are linked to one another by high-energy bonds that can be easily broken. It is within these bonds between the three phosphate groups that the actual power source of ATP is stored. When energy is needed immediately, the covalent bond between the second and third phosphate groups of ATP, shown in Figure 3, is broken.

Example 2: Identifying the Energy Source in the Chemical Bonds of ATP

The main function of ATP is to act as a source of energy for cellular processes. The diagram shows a simple outline of the structure of an ATP molecule. Which bond breaks to release the energy stored in it?

Answer

All nucleotides have a specific structure composed of three subunit molecules: a nitrogen-containing base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. ATP is a nucleotide made of an adenine nitrogenous base combined with a ribose sugar and three phosphate groups. Given the full name of ATP, adenosine triphosphate, and the typical structure of a nucleotide, we can deduce a few things about the structure of ATP.

The bonds that link the phosphate groups are high-energy bonds that when broken release enough energy to power up different reactions. So, when energy is needed immediately in the cell, the bond between the second and third phosphate groups (seen as number 1 in the diagram of ATP below) is broken to release the energy needed. This means that it is the chemical bond between the second and third phosphate groups of ATP that is the actual power source in ATP. When energy is needed immediately, the covalent bond is broken between the phosphate group in the middle and the one located farthest from the ribose in ATP. The broken bond is indicated by the number 1 in the figure.

Therefore, for ATP to act as a source of energy for cellular processes, the bond that breaks down to release the energy stored is indicated by the number 1 in the figure.

Let’s take a closer look at the reaction that breaks down bonds between phosphate groups.

When energy is needed immediately in the cell, the bond between the second and third phosphate groups is broken to convert ATP into adenosine diphosphate (ADP) and an inorganic phosphate group. This breakdown of the bond between the second and third phosphate groups in ATP is called hydrolysis, because it consumes a water molecule, as seen in Figure 4.

The hydrolysis of ATP produces ADP and an inorganic phosphate group and releases free energy. The word hydrolysis contains the prefix hydro-, meaning “water,” and the term lysis, meaning “separation.” During hydrolysis, water is split, resulting in the release of a hydrogen atom () and a hydroxyl group ().

The structure of ADP is the same as that of ATP, except that ADP has one less phosphate group attached at the end. The word diphosphate, which contains the prefix di-, indicates the two phosphate groups in ADP. Removal of a phosphate group from ATP is catalyzed by the enzyme ATP hydrolase.

During hydrolysis, only the outer phosphate group is usually removed from ATP to release the energy needed in a reaction. The phosphate group released from ATP is called an inorganic phosphate, which is another name for a free phosphate in the cell and is symbolized as . Such conversion from ATP to ADP provides about 7‎ ‎300 calories per mole of ATP, which is about the same amount of energy that is found in a single peanut.

When ATP is hydrolyzed into ADP and , unless the energy released is used quickly, it is lost as heat (thermal energy). To avoid this loss, ATP hydrolysis is coupled to other energy-requiring reactions in the cell. This way, the energy released by ATP hydrolysis can be used to power up other reactions in the cell, rather than being lost as heat.

The inorganic phosphate that is released from ATP hydrolysis is not just left floating in the cell but is instead put to good use. This inorganic phosphate can be added to other molecules in the cell in a process called phosphorylation. Adding another phosphate to other molecules can make the compound more reactive.

Although ATP is continuously broken down to use the energy released in hydrolysis and the inorganic phosphate group, it is constantly being replenished. Such regeneration of ATP is important because cells tend to use ATP very quickly and rely on this constantly replenished ATP supply to power up the cell.

ATP is easily resynthesized in a condensation reaction that adds an inorganic phosphate group to ADP. Generally, a condensation reaction (which is also called dehydration synthesis) is a reaction that joins two molecules in a chemical bond and results in the formation of a water molecule, as seen in Figure 4. So, the water that was lost during ATP hydrolysis is reformed when a third phosphate group is added to the ADP molecule. The addition of a third phosphate group to ADP is catalyzed by the enzyme ATP synthase. If the addition of a phosphate group to another molecule sounds a lot like the process of phosphorylation, which has been discussed earlier, it is because it is. Since ATP is produced by the addition of a phosphate group to ADP, ATP can be thought of as a phosphorylated nucleotide.

In plants, ATP is synthesized in cells with chlorophyll during photosynthesis through photophosphorylation. In both plant and animal cells, ATP is also regenerated during respiration. While ATP can help power up reactions, it is not a storage molecule for chemical energy. Although six-carbon sugars like glucose are considered excellent long-term storage sites of energy for the cell, they take a long time (and a lot of energy) to break down. So, instead, to provide the cells with quick access to energy, cells can convert glucose into ATP during cellular respiration in order to have more immediate access to stored energy.

Example 3: The Difference between ATP Breakdown and Synthesis

The diagram provided gives a basic outline of the relationship between ATP and ADP.

1. What type of reaction is reaction X?
   1. Hydrolysis
   2. Condensation
   3. Reduction
   4. Oxidation
   5. Polymerization
2. What type of reaction is reaction Y?
   1. Condensation
   2. Hydrolysis
   3. Reduction
   4. Oxidation
   5. Monomerization

Answer

In the diagram shown in the question, the relationship between ATP synthesis and breakdown is shown. The first arrow (pointing down) shows the reaction that converts ATP into ADP, which is hydrolysis. The second arrow (pointing up) shows the reaction that resynthesizes ADP to ATP, which is condensation.

Part 1

When energy is needed immediately in the cell, ATP can be converted into ADP and an inorganic phosphate group. Breaking down the bond between the second and third phosphate groups converts ATP into adenosine diphosphate (ADP) and an inorganic phosphate group. Such breakdown of the bond between the second and third phosphate groups in ATP is called hydrolysis, because it consumes a water molecule, as seen in the figure above. The word hydrolysis contains the prefix hydro-, meaning “water,” and the term lysis, meaning “separation.”

The hydrolysis of ATP also releases free energy. Inorganic phosphate is another name for a free phosphate group in the cell, and it is symbolized as . The removal of the last phosphate group is catalyzed by the enzyme ATP hydrolase.

Reaction X is a hydrolysis reaction.

Part 2

ATP is easily resynthesized in a condensation reaction that adds an inorganic phosphate group to ADP. A condensation reaction is a reaction that joins molecules in a chemical bond and results in the formation of a water molecule, as seen in the figure above. The addition of a third phosphate group is catalyzed by the enzyme ATP synthase.

Reaction Y is a condensation reaction.

The properties of ATP highlight why this molecule is so important for living organisms. ATP is a small soluble molecule that can be easily transported around the cell. Even though ATP is small, its hydrolysis releases just enough energy to power up reactions in the cell without too much wasted energy. This same reaction also helps release an inorganic phosphate that can make other molecules more reactive through phosphorylation. Finally, given the importance of ATP, the fact that it can be quickly remade is very useful.

It is these properties of ATP that make it an excellent resource for powering up reactions. Hence, ATP serves as a shuttle, delivering energy to places within the cell where energy-consuming activities are taking place. There are three general types of tasks in cells where ATP is required:

1. To drive metabolic reactions that cannot occur automatically through phosphorylation and activation of a molecule by the inorganic phosphate
2. To transport needed substances across membranes, where ATP helps move molecules and ions against the concentration gradient
3. To do mechanical work, where ATP provides the energy for actions like muscle contraction.

Let’s summarize what we have learned in this explainer.