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ATP

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Biology

ATP or adenosine triphosphate is the energy-carrying molecule essential for all living organisms. It is used to transfer the chemical energy necessary for cellular processes.

You already know that energy is one of the most important requirements for the normal functioning of all living cells. Without it, there is no life, as essential chemical processes inside and outside cells couldn’t be performed. That is why humans and plants use energy, storing the excess.

To be used, this energy needs to be transferred first. ATP is responsible for the transfer. That is why it is often called the energy currency of cells in living organisms.

What does it mean when we say “energy currency”? It means that ATP carries energy from one cell to another. It is sometimes compared to money. Money is referred to as currency most accurately when used as a medium of exchange. The same can be said of ATP - it is used as a medium of exchange as well, but the exchange of energy. It is used for various reactions and can be reused.

The structure of ATP

ATP is a phosphorylated nucleotide. Nucleotides are organic molecules consisting of a nucleoside (a subunit composed of a nitrogenous base and sugar) and a phosphate. When we say that a nucleotide is phosphorylated, it means that phosphate is added to its structure. Therefore, ATP consists of three parts:

  • Adenine - an organic compound containing nitrogen = nitrogenous base
  • Ribose - a pentose sugar to which other groups are attached
  • Phosphates - a chain of three phosphate groups.

ATP is an organic compound like carbohydrates and nucleic acids, for instance. Note the ring structure of ribose, which contains carbon atoms, and the two other groups that contain hydrogen (H), oxygen (O), nitrogen (N) and phosphorus (P).

ATP is a nucleotide, and it contains ribose, a pentose sugar to which other groups attach. Does this sound familiar? It might do if you have already studied the nucleic acids DNA and RNA. Their monomers are nucleotides with a pentose sugar (either ribose or deoxyribose) as a base. ATP is therefore similar to the nucleotides in DNA and RNA.

How does ATP store energy?

The energy in ATP is stored in the high-energy bonds between the phosphate groups. Usually, the bond between the 2nd and the 3rd phosphate group (counted from the ribose base) is broken to release energy during hydrolysis.

Don’t confuse the storing of energy in ATP with storing energy in carbohydrates and lipids. Rather than actually storing energy long-term like starch or glycogen, ATP catches the energy, stores it in the high-energy bonds, and quickly releases it where needed. Actual storage molecules such as starch cannot simply release energy; they need ATP to carry the energy further.

The hydrolysis of ATP

The energy stored in the high-energy bonds between the phosphate molecules is released during hydrolysis. It is usually the 3rd or the last phosphate molecule (counting from the ribose base) that is detached from the rest of the compound.

The reaction goes as follows:

  1. The bonds between the phosphate molecules break with the addition of water. These bonds are unstable and therefore easily broken.
  2. The reaction is catalysed by the enzyme ATP hydrolase (ATPase).
  3. The reaction results are adenosine diphosphate (ADP), one inorganic phosphate group (Pi) and the release of energy.

ATP Hydrolysis of ATP results in the formation of ADP, Pi and the release of energy StudySmarterFigure 3. Hydrolysis of ATP results in the formation of ADP, Pi, and the release of energy. Source: commons.wikimedia.org

The other two phosphate groups can be detached as well. If another (second) phosphate group is removed, the result is the formation of AMP or adenosine monophosphate. This way, more energy is released. If the third (final) phosphate group is removed, the result is the molecule adenosine. This, too, releases energy.

The synthesis of ATP

The hydrolysis of ATP is reversible, meaning that the phosphate group can be reattached to form the complete ATP molecule. This is called the synthesis of ATP.

Therefore, we can conclude that the synthesis of ATP is the addition of a phosphate molecule to ADP to form ATP.

ATP is produced during cellular respiration and photosynthesis when protons (H+ ions) move down across the cell membrane (down an electrochemical gradient) through a channel protein ATP synthase. ATP synthase also serves as the enzyme that catalyses ATP synthesis. It is embedded in the thylakoid membrane of chloroplasts and the inner membrane of mitochondria, where ATP is synthesised.

Water is removed during this reaction as the bonds between phosphate molecules are created. That is why you may come across the term condensation reaction used since it is interchangeable with the term synthesis.

ATP simplified representation of ATP synthase, which serves as a channel protein for H+ ions and an enzyme that catalyses the ATP synthesis StudySmarterFigure 4. Simplified representation of ATP synthase, which serves as a channel protein for H+ ions and an enzyme that catalyses the ATP synthesis. commons.wikimedia.org

Bear in mind that ATP synthesis and ATP synthase are two different things and therefore should not be used interchangeably. The first is the reaction, and the latter is the enzyme.

ATP synthesis happens during three processes: oxidative phosphorylation, substrate-level phosphorylation and photosynthesis.

ATP in oxidative phosphorylation

The largest amount of ATP is produced during oxidative phosphorylation. This is a process in which ATP is formed using the energy released after cells oxidise nutrients with the help of enzymes. Oxidative phosphorylation takes place in the membrane of mitochondria. It is one of four stages in cellular aerobic respiration.

ATP in substrate-level phosphorylation

Substrate-level phosphorylation is the process by which phosphate molecules are transferred to form ATP. It takes place in the cytoplasm of cells during glycolysis, the process that extracts energy from glucose, and in mitochondria during the Krebs cycle, the cycle in which the energy released after oxidation of acetic acid is used.

ATP in photosynthesis

ATP is also produced during photosynthesis in plant cells that contain chlorophyll. This synthesis happens in the organelle called chloroplast, where ATP is produced during the transport of electrons from chlorophyll to thylakoid membranes. This process is called photophosphorylation, and it takes place during the light-dependent reaction of photosynthesis. You can read more about this in the article on photosynthesis and the light-dependent reaction.

The function of ATP

As already mentioned, ATP transfers energy from one cell to another. It is an immediate source of energy that cells can access fast.

If we compare ATP to other energy sources, for instance, glucose, we see that ATP stores a smaller quantity of energy. Glucose is an energy giant in comparison to ATP. It can release a large amount of energy. However, this isn’t as easily manageable as the release of energy from ATP. Cells need their energy quick to keep their engines constantly roaring, and ATP supplies energy to needy cells faster and easier than glucose can. Therefore, ATP functions much more efficiently as an immediate energy source than other storage molecules such as glucose.

ATP is also used in various energy-fueled processes in cells:

  • Metabolic processes, such as the synthesis of macromolecules, for instance, proteins and starch, rely on ATP. It releases energy used to join the bases of the macromolecules, namely amino acids for proteins and glucose for starch.
  • ATP provides energy for muscle contraction or, more precisely, the sliding filament mechanism of muscle contraction. Myosin is a protein that converts chemical energy stored in ATP to mechanical energy to generate force and movement. Read more about this in our article on the sliding filament mechanism.
  • ATP functions as an energy source for active transport as well. It is crucial in the transport of macromolecules across a concentration gradient. It is used in significant amounts by the epithelial cells in the intestines. They cannot absorb substances from the intestines by active transport without ATP.
  • ATP provides energy for synthesising nucleic acids DNA and RNA, more precisely during translation. ATP provides energy for amino acids on the tRNA to join together by peptide bonds and attach amino acids to tRNA.
  • ATP is required to form the lysosomes that have a role in the secretion of cell products.
  • ATP is used in synaptic signalling. It recombines choline and ethanoic acid into acetylcholine, a neurotransmitter. Explore the article on the transmission across a synapse for more information on this complex yet interesting topic.
  • ATP helps enzyme-catalysed reactions take place more quickly. As we have explored above, the inorganic phosphate (Pi) is released during the hydrolysis of ATP. Pi can attach to other compounds to make them more reactive and lower the activation energy in enzyme-catalysed reactions.

ATP - Key takeaways

  • ATP or adenosine triphosphate is the energy-carrying molecule essential for all living organisms. It transfers the chemical energy necessary for cellular processes. ATP is a phosphorylated nucleotide. It consists of adenine - an organic compound containing nitrogen, ribose - a pentose sugar to which other groups are attached and phosphates - a chain of three phosphate groups.
  • The energy in ATP is stored in the high-energy bonds between the phosphate groups that are broken to release energy during hydrolysis.
  • The synthesis of ATP is the addition of a phosphate molecule to ADP to form ATP. The process is catalysed by ATP synthase.
  • ATP synthesis happens during three processes: oxidative phosphorylation, substrate-level phosphorylation and photosynthesis.
  • ATP helps in muscle contraction, active transport, synthesis of nucleic acids, DNA and RNA, formation of the lysosomes, and synaptic signalling. It allows enzyme-catalysed reactions to take place more quickly.

ATP

No, ATP is classed as a nucleotide (although sometimes referred to as a nucleic acid) because of its similar structure to the nucleotides of DNA and RNA.

ATP is produced in the chloroplasts and the membrane of mitochondria.

ATP has various functions in living organisms. It functions as an immediate source of energy, providing energy for the cellular processes, including metabolic processes, muscle contraction, active transport, synthesis of nucleic acids DNA and RNA, the formation of the lysosomes, synaptic signalling, and it helps enzyme-catalysed reactions take place more quickly.

ATP stands for adenosine triphosphate.

The biological role of ATP is the transport of chemical energy for cellular processes.

Final ATP Quiz

Question

What is the definition of ATP?

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Answer

ATP or adenosine triphosphate is the energy-carrying molecule essential for all living organisms. It is used to transfer the chemical energy necessary for cellular processes.

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Question

Why is ATP called the ‘energy currency’? 

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Answer

ATP is called energy currency as it is used to carry energy from one cell to another. Like money (currency), it is used as the medium of exchange, but the exchange of energy. It is used for various reactions and can be reused.

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Question

Structurally, ATP consists of three parts. What are they?


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Answer

Adenosine

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Question

ATP is a phosphorylated nucleotide. What does that mean?

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Answer

It means that it has phosphates added.

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Question

How is ATP similar to DNA and RNA?

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Answer

ATP is a nucleotide, same as the monomers of DNA and RNA. They all have a pentose sugar (either ribose or deoxyribose) as a base. ATP is therefore similar to nucleotides in DNA and RNA.

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Question

Where is the energy stored in ATP?

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Answer

The energy in ATP is stored in the high-energy bonds between the phosphate groups. Usually, it is the bond between the 2nd and the 3rd phosphate group (counted from the ribose base).

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Question

How does energy storage in ATP differ from the energy storing in molecules such as starch?


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Answer

Rather than actually storing energy like starch or glycogen and storing it long term, ATP catches the energy, stores it in the high-energy bonds, and quickly releases it where needed. Actual storage molecules such as starch cannot simply release energy; it needs ATP to carry its energy further.

Show question

Question

What happens during the hydrolysis of ATP?

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Answer

  1. The bonds between the phosphate molecules break with the addition of water. These bonds are unstable and therefore easily broken. 
  2. The reaction is catalysed by the enzyme ATP hydrolase (ATPase). 
  3. The results of the reaction are adenosine diphosphate (ADP), one inorganic phosphate group (Pi) and the release of energy.


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Question

What happens when the other two phosphate groups detach from ATP?

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Answer

If the second phosphate group is removed, the result is the formation of AMP or adenosine monophosphate. This way, more energy is released. If the third (final) phosphate group is removed, the result is the molecule of adenosine. This, too, releases energy. 

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Question

Which molecule is added to ADP to form ATP during ATP synthesis?

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Answer

A phosphate molecule.

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Question

How is ATP produced? Fill in the gaps.

ATP is produced during _______and _______when protons (H+ ions) move down across the _________ (down an electrochemical gradient) through a channel protein ___________. __________ also serves as the enzyme that catalyses ATP synthesis.

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Answer

ATP is produced during cellular respiration and photosynthesis when protons (H+ ions) move down across the cell membrane (down an electrochemical gradient) through a channel protein ATP synthase

ATP synthase also serves as the enzyme that catalyses ATP synthesis.

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Question

Where is ATP synthase found?

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Answer

It is embedded in the thylakoid membrane of chloroplasts and the inner membrane of mitochondria, where ATP is synthesised.

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Question

ATP synthesis happens during three processes: oxidative phosphorylation, substrate-level phosphorylation and _________.

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Answer

Photosynthesis. 

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Question

ATP serves as an immediate source of energy. True or False.

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Answer

True. Cells need their energy quick to keep their engines constantly roaring, and ATP brings energy faster and easier than glucose can to needy cells. Therefore, ATP functions much more efficiently as an immediate source of energy than other storage molecules such as glucose. 

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Question

What are the functions of ATP?

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Answer

ATP is used in various energy-fueled processes in cells:

  • Metabolic processes, such as the synthesis of macromolecules.
  • ATP provides energy for muscle contraction.
  • ATP functions as an energy source for the active transport of macromolecules. 
  • ATP provides energy for the synthesis of nucleic acids DNA and RNA, more precisely translation.
  • ATP is required for the formation of the lysosomes that have a role in the secretion of cell products. 
  • ATP is used in synaptic signalling.  
  • ATP helps enzyme-catalysed reactions take place more quickly.

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