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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.
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:
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.
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 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:
Figure 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 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.
Figure 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.
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.
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 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.
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:
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.
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.
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