ATP

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.

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).

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.

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.

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 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.

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

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.