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7.1.1. Energetic Aspects of Cellular Chemical Reactions

Energetic Aspects of Cellular Chemical Reactions

Adenosine triphosphate (ATP)

The adenosine triphosphate (ATP) is the molecule responsible for its links stored in the energy produced in chemical reactions, but last only a short time. A person expends about 45 kg of ATP a day, although he has less than 1 gram at any one time. 

As we saw when we talked about nucleotides, these energy-rich phosphate bonds are symbolized by the sign ~, little virgulillaThe inorganic phosphate is represented by Pi.                       

Through dephosphorylation, ATP transfers a phosphate group to another compound, leaving ADP (adenosine diphosphate) as the resulting molecule, and if another phosphate is lost, AMP (adenosine monophosphate). These reactions are reversible:

ATP ↔ ADP + Pi + energy

ADP ↔ AMP + Pi + energy

In the cell, ATP can be obtained by:

  • Substrate level phosphorylation. A substrate molecule that contains a phosphate group gives it to ADP to form ATP.
  • Oxidative phosphorylation. In mitochondria, through the ATP synthase complex , most of the ATP of the cell is obtained.
  • Photophosphorylation or photosynthetic phosphorylation. In the chloroplasts  of eukaryotic plant cells , through the ATP synthase complex.

Video: ATP: Hydrolysis, Phosphorylation and Structure.

Molecules involved in metabolism

In addition to the specific enzymes of each metabolic pathway, four other types of molecules are necessary:

  • Metabolites. A metabolite is any molecule used, or produced during metabolism. The first metabolite is the substrate ,and the final metabolite is the product. The molecules produced between the two are the intermediate metabolites .Metabolites can follow the different metabolic pathways, both for their degradation (catabolism) or for synthesizing other more complex molecules (anabolism).
  • NucleotidesSome molecules, such as the NAD +, the NADP +, the FAD, the FMN , the Coenzyme Q  (ubiquinone) and cytochromes allow oxidation or reduction metabolites. They are usually coenzymes associated with the protein part of enzymes (apoenzyme) and act as electron transporters, with two fairly close oxidation states.
  • Molecules with energy-rich bonds. As has already been seen when talking about ATP or GTP, the bonds of the phosphate groups are rich in energy. When it is formed, chemical energy is stored, and when it breaks, the same amount of energy is released.
  • Extreme environmental molecules. Some simple molecules such as oxygen, the water and carbon dioxide, or more complex, such as ethyl alcohol or lactic acid, are at the beginning or end of some metabolic process, allowing the system open. 

Performance and energy balance of metabolism

In metabolism, oxidation and reduction processes always occur in a simultaneous and coupled way.

  • In oxidation, a compound is oxidized, losing electrons and sometimes hydrogens as well.
  • In reduction, a compound is reduced by gaining electrons and sometimes hydrogens as well.

In catabolism, the oxidation reactions of the substrates are coupled with the reduction of coenzymes such as NAD+ and FAD, which are reduced to NADH and FADH2. As a result of oxidative degradation, energy is obtained that will allow the cell to live and use it in anabolism.

The amount of energy obtained in an exergonic process depends on the difference between the reduction potential of the substrate and of the last molecule that is reduced allowing the oxidation of the others.

In anabolism, reduction reactions require the electrons of NADH and FADH2 synthesized in catabolism.

The energy balance allows to measure the amount of energy released or used in a metabolic process, according to the amount of energy in the form of energy-rich bonds (ATP) that is obtained. If the pathway is anabolic, the balance will be negative, and if it is catabolic, positive.