Adenosine triphosphate (ATP): Introduction, Structure, Biosynthesis, and Role in the cell

Adenosine triphosphate (ATP)

Introduction:

• Living cells acquire energy by the breakdown of complex molecules such as polysaccharides, fats, proteins, alcohols etc.
• Phototrophs utilizes sunlight as the source of energy whereas chemotrops utilizes the chemical compounds as the source of energy.
• By the degradation of complex molecules a lot of energy is released which is captured in the cell in the form of chemical bonds that can be utilized readily when needed.
• The rest of the energy which is not captured in lost as heat.
• In the cell, the energy is captured in the form of adenosine triphosphate (ATP)
• ATP is nucleotide and it consists of adenine, pentose sugar ribose, and a triphosphate unit.

• The ATP is activated by cations such as Mg ²⁺ and Mn²⁺. ATP is complexed with these cations and activated.
• The energy is stored in its triphosphate moiety.
• Heterotrophic cells obtain free energy in chemical form by the catabolism of nutrient molecules and they use that energy to make ATP from ADP and Pi.
• The ATP thus formed, undergoes hydrolysis to ADP and Pi and release energy which is utilized for various purposes.
• Sometimes, ATP is hydrolysed to AMP and pyrophosphate(PP).

Biosynthesis of ATP:

ATP can be synthesized in three ways:

• Oxidative phosphorylation :
  • The process of synthesis of ATP via electron transport chain is known as oxidative phosphorylation.
  • In the electron transport chain, electrons are transferred through a series of electron acceptors by redox reactions.
  • The electrons carried by a molecule of NADH+H+ release 3 ATP whereas the electron carried by a molecule of FADH2 releases 2 ATP through electron transport chain.
  • The ATP is synthesized by the ATPase complex.

• Substrate level phosphorylation :
  • When energy is trapped directly from the substrate without the help of complicated electron transport chain, it is called substrate level phosphorylation.

• Photophosphorylation:
  • There are two types of photophorylation

• Cyclic Photophosphorylation:
  • In Cyclic Photophosphorylation, the molecule of baacterila chlorophyll absorb the quantum of light.
  • The energy of light raises the molecule to an excited state.
  • The excited bacterial chlorophyll gives electron to ferredoxin and then to ubiquinone.
  • The electron successively passes to Cytochrome b and to Cytochrome f and finally back to the positively charged bacteriochlorophyll.
  • The energy released between Cyt b and Cyt f is used for photophosphorylation.

• Non-Cyclic Photophosphorylation:
  • In plants, algae, and cyanobacteria, non cyclic photophosphorylation takes place.
  • When a molecule in pigment system II absorbs light, the molecule is excited and releases electrons.
  • The released electrons passes to plastoquinone to Cyt b and then to Cyt f and finally to pigment system I.
  • Photophosphorylation occurs generating ATP from ADP and iP between Cyt b and Cyt f.
  • The photo-system I release that electron when it absorbs light.
  • The electron is then transferred to ferredoxin to flavoprotein and finally to NADP⁺.
  • Photophosphorylation occurs when electrons passes to ferredoxin.
  • The electron loss by pigment system I is replaced from photolysis of H₂O.

Difference between Cyclic and Non-Cyclic photophoshorylation

Role of ATP in the cell

• ATP plays a major role in anabolic reactions such as the synthesis of macromolecules from simple precursors. In the process, the terminal phosphate group or groups of ATP are transferred enzymatically to precursor building block molecules which thus become energized and prepared for their assembly into macromolecules.
• The performance of mechanical work in muscle contraction and other cellular movements also require ATP.
• The active transport of molecules and ions through membranes against the concentration gradient is only possible by the involvement of ATP i.e. ABC transport system (ATP binding cassette).

• ATP is required for the biosynthesis of DNA [ DNA replication → DNA topoisomerase II or DNA gyrase of prokaryotes require ATP].
• It is also required for the synthesis of protein (during activation of amino acids).

• ATP plays a very important role in preserving the structure of the cell by helping the assembly of the cytoskeletal elements. It also supplies energy to the flagella and chromosomes to maintain their appropriate functioning.

References:

• http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/atp.html
• https://www.ncbi.nlm.nih.gov/books/NBK553175/
• http://www.chm.bris.ac.uk/motm/atp/atp1.htm
• https://biologydictionary.net/atp/
• https://www.news-medical.net/life-sciences/Adenosine-Triphosphate-(ATP)-Function-in-Cells.aspx
• http://www.biosciencenotes.com/synthesis-of-adenosine-triphosphate-atp/