Synthesis of (Adenosine triphosphate) ATP
During aerobic respiration, a glucose molecule gives rise to 6 or 8 ATP molecules from glycolytic process depending upon the shuttle pathway that operates while under anaerobic condition only 2 molecules of ATP are produced. When glycolysis occurs from glycogen 1 more ATP is synthesized. This is because ATP is consumed for activation of glucose while glycogen directly produces glucose-1-phosphate which converts into glucose-6-phosphate. Glycolysis from glycogen in anaerobic respiration gives 3 ATP molecules.
ATP can be synthesized in following ways:
- Oxidative phosphorylation
- Substrate level phosphorylation
Oxidative phosphorylation: It is the process of synthesis of ATP via electron transport chain, in which electron are transferred through a series of electron acceptors by redox reactions. ATP is synthesized by ATP Synthase enzyme. In eukaryotes, it occurs in mitochondria while in prokaryotes it occurs in cytosol.
- The electrons carried by a molecule of NADH+H+ release 3 ATP whereas the electron carried by a molecule of FADH2 release 2 ATP through electron transport chain.
Substrate level phosphorylation: It is the process in which energy or ATP is synthesized directly from substrate rather than complex reactions of electron transport chain.
Here, direct transfer of phosphoryl group to Adenosine diphosphate (ADP) or guanosine diphosphate (GDP) occurs from other phosphorylated compound to yield ATP or Guanosine triphosphate (GTP). It takes place in cytoplasm (In glycolysis) and mitochondria (In Krebs’s or TCA cycle).
Fig: Examples of Substrate level phosphorylation
Photophosphorylation: it is the phosphorylation of ADP using sunlight as energy during photosynthesis to yield ATP. Production of ATP from light dependent reactions is photophosphorylation. It is of two types:
- Cyclic photophosphorylation
- Non- cyclic photophosphorylation
Cyclic phosphorylation involves cyclic movement of electron to yield ATP. It is most dominant in photosynthetic bacteria such as Cyanobacteria.
Non cyclic photophosphorylation involves unidirectional movement of electrons. It is dominant in green plants.
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