ATP Synthesis Mechanisms

ATP synthesis occurs during several cellular processes, including phosphorylation reactions. It can occur in both aerobic and anaerobic conditions. The significant ways of ATP production are; cellular respiration (oxidative phosphorylation, substrate-level phosphorylation), beta-oxidation and lipid catabolism, protein catabolism, photo-phosphorylation, and fermentation. 

1. Cellular Respiration

It is the process where glucose is catabolized into acetyl – CoA and subjected to oxidative phosphorylation for ATP synthesis. It is the major mechanism for synthesizing most of the ATP required for a cell. The ATP production via cellular respiration occurs in two different stages;

a. Substrate level phosphorylation

ATP production occurs directly during glycolysis. In the glycolytic pathway, oxidation of G-3-P by G-3-P dehydrogenase enzyme adds a high energy phosphate group which is transferred to ADP in the next reaction generating ATP molecule.

In another reaction, the energy released during dehydration of 2-phosphoglycerate converts the low energy phosphate bond into a high energy phosphate bond which is transferred to ADP in the next reaction producing an ATP molecule.

Pyruvate is then oxidized to acetyl – CoA molecule by pyruvate dehydrogenase complexes. Thus formed acetyl – CoA is then subjected to the Krebs cycle, where it is oxidized to produce one equivalent of ATP, i.e., GTP molecule, three molecules of NADH, and one molecule of FADH₂. These NADH and FADH₂ molecules are electron carriers that will enter the ETC (electron transport chain) and produce ATP molecules.  

b. Oxidative phosphorylation

The intermediate compounds like NADH and FADH₂ produced during glycolysis, pyruvate decarboxylation, and Krebs cycle are used as electron carriers and subjected as the substrate to the electron transport chain (ETC) generating proton gradient. The proton gradient is coupled with chemiosmosis, where the ATP synthase enzyme synthesizes ATP.

2. Photo-phosphorylation

It is the process where the light energy is used to phosphorylate ADP to ATP inside chlorophyll-containing cells. The general reaction of photophosphorylation can be expressed as:

ADP + light energy + Pi → ATP

 It is of two types; cyclic and non-cyclic photophosphorylation.

a. Cyclic Photo-phosphorylation

It is the photo-phosphorylation process where electrons released by the P700 pigment of Photosystem-I are recycled back to Photosystem-I. The electron released is subjected to an ETC which generates a proton gradient that is used to produce ATP by ATP synthase in a process called chemiosmosis. It predominately occurs in bacterial cells.   

b. Non-cyclic Photo-phosphorylation 

It is the photo-phosphorylation process where the released electrons are not recycled back to the photosystem which produces them. In this mechanism, both photosystem-I and –II are excited simultaneously. Electrons released by P680 of photosystem-II are passed through an ETC generating ATP by phosphorylation of ADP by ATP synthase enzyme in chemiosmosis. The electrons are then used to replace the electrons lost by P700 of photosystem-II during photoexcitation. The electrons released by photosystem-II are then used to reduce NADP+ to NADPH. It predominately occurs in plant cells and causes the release of one O₂ molecule in each step.