It predominately occurs in plant cells and causes the release of one O 2 molecule in each step. The electrons released by photosystem-II are then used to reduce NADP+ to NADPH. The electrons are then used to replace the electrons lost by P700 of photosystem-II during photoexcitation.

Electrons released by P680 of photosystem-II are passed through an ETC generating ATP by phosphorylation of ADP by ATP synthase enzyme in chemiosmosis. In this mechanism, both photosystem-I and –II are excited simultaneously. It is the photo-phosphorylation process where the released electrons are not recycled back to the photosystem which produces them. It predominately occurs in bacterial cells. 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 is the photo-phosphorylation process where electrons released by the P700 pigment of Photosystem-I are recycled back to Photosystem-I. It is of two types cyclic and non-cyclic photophosphorylation. The general reaction of photophosphorylation can be expressed as: It is the process where the light energy is used to phosphorylate ADP to ATP inside chlorophyll-containing cells. The proton gradient is coupled with chemiosmosis, where the ATP synthase enzyme synthesizes ATP. The intermediate compounds like NADH and FADH 2 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. These NADH and FADH 2 molecules are electron carriers that will enter the ETC (electron transport chain) and produce ATP molecules. 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 FADH 2. Pyruvate is then oxidized to acetyl – CoA molecule by pyruvate dehydrogenase complexes. 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. 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.

Substrate level phosphorylationĪTP production occurs directly during glycolysis. The ATP production via cellular respiration occurs in two different stages a. It is the major mechanism for synthesizing most of the ATP required for a cell. It is the process where glucose is catabolized into acetyl – CoA and subjected to oxidative phosphorylation for ATP synthesis. 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. It can occur in both aerobic and anaerobic conditions.

Similarly, proteins and lipids metabolism also produce simple end products like acetyl CoA, succinyl CoA, keto-acids, ammonia, etc., which are then subjected to the Krebs cycle and oxidative phosphorylation to yield ATP molecules.ĪTP synthesis occurs during several cellular processes, including phosphorylation reactions. This released energy is trapped and stored in the form of ATP. Carbohydrates consumed in different forms (starch, sucrose, dextrose, lactose, fructose, etc.) are mostly broken down to monosaccharide form ‘glucose.’ Glucose is then subjected to metabolic reactions, glycolysis, Krebs cycle, and oxidative phosphorylation and is oxidized to release energy. Glucose and ATPĬarbohydrate is the primary source of energy. The energy released during the oxidation of these nutrients is trapped in the form of the high-energy phosphodiester bond in the ATP molecule. Oxidation of glucose, lipids (fats), and amino acids produce the ATP molecules inside cells. ATP is an energy-rich compound primarily synthesized during cellular respiration in aerobic and anaerobic cells.

Atp production