Oxidative Phosphorylation – ATP Synthesis
Oxidative phosphorylation occurs in mitochondria here transfer of electrons is coupled with ATP formation from ADP and Pi. It is driven by the strong thermodynamics tendency of O2 to become reduced, therefore, called oxidative phosphorylation.
Oxidative phosphorylation accounts for only a small fraction of ATP generated in living cells of higher plants. The number of ATP molecules synthesized depends upon the nature of electron donor. Electrons derived from oxidation of NADH2 molecules produced in the matrix results in generation of three ATP molecules. FADH2 enters the respiratory chain at a different point then NADH2, therefore only two molecules of ATP are generated for each molecules of succinic acid oxidized to fumaric acid in the respiratory chain. The ATP generation takes place at three sites that exist along electron transport chain. The currently accepted mechanism of mitochondrial ATP synthesis is based on the chemiosmotic hypothesis, proposed by Peter Mitchell.
Elementary Particles or Fo-F1 ATP Synthase Complex
The ATP synthesis takes place by a protein complex associated with the inner membrane, the Fo-F1 ATP synthase. It consists of two major components Fi and F. Fi is a peripheral membrane protein complex that is composed of at least five different sub-units and contains the catalytic site for converting ADP and Pi to ATP. This complex is attached to the matrix side of the inner membrane. Fo is an integral protein complex that consists of at least three different polypeptides that form channel through which protons are able to cross the inner membrane. The passage of proton through the channel activates the ATP synthase to synthesize ATP and simultaneously dissipates the proton electrochemical gradient.
Energy Balance of Respiration – ATP Yield Per Molecule of Glucose
Complete oxidation of glucose molecule through glycolysis. Conversion of pyruvic acid to acetyl Co A and Krebs cycle leads to net formation of four molecules of ATP by substrate level phosphorylation (two during glycolysis and two in Krebs cycle); two molecules of NADH2 in the cytosol (during glycoysis), eight molecules of NADH2 and two molecules of FADH2 in the mitochondrial matrix.
- The oxidation of 8 NADH2 molecules produced in mitochondrial matrix lead to generation of 24 ATP molecules, i.e., three per NADH2
- Four molecules of ATP are generated in the oxidation of succinic acid fumaric acid via FADH2. These make the total 28 ATP molecules.
- Addition of two ATP molecules formed during substrate level phosphorylation in Krebs cycle via GTP brings the total ATP synthesized to 30.
- Two ATP molecules are generated during substrate level phosphorylation in glycolysis and four from oxidation of NADH2 produced during glycolysis. Addition these six ATP molecules brings the total ATP synthesis to 36 molecules.
Thus, there is a total yield of 36 ATP molecules per glucose molecule completely oxidized to CO2 and water.
In the absence of oxygen, the Krebs cycle and electron transport chain cannot function, therefore oxidation of NADH2 stops and supply of NAD+ necessary for dehydrogenase reactions is limited. So, continued operation of glycolysis stops. To overcome this problem plants (especially fungi) and other organisms metabolize pyruvates by a process known as fermentation.
Fermentation is a sequential series of reactions that occurs in the absence of oxygen. The pyruvate is converted to ethanol and CO2 or to lactic or other organic acids depending the organisms are involved. Under extreme low O2 concentrations, for example in plants growing in waterlogged or flooded conditions, the plant tissues are forced to carry out fermentative metabolism.
Fermentation liberates a fraction of energy available in each molecule of glucose, i.e., approximately 53 Kcal per mole. There is net gain of two ATP molecules during fermentation.
Types of Fermentations
The fermentation may be: Lactic Acid Fermentation Or Alcoholic Fermentation
Lactic Acid Fermentation
This type of fermentation is common to mammalian muscle as well as plants (fungi). These organisms use NADH2 in the presence of enzyme lactate dehydrogenase to reduce pyruvate to lactate. NAD+ is regenerated during this process. Carbon dioxide is not released during lactic acid fermentation.
Pyruvic Acid + NADH2 ————————————————— Lactic Acid
This type of fermentation is more widely known to occur in brewer’s yeast but also common in plants. The pyruvate is acted upon by two enzymes, pyruvate decarboxylase and alcohol dehydrogenase producing ethanol and CO2. The NADH2 is oxidized to NAD+.
Pyruvic Acid ————————————————— Acetaldehyde + CO2
Acetaldehyde + NADH ————————————————— Ethyl alcohol + NAD+
Ethanol is thought to be a less harmful end product of fermentation than lactic acid, because accumulation of lactic acid promotes acidification of the cytosol.