Pentose Phosphate Pathway or HMP-Pathway
Besides the aerobic respiration as the major pathway of oxidation of glucose via glycolysis and Krebs’ cycle, there exists an alternative pathway in several organisms, plants and in the liver and adipose tissue cells and lactating mammary glands. This alternative pathway is called pentose phosphate pathway. It is also known as hexose monophosphate pathway. This pathway is a major source of 5-carbon sugars which are components on nucleotides (ATP, NAD) and nucleic acids. This may operate simultaneously along with normal glycolytic pathway and can contribute between of energy supplied by carbohydrate respiration.
Step-1: Phosphorylation: Pentose phosphate pathway starts with the phosphorylation of six molecules of glucose in presence of enzyme hexokinase forming 6 molecules of glucose 6 phosphate.
Step-2: Oxidation: Glucose 6-phosphate is immediately oxidized by enzyme glucose 6-phosphate dehydrogenase to 6-phosphogluconic acid and NADP is reduced to NADHP2. Thus NADP acts as hydrogen acceptor.
Step-3: Dehydrogenation and Decarboxylation: 6-phosphogluconic acid undergoes dehydrogenation and decarboxylation in the presence of enzyme glucose 6-phosphogluconic acid dehydrogenase. This results is in 5-carbon compound ribulose 5-phosphate, CO2 and NADPH-H.
In the above three steps, 6 molecules of glucose-6-phosphate are oxidized to 6 molecules of ribulose 5-phosphate producing 6 molecules of CO2 and 12 molecules of NADPH2. These are oxidized to 12 NADP in presence of cytochrome system and oxygen. The end results can be summarized by the following equation:
PPP is also called HMP-pathway because the raw material for glucose oxidation is Glucose-6-P which is a hexose sugar produced after consuming only one ATP in contrast to Glycolytic pathway, where two ATP-molecules are consumed during the respiratory-oxidation of Glucose under aerobic condition.
-In glycolytic oxidation of one molecule of glucose, 12NADPH+ are formed from which 3 6ATP molecules are produced. This pathway is also as efficient as the glycolysis-Krebs’ cycle pathway in trapping the energy released in the oxidation of glucose molecule.
-This pathway occurs in the cytoplasm.
-Hexose sugar is broken down to CO2 and water without glycolysis and Krebs’ cycle.
-It is the source of 5-carbon sugars which are the constituents of nucleotides and nucleic acids.
Activity-I: To demonstrate anaerobic respiration in germinating seeds
Take eight or ten water-soaked pea seeds with the seed coats removed and Fill a test tube with mercury and invent it over a beaker containing mercury. The seeds will immediately rise to top. Leave the set up for about a day.
Observation: After some time the mercury level in test tube goes down. Now introduce some KOH in the above test tube. After a little while the mercury level will go up to the former position.
Explanation: The lowering of mercury level in test tube indicates that some gas has been released by germinating seeds. Only CO2 gas may be absorbed by KOH. We see that with the introduction of KOH the level of mercury goes high; hence the seeds have released CO2 under anaerobic conditions.
Activity-II: Anaerobic respiration in yeast
Take a test tube and add about 10 ml of 10% glucose solution in it and add a pinch of dry baker’s yeast into the glucose solution and cover the surface of the liquid carefully with an oily layer to prevent the contact with air. Fix a holed cork into the mouth of the test tube and pass a delivery tube through it. The other end of the delivery tube is dipped in lime water. The whole apparatus is made air tight. The test tube with glucose is kept in warm water in a beaker. It is observed that after sometime, sir bubbles arise and they turn the lime water milky showing that the air evolved is carbon dioxide. Open the cork of the test tube containing sugar and yeast. The smell of alcohol is observed. This shows that the products of anaerobic respiration by yeast are ethyl alcohol and carbon dioxide. Equation for the alcoholic fermentation: