Question

Write explanatory notes on :
(a) Glycolysis
(b) Fermentation by yeast
(c) Electron transport chain.

Answer 1

(a) Glycolysis is the metabolic process that takes place in the absence of oxygen. It is the foundation for both aerobic and anaerobic cellular respiration. In this, glucose is converted into pyruvate. It occurs in the cytoplasm. A net of 2 ATP is produced by this process. It comprises of 10 steps.

Step I: Phosphorylation of glucose

Step II: Isomerization of glucose-6 phosphate to fructose-6- Phosphate

Step III: Phosphorylation of fructose-6-phosphate

Step IV: Cleavage of Fructose-1,6-bisphosphate

Step V: Conversion of dihydroxy-acetone phosphate to glyceraldehyde-3-phosphate

Step VI: Oxidation of glyceraldehyde-3-phosphate

Step VII: Transfer of phosphoryl group from 1,3-bisphosphoglycerate to ADP

Step VIII: Conversion of 3-phosphoglycerate to 2-phoshoglycerate

Step IX: Dehydration of 2-phosphoglycerate (Removal of H2O from 2-phosphoglycerate)

Step X: Transfer of phosphoryl group from PEP to ADP.

(b) Fermentation by yeast is an anaerobic process. It occurs in the absence of oxygen. In the absence of oxygen, yeast break down sugar to yield energy in the form of ATP and alcohol. Carbon dioxide is produced as the waste product. Enzyme involved in this process is zymase which is present in yeast. Its activity varies among yeast strains.

(c) ETC (electron transport chain) is located in the inner mitochondrial membrane. Its main function is to release and utilize the energy stored in NADH + H${}^{+}$ and FADH${}_2$. NADH + H${}^{+}$ is formed during glycolysis and citric acid cycle. It is oxidized by NADH dehydrogenase enzyme (complex I). The electrons generated through this oxidation are transferred to ubiquinone through FMN. FADH${}_2$ (Complex II) formed during the citric acid cycle are transferred to ubiquinone. The electrons from ubiquinone are then transferred to cytochrome c (complex III) by cytochrome bc1. From cytochrome c, electrons are transferred to cytochrome c oxidase complex (complex IV). During each step, ATP is produced from ADP by enzyme ATP synthase (complex V). Total of 3 ATP molecules are produced by the oxidation of one molecule of NADH and 2 ATP molecules are produced by oxidation of one molecule of FADH${}_2$.