Question

Give an account of glycolysis. Where does it occur? What are the end products? Trace the fate of these products in both aerobic and anaerobic respiration.

Answer 1

Hint: Glycolysis is an important metabolic process that occurs inside the cells to provide energy for growth and development. It is characterized by the breakdown of glucose into an energy molecule called ATP. Glycolysis is an essential step of cellular respiration. Cellular respiration can be aerobic or anaerobic.

Complete answer: Glycolysis is the primary process under cellular respiration. It is defined as the glucose breaking down for energy production. It results in the formation of two pyruvate molecules along with NADH, ATP and water. The location of glycolysis is in the cytosol of the cell cytoplasm. If it occurs in presence of oxygen, it is called aerobic and if it occurs in the absence of oxygen, it is called anaerobic. The glycolysis process is known as EMP because it was discovered by Gustav Embden, Otto Meyerhof and Karol Parnas. The glycolysis is a multi-step process. It occurs in the following steps.
a) The first step is the addition of a phosphate group from ATP to a glucose molecule. It is catalyzed by the enzyme hexokinase. The product is called glucose, \[6-\]phosphate.
b) The second step is the isomerization of glucose$-6-$phosphate into fructose, $6-$phosphate by the action of the phosphoglucomutase enzyme.
c) The third step involves the transfer of another phosphate group from ATP to fructose, $6-$phosphate. This results in the formation of 1, $6-$bisphosphate. This reaction is catalyzed by phosphofructokinase enzymes.
d) Step 4 is the production of glyceraldehydes $3-$phosphate and dihydroxyacetone by conversion of fructose 1, $6-$bisphosphate. It is catalyzed by the aldolase enzyme.
e) Step 5 is the conversion of dihydroxyacetone phosphate into glyceraldehyde $3-$phosphate by the action of triose-phosphate isomerase.
f) Step 6 involves the transfer of 1 hydrogen molecule to nicotinamide adenine dinucleotide or NAD from glyceraldehydes phosphate. It is catalyzed by an enzyme named glyceraldehyde $3-$phosphate dehydrogenase. The product formed is NADH $+$ \[{{H}^{+}}\]. The same enzyme adds a phosphate to the oxidized glyceraldehydes phosphate to make 1, $3-$bisphosphoglycerate.
g) Step 7 is the transfer of phosphate from 1, $3-$bisphosphoglycerate to ADP. Phosphoglycerolinase converts it into ATP. Two molecules of phosphoglycerate and ATP are formed at the end of the reaction.
In step 8, the phosphoglyceromutase relocates the phosphate of both phosphoglycerate molecules to the second carbon from the third. Hence, two molecules of $2-$phosphoglycerate are formed.
h) Step 9 involves the removal of a water molecule from $2-$ phosphoglycerate to make phosphoenolpyruvate. The reaction is catalyzed by the enolase enzyme.
i) Step 10 is the last step in which two molecules of pyruvate and ATP are formed by transfer of a phosphate from phosphoenolpyruvate to ADP.
Now, the two molecules of pyruvate can have two fates. In presence of oxygen (aerobic), it enters the Krebs cycle, which will form 38 ATP molecules. On the other hand, in absence of oxygen (anaerobic), it is broken down into lactic acid or alcohol. Only 2 ATP are produced in this reaction.

Note: Anaerobic breakdown of pyruvate can be seen in muscles during extensive exercise. Lactic acid is produced in the muscle cells which may cause cramps. Yeast cells also perform anaerobic respiration during the fermentation process. Along with the production of ethanol during brewing, carbon dioxide is formed as a by-product.