Glycolysis Pathway - Practice Questions & MCQ

• The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting.
• The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.
• In anaerobic organisms, it is the only process in respiration.

Place of Occurrence: Glycolysis occurs in the cytoplasm of the cell and is present in all living
organisms. 

Process of Glycolysis: 

• In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. 
• Glycolysis begins with the six-carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate.
• Glycolysis consists of two distinct phases. 
• The first part of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into two three-carbon molecules. This is the energy-requiring step.
• The second part of glycolysis extracts energy from the molecules and stores it in the form of ATP and NADH, the reduced form of NAD. This is the energy-releasing step.

Glycolysis - Energy Requiring Step

• The first step in glycolysis is phosphorylation of glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. Hexokinase catalyses this reaction.

• In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, fructose-6-phosphate.

• The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase producing fructose-1,6-bisphosphate.

• The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate. 

• In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. 

• At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule.

• The sixth step involves oxidation of glyceraldehyde-3-phosphate, extracting high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH.

• Glyceraldehyde-3-phosphate is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.
• In the seventh step, catalyzed by phosphoglycerate kinase 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP and 3-phosphoglycerate is formed. This is called substrate-level phosphorylation.

• In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing this step is a mutase (an isomerase).

• Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP). This is a dehydration reaction.

• The last step in glycolysis is catalyzed by the enzyme pyruvate kinase and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid.

• Glycolysis starts with glucose and ends with two pyruvate molecules, a total of four ATP molecules and two molecules of NADH. 
• Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use.
• If the cell cannot catalyze the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose.