The Pathway And Key Events In Glycolysis Process: Pathway, Steps & Products

What Is Glycolysis?

Glycolysis is the metabolic pathway by which the six-carbon sugar glucose is broken down into two molecules of the three-carbon compound, pyruvate. It akes place in the cytoplasm of a cell and is also the first stage of cellular respiration. This process is important in that it yields a net gain of two ATP molecules and two NADH molecules to the cell and meanwhile injects energy and reduces the power that these end products can easily be converted to other metabolic processes.

The first stage is glycolysis of cellular respiration. It takes place under both aerobic and anaerobic conditions. In the presence of oxygen, through glycolysis and then into a second pathway in the mitochondria called the citric acid cycle, followed by oxidative phosphorylation, in which glucose is fully oxidised with a high yield of ATP. Under anaerobic conditions, glycolysis leads to fermentation, where the cells can produce ATP without oxygen. This flexibility makes glycolysis a central pathway to energy production in an extraordinarily large number of organisms.

Steps of Glycolysis (10 Steps, Enzyme-Wise)

The pathway of glycolysis is further divided into two phases: energy investment phase and energy payoff phase.

The first 5 of which constitute the energy investment phase, in which the energy of ATP is invested. The energy gain comes in the energy payoff phase of glycolysis, i.e. from step 6 to 10.

Phase 1 – Energy Investment Phase (Steps 1–5)

Step 1: Glucose to Glucose-6-Phosphate

• Enzyme: Hexokinase

• Catalyses the phosphorylation of glucose to glucose-6-phosphate.

• 1 ATP consumed, ATP donates a phosphate group, converting it to ADP.

Step 2: Glucose-6-Phosphate to Fructose-6-Phosphate

• Enzyme: Phosphohexose isomerase

• 1 ATP consumed, ATP is spent to phosphorylate a molecule and it is regenerated back to ADP.

• Key regulatory step in glycolysis

• This reaction step is tightly regulated by intracellular levels of ATP.

Step 3: Fructose-6-Phosphate to Fructose-1,6-Bisphosphate

• Enzyme: Phosphofructokinase

• Adds a second phosphate group to fructose-6-phosphate, forming fructose-1,6-bisphosphate.

• Key regulatory step in glycolysis

• This step is highly regulated by cellular energy levels.

Step 4: Fructose-1,6-Bisphosphate to DHAP and G3P

• Enzyme: Aldolase

• Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

• Products is dihydroxyacetone

• Converts DHAP to G3P, so that two G3P passes through glycolysis.

Step 5: Conversion of DHAP to G3P

• Enzyme: Triose phosphate isomerase

• Converts DHAP to G3P, ensuring that two molecules of G3P proceed through glycolysis.

Phase 2 – Energy Payoff Phase (Steps 6–10)

Step 6: G3P to 1,3-Bisphosphoglycerate

• Enzyme: glyceraldehyde-3-phosphate dehydrogenase

• Catalyzes the oxidation and phosphorylation of G3P to produce 1,3-bisphosphoglycerate

• 1NADH per G3P

• NAD+ is reduced to NADH at this step

Step 7: 1,3-Bisphosphoglycerate To 3-Phosphoglycerate

• Enzyme: Phosphoglycerate kinase

• Transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.

• 1 ATP per G3P

• This step produces one molecule of ATP for each G3P.

Step 8: 3-Phosphoglycerate To 2-Phosphoglycerate

• Enzyme: Phosphoglycerate mutase

• Converts 3-phosphoglycerate reversibly to 2-phosphoglycerate by shifting the position of a phosphate group from the third to the second carbon.

Step 9: 2-Phosphoglycerate To Phosphoenolpyruvate

• Enzyme: Enolase

• Dehydrates 2-phosphoglycerate to form phosphoenolpyruvate (PEP), removing a molecule of water.

• 1 Water molecule removed per G3P and released as a byproduct.

Step 10: Phosphoenolpyruvate To Pyruvate

• Enzyme: Pyruvate kinase

• Transfers a phosphate group from PEP to ADP, forming ATP and pyruvate.

• 1 ATP per G3P

• This step produces one molecule of ATP for each G3P.

• Regulation point: Second key regulatory step

• This step is regulated to ensure efficient energy production.

Regulation of Glycolysis

Glycolysis is closely regulated to ensure that the generation of energy is highly efficient and integrated with the cell.

Regulatory Enzymes

Hexokinase: The enzyme catalysing the phosphorylation of glucose to glucose-6-phosphate is inhibited by the product of the reaction, glucose-6-phosphate.

Phosphofructokinase: This enzyme controls the step of conversion from fructose-6-phosphate to fructose-1,6-bisphosphate; a major regulatory step affected by ATP, AMP, and citrate levels.

Pyruvate kinase: This is the final step in the conversion of phosphoenolpyruvate to pyruvate and is regulated by the levels of ATP and alanine, while it is activated by fructose-1,6-bisphosphate.

Mechanisms Of Regulation

Allosteric regulation: Enzymes like phosphofructokinase are regulated through binding at places other than the active site of the enzyme, thereby modifying its activity.

Feedback inhibition: The end products like ATP will inhibit the early steps of glycolysis so that overproduction can be avoided.

Hormonal regulation: Insulin enhances glycolysis by increasing the expression of most glycolytic enzymes, while glucagon inhibits glycolysis to enhance gluconeogenesis.

Energetics of Glycolysis

Glycolysis generates energy in the form of ATP and NADH through its activity and is, therefore, an important aspect of cellular metabolism.

ATP Yield

In glycolysis, while 4 ATP molecules per glucose are produced, 2 ATP are consumed. Hence, there is a net gain of 2 ATP.

NADH Production

The NADH produced during glycolysis in each glucose molecule passes high-energy electrons into the electron transport chain in the mitochondria, which leads to additional production of ATP.

Overall Energy Efficiency

Part of the energy from the glucose is captured and stored as ATP and NADH during glycolysis while still having some energy remaining in pyruvate, which can be further oxidised in the presence of oxygen.

Fate of Pyruvate After Glycolysis

The metabolic fate of pyruvate after glycolysis is:

Aerobic Conditions

In the presence of sufficient oxygen, pyruvate is transported into the mitochondria and then converted to acetyl-CoA by the pyruvate dehydrogenase complex. The produced acetyl-CoA will enter the citric acid cycle (Krebs cycle), where it becomes further oxidized into ATP, NADH, and FADH2 required for the electron transport chain.

Anaerobic Conditions

In the absence of, or when oxygen is limited, the cells resort back to anaerobic pathways for energy generation.

• In lactic acid fermentation, when the amount of oxygen is inadequate during exercise or when oxygen demand is high, pyruvate is converted to lactate.

• Alcoholic fermentation in yeast and some bacteria, pyruvate is converted to ethanol and carbon dioxide through alcoholic fermentation.

Importance of Glycolysis

The importance of glycolysis is:

• It is a universal metabolic pathway occurring in both prokaryotes and eukaryotes.

• It helps in the rapid generation of the ATP.

• It helps prepare the substrate that will be further used in the Krebs cycle.

• It maintains the supply of oxygen during low oxygen conditions

Glycolysis NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Steps in Glycolysis

• Regulation of Glycolysis

• Fate of Pyruvate

Practice Questions for NEET

Q1. What is the role of NAD+ in cellular respiration?

1. It is a nucleotide source for ATP synthesis.

2. It functions as an electron carrier.

3. It functions as an enzyme.

4. It is the final electron acceptor for anaerobic respiration.

Correct answer: 2) It functions as an electron carrier.

Explanation:

At various chemical reactions, the NAD+ picks up an electron from glucose, at which point it becomes NADH. Then NADH, along with another molecule, flavin adenine dinucleotide (FADH2), will ultimately transport the electrons to the mitochondria, where the cell can harvest energy stored in the electrons.

Hence, the correct answer is Option (2) It functions as an electron carrier.

Q2. In terms of quantity, oxidation of 1 glucose molecule yields

1. 686 Kcal energy

2. 782 Kcal energy

3. 656 Kcal energy

4. 608 Kcal energy

Correct answer: 1) 686 Kcal energy

Explanation:

There is about 686 kcal oxidation for each glucose molecule. Cellular respiration breaks glucose into carbon dioxide and water. The theoretical maximum yield of ATP in this process is about 30 to 38 ATP molecules depending on the efficiency of electron transport and conditions prevailing in the cell. Practical yields are usually much lower because there is a loss of energy in transport and other metabolic processes.

Hence the correct answer is Option 1. 686 Kcal energy.

Q3. The number of ‘ends’ in a glycogen molecule would be

1. Equal to the number of branches plus one

2. Equal to the number of branch points

3. One

4. Two, one on the left side and another on the right side

Correct answer: 4) Two, one on the left side and another on the right side

Explanation:

One should remember that the number of ‘ends’ in a glycogen molecule would be equal to the number of branches plus one. This is because glycogen is a branched polysaccharide, with each branch terminating in a free non-reducing end. The main chain contributes one end, while each additional branch adds another. These ends are crucial for enzymatic action during glycogen synthesis and breakdown. Enzymes like glycogen phosphorylase act on these non-reducing ends to release glucose-1-phosphate during glycogenolysis.

Hence, the correct answer is option 4)Two, one on the left side and another on the right side.

Also Read:

• MCQs on Glycolysis Pathway

• TCA Cycle

• Lactic Acid Fermentation

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