In-Depth Glycolysis Notes

Glycolysis Overview

  • Glycolysis is a catabolic pathway involved in the breakdown of glucose.

  • It occurs in the cytosol and is ubiquitous across all living cells.

  • The process does not require oxygen (anaerobic) and results in the production of ATP and NADH.

  • Pyruvate is the final product, which can be further metabolized under aerobic or anaerobic conditions.

Key Pathways in Carbohydrate Catabolism

  • Glycolysis: Breakdown of glucose into pyruvate.

  • Krebs Cycle: Also known as the citric acid cycle; it further processes pyruvate.

  • Electron Transport Chain (ETC): Utilizes electrons from NADH to produce ATP through oxidative phosphorylation.

Steps of Glycolysis

General Phases

  1. Energy Investment Phase:

    • Uses 2 ATP molecules to phosphorylate glucose and its derivatives.

    • Converts glucose to fructose-1,6-bisphosphate facilitated by hexokinase.

    • Key Enzymes: Hexokinase, Phosphofructokinase.

  2. Energy Payoff Phase:

    • Produces a net gain of 2 ATP and 2 NADH.

    • Converts glyceraldehyde-3-phosphate to pyruvate.

    • Key Enzymes: Glyceraldehyde-3-phosphate dehydrogenase, Pyruvate kinase.

Major Enzymatic Steps

  • Preparatory Phase Enzymes:

    • Hexokinase: Catalyzes the first step, phosphorylating glucose to glucose-6-phosphate.

    • Phosphoglucose Isomerase: Converts glucose-6-phosphate to fructose-6-phosphate.

    • Phosphofructokinase: Converts fructose-6-phosphate to fructose-1,6-bisphosphate; critical regulatory step.

    • Aldolase: Cleaves fructose-1,6-bisphosphate into two three-carbon molecules.

    • Triose Phosphate Isomerase: Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.

  • Pay-off Phase Enzymes:

    • Glyceraldehyde-3-phosphate Dehydrogenase: Converts GAP, producing NADH.

    • Phosphoglycerate Kinase: Transfers a phosphate to ADP, producing ATP.

    • Enolase: Prepares the substrate for conversion to pyruvate.

    • Pyruvate Kinase: Final step, converting phosphoenolpyruvate to pyruvate with ATP production.

Energy Yield from Glycolysis

  • Overall reaction:
    Glucose+2NAD++2ADP+2Pi→2ATP+2NADH+2pyruvate+2H2O\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{Pi} \rightarrow 2\text{ATP} + 2\text{NADH} + 2\text{pyruvate} + 2\text{H}_2\text{O}

  • Net Gain: 2 ATP molecules from glycolysis.

Regulation of Glycolysis

  • Key regulatory steps include:

    • Hexokinase: Feedback inhibited by glucose-6-phosphate.

    • Phosphofructokinase (PFK): Activated by AMP (low energy) and inhibited by ATP (high energy).

    • Pyruvate Kinase: Allosterically activated by fructose-1,6-bisphosphate; regulated by hormones such as insulin and glucagon.

Fate of Pyruvate

  • Under aerobic conditions:

    • Converted to acetyl-CoA for the Krebs cycle.

  • Under anaerobic conditions:

    • Converted to lactate in muscles or ethanol in yeast.

Importance of Glycolysis

  • Glycolysis is critically important for cells that have limited access to oxygen, such as red blood cells.

  • It supports energy production in high-intensity exercise and is involved in both the catabolism of glucose and the production of intermediates for other metabolic pathways.


  1. What is the final product of glycolysis?

    • A) Glucose

    • B) Fructose-1,6-bisphosphate

    • C) Pyruvate

    • D) Acetyl-CoA
      Answer: C) Pyruvate

  2. Where does glycolysis take place in the cell?

    • A) Mitochondria

    • B) Cytosol

    • C) Nucleus

    • D) Endoplasmic reticulum
      Answer: B) Cytosol

  3. Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the preparatory phase of glycolysis?

    • A) Phosphoglucose isomerase

    • B) Hexokinase

    • C) Phosphofructokinase

    • D) Pyruvate kinase
      Answer: B) Hexokinase

  4. In the energy payoff phase of glycolysis, what is produced alongside ATP?

    • A) Oxygen

    • B) NADH

    • C) FADH2

    • D) Acetyl-CoA
      Answer: B) NADH

  5. Which of the following molecules inhibits phosphofructokinase (PFK)?

    • A) ATP

    • B) ADP

    • C) Fructose-1,6-bisphosphate

    • D) NADH
      Answer: A) ATP

  6. What happens to pyruvate under aerobic conditions?

    • A) Converted to lactate

    • B) Converted to ethanol

    • C) Converted to acetyl-CoA

    • D) Converted to glucose
      Answer: C) Converted to acetyl-CoA

  7. During glycolysis, how many ATP molecules are used in the energy investment phase?

    • A) 1

    • B) 2

    • C) 4

    • D) 0
      Answer: B) 2

  8. Which of the following statements about glycolysis is true?

    • A) It requires oxygen to proceed.

    • B) It is the only pathway for glucose metabolism.

    • C) It occurs in all living cells.

    • D) It produces more ATP than the Krebs cycle.
      Answer: C) It occurs in all living cells.


  1. What is the final product of glycolysis?

    • A) Glucose

    • B) Fructose-1,6-bisphosphate

    • C) Pyruvate

    • D) Acetyl-CoA
      Answer: C) Pyruvate

  2. Where does glycolysis take place in the cell?

    • A) Mitochondria

    • B) Cytosol

    • C) Nucleus

    • D) Endoplasmic reticulum
      Answer: B) Cytosol

  3. Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the preparatory phase of glycolysis?

    • A) Phosphoglucose isomerase

    • B) Hexokinase

    • C) Phosphofructokinase

    • D) Pyruvate kinase
      Answer: B) Hexokinase

  4. In the energy payoff phase of glycolysis, what is produced alongside ATP?

    • A) Oxygen

    • B) NADH

    • C) FADH2

    • D) Acetyl-CoA
      Answer: B) NADH

  5. Which of the following molecules inhibits phosphofructokinase (PFK)?

    • A) ATP

    • B) ADP

    • C) Fructose-1,6-bisphosphate

    • D) NADH
      Answer: A) ATP

  6. What happens to pyruvate under aerobic conditions?

    • A) Converted to lactate

    • B) Converted to ethanol

    • C) Converted to acetyl-CoA

    • D) Converted to glucose
      Answer: C) Converted to acetyl-CoA

  7. During glycolysis, how many ATP molecules are used in the energy investment phase?

    • A) 1

    • B) 2

    • C) 4

    • D) 0
      Answer: B) 2

  8. Which of the following statements about glycolysis is true?

    • A) It requires oxygen to proceed.

    • B) It is the only pathway for glucose metabolism.

    • C) It occurs in all living cells.

    • D) It produces more ATP than the Krebs cycle.
      Answer: C) It occurs in all living cells.


  1. What is the final product of glycolysis?

    • A) Glucose

    • B) Fructose-1,6-bisphosphate

    • C) Pyruvate

    • D) Acetyl-CoA
      Answer: C) Pyruvate

  2. Where does glycolysis take place in the cell?

    • A) Mitochondria

    • B) Cytosol

    • C) Nucleus

    • D) Endoplasmic reticulum
      Answer: B) Cytosol

  3. Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the preparatory phase of glycolysis?

    • A) Phosphoglucose isomerase

    • B) Hexokinase

    • C) Phosphofructokinase

    • D) Pyruvate kinase
      Answer: B) Hexokinase

  4. In the energy payoff phase of glycolysis, what is produced alongside ATP?

    • A) Oxygen

    • B) NADH

    • C) FADH2

    • D) Acetyl-CoA
      Answer: B) NADH

  5. Which of the following molecules inhibits phosphofructokinase (PFK)?

    • A) ATP

    • B) ADP

    • C) Fructose-1,6-bisphosphate

    • D) NADH
      Answer: A) ATP

  6. What happens to pyruvate under aerobic conditions?

    • A) Converted to lactate

    • B) Converted to ethanol

    • C) Converted to acetyl-CoA

    • D) Converted to glucose
      Answer: C) Converted to acetyl-CoA

  7. During glycolysis, how many ATP molecules are used in the energy investment phase?

    • A) 1

    • B) 2

    • C) 4

    • D) 0
      Answer: B) 2

  8. Which of the following statements about glycolysis is true?

    • A) It requires oxygen to proceed.

    • B) It is the only pathway for glucose metabolism.

    • C) It occurs in all living cells.

    • D) It produces more ATP than the Krebs cycle.
      Answer: C) It occurs in all living cells.


  1. What is the final product of glycolysis?

    • A) Glucose

    • B) Fructose-1,6-bisphosphate

    • C) Pyruvate

    • D) Acetyl-CoA
      Answer: C) Pyruvate

  2. Where does glycolysis take place in the cell?

    • A) Mitochondria

    • B) Cytosol

    • C) Nucleus

    • D) Endoplasmic reticulum
      Answer: B) Cytosol

  3. Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the preparatory phase of glycolysis?

    • A) Phosphoglucose isomerase

    • B) Hexokinase

    • C) Phosphofructokinase

    • D) Pyruvate kinase
      Answer: B) Hexokinase

  4. In the energy payoff phase of glycolysis, what is produced alongside ATP?

    • A) Oxygen

    • B) NADH

    • C) FADH2

    • D) Acetyl-CoA
      Answer: B) NADH

  5. Which of the following molecules inhibits phosphofructokinase (PFK)?

    • A) ATP

    • B) ADP

    • C) Fructose-1,6-bisphosphate

    • D) NADH
      Answer: A) ATP

  6. What happens to pyruvate under aerobic conditions?

    • A) Converted to lactate

    • B) Converted to ethanol

    • C) Converted to acetyl-CoA

    • D) Converted to glucose
      Answer: C) Converted to acetyl-CoA

  7. During glycolysis, how many ATP molecules are used in the energy investment phase?

    • A) 1

    • B) 2

    • C) 4

    • D) 0
      Answer: B) 2

  8. Which of the following statements about glycolysis is true?

    • A) It requires oxygen to proceed.

    • B) It is the only pathway for glucose metabolism.

    • C) It occurs in all living cells.

    • D) It produces more ATP than the Krebs cycle.
      Answer: C) It occurs in all living cells.


  1. What is the final product of glycolysis?

    • A) Glucose

    • B) Fructose-1,6-bisphosphate

    • C) Pyruvate

    • D) Acetyl-CoA
      Answer: C) Pyruvate

  2. Where does glycolysis take place in the cell?

    • A) Mitochondria

    • B) Cytosol

    • C) Nucleus

    • D) Endoplasmic reticulum
      Answer: B) Cytosol

  3. Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the preparatory phase of glycolysis?

    • A) Phosphoglucose isomerase

    • B) Hexokinase

    • C) Phosphofructokinase

    • D) Pyruvate kinase
      Answer: B) Hexokinase

  4. In the energy payoff phase of glycolysis, what is produced alongside ATP?

    • A) Oxygen

    • B) NADH

    • C) FADH2

    • D) Acetyl-CoA
      Answer: B) NADH

  5. Which of the following molecules inhibits phosphofructokinase (PFK)?

    • A) ATP

    • B) ADP

    • C) Fructose-1,6-bisphosphate

    • D) NADH
      Answer: A) ATP

  6. What happens to pyruvate under aerobic conditions?

    • A) Converted to lactate

    • B) Converted to ethanol

    • C) Converted to acetyl-CoA

    • D) Converted to glucose
      Answer: C) Converted to acetyl-CoA

  7. During glycolysis, how many ATP molecules are used in the energy investment phase?

    • A) 1

    • B) 2

    • C) 4

    • D) 0
      Answer: B) 2

  8. Which of the following statements about glycolysis is true?

    • A) It requires oxygen to proceed.

    • B) It is the only pathway for glucose metabolism.

    • C) It occurs in all living cells.

    • D) It produces more ATP than the Krebs cycle.
      Answer: C) It occurs in all living cells.