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Questions and Answers
What is the primary function of glycolysis in cellular metabolism?
What is the primary function of glycolysis in cellular metabolism?
Which statement accurately describes the phases of glycolysis?
Which statement accurately describes the phases of glycolysis?
How many ATP molecules are utilized and generated during glycolysis?
How many ATP molecules are utilized and generated during glycolysis?
What are the final products of glycolysis from one molecule of glucose?
What are the final products of glycolysis from one molecule of glucose?
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Which of the following metabolic pathways is NOT directly associated with glycolysis?
Which of the following metabolic pathways is NOT directly associated with glycolysis?
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Which enzyme is primarily responsible for phosphorylating glucose to glucose-6-phosphate in glycolysis?
Which enzyme is primarily responsible for phosphorylating glucose to glucose-6-phosphate in glycolysis?
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Which of the following statements about the glycolytic pathway is true?
Which of the following statements about the glycolytic pathway is true?
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What is the role of glucose-6-phosphate (G6P) in muscle hexokinase regulation?
What is the role of glucose-6-phosphate (G6P) in muscle hexokinase regulation?
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Which molecule is produced as a result of the hexokinase-catalyzed reaction in glycolysis?
Which molecule is produced as a result of the hexokinase-catalyzed reaction in glycolysis?
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What distinguishes glucokinase from hexokinase?
What distinguishes glucokinase from hexokinase?
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Study Notes
Glycolysis: A Starting Point of Metabolism
- Glycolysis is the central pathway for glucose breakdown in most organisms.
- Glucose is a primary energy source.
- Complete oxidation of glucose to carbon dioxide and water releases significant energy (ΔG' = -2840 kJ/mol).
- Glycolysis is a central pathway for glucose catabolism. It's the sole energy source for some tissues (e.g., brain, erythrocytes).
- Glycolysis was the first metabolic pathway elucidated.
- The pathway consists of 10 enzyme-catalyzed reactions.
- The overall reaction of glycolysis is: Glucose + 2NAD+ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2ATP + 2H₂O + 2H+
General Organisation of Metabolic Pathways
- Metabolic pathways are sequences of reactions.
- Small changes accumulate to cause significant alterations.
- Location within the cell influences the reaction. (e.g., brain, muscle, gut, cytosol, mitochondria, nucleus).
- Most pathways are linear, while some are cyclic.
General Overview of Glycolysis
- Molecules are broken down into smaller molecules gradually. (e.g., polysaccharides are broken down into simple sugars).
- The end product of stage 2 is Acetyl-CoA.
- Stage 3 involves oxidation of Acetyl CoA in the TCA cycle which generates electrons.
- These electrons are transferred via carriers (e.g., NADH) to the mitochondrial membrane, initiating the electron transport chain.
- Oxygen is the final electron acceptor, reduced to water.
- This process drives ATP generation (oxidative phosphorylation).
- Carbohydrates, proteins, and lipids all feed into the common pathways for energy metabolism via Acetyl CoA.
Glycolysis: A Set of Sequential Enzyme Reactions
- Glycolysis breaks down glucose into two 3-carbon molecules.
- The released free energy is stored as ATP and NADH.
- The process consists of two phases:
- Phase I (steps 1-5): ATP is used to raise the free energy of intermediates.
- Phase II (steps 6-10): ATP and NADH are generated.
Glucose
- Glucose is a major energy source in most organisms.
- Starch and glycogen store large amounts of glucose.
- Glucose is delivered to cells via specific transport proteins.
Catabolism of Other Sugars
- Glycogen releases glucose residues as glucose-1-phosphate, which is converted to glucose-6-phosphate.
- Other hexose sugars (e.g., fructose, mannose, galactose) are converted to glycolytic intermediates.
Glycolysis: ATP Production and Energy Transduction
- Glycolysis produces a net gain of 2 ATP.
- Energy is released and stored by transferring electrons via NADH.
- Glycolysis is a process of energy transduction.
Fate of Pyruvate in Aerobic and Anaerobic Conditions
- Aerobic conditions: Pyruvate is converted to Acetyl-CoA, which enters the TCA cycle.
- Sufficient oxygen present
- Anaerobic conditions (fermentation): Pyruvate is used to regenerate NAD+ to keep glycolysis going.
- Lactate (muscle cells) or ethanol (yeast) form.
Gluconeogenesis
- Gluconeogenesis is the synthesis of glucose from simpler precursors (e.g., pyruvate or lactate).
- Primarily occurs in the liver and supplies glucose to other tissues when glucose sources are depleted.
- Relies on some glycolysis enzymes (reversible ones), and other enzymes for the irreversible steps.
- Replacing glycolysis enzymes controls these reactions
Control of the Glycolytic Pathway
- Certain steps are essentially irreversible due to highly negative ΔG values, making them important control points.
- Key enzymes in controlling glycolysis are regulated: Hexokinase, Phosphofructokinase, and Pyruvate Kinase.
Hexokinase
- Hexokinase phosphorylates hexose sugars (glucose being the preferred one, along with fructose and mannose).
- The reaction is essentially irreversible and a key control point.
- Muscle hexokinase is inhibited allosterically by its own product, glucose-6-phosphate.
Isoenzymes: Hexokinase and Glucokinase
- Liver cells contain glucokinase, a specific isoenzyme of hexokinase.
- Glucokinase has a different regulatory mechanism than hexokinase.
- Km values differ in order for each isoenzyme to regulate the reaction.
Phosphofructokinase
- Phosphofructokinase converts fructose-6-phosphate to fructose-1,6-bisphosphate.
- This step is unique to glycolysis (the commitment step).
- ATP is an allosteric inhibitor; high levels mean the cell doesn't need more.
- Citrate is a negative allosteric regulator of phosphofructokinase.
Pyruvate Kinase
- Pyruvate kinase converts phosphoenolpyruvate to pyruvate.
- It's in the pay-off phase of glycolysis.
- It's activated by fructose 1,6 bisphosphate (feed-forward control).
- Inhibited by ATP.
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Description
Explore the intricacies of glycolysis, a pivotal metabolic pathway for glucose breakdown essential for energy production in many organisms. This quiz covers key reactions, significance, and the general organization of metabolic pathways, emphasizing their cellular locations and overall functions.
