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Questions and Answers
What regulates the rate of glycolysis?
What regulates the rate of glycolysis?
Which enzyme is considered the rate-limiting step in the pentose-phosphate pathway?
Which enzyme is considered the rate-limiting step in the pentose-phosphate pathway?
When does glycogenesis primarily occur?
When does glycogenesis primarily occur?
What is the primary function of NADPH produced in the pentose-phosphate pathway?
What is the primary function of NADPH produced in the pentose-phosphate pathway?
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Which statement best describes how the pentose-phosphate pathway is regulated?
Which statement best describes how the pentose-phosphate pathway is regulated?
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What are the main products of the oxidative phase of the pentose-phosphate pathway?
What are the main products of the oxidative phase of the pentose-phosphate pathway?
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Which of the following conditions would likely favor the pentose-phosphate pathway over glycolysis?
Which of the following conditions would likely favor the pentose-phosphate pathway over glycolysis?
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Which metabolic process is described as glycogen breakdown?
Which metabolic process is described as glycogen breakdown?
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What is the role of acetyl-CoA in gluconeogenesis regulation?
What is the role of acetyl-CoA in gluconeogenesis regulation?
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How does fructose-2,6-bisphosphate (F-2,6-BP) influence glycolysis and gluconeogenesis?
How does fructose-2,6-bisphosphate (F-2,6-BP) influence glycolysis and gluconeogenesis?
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What effect does glucagon have on fructose-2,6-bisphosphate levels?
What effect does glucagon have on fructose-2,6-bisphosphate levels?
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Which hormone inhibits gluconeogenesis by downregulating key gluconeogenic enzymes?
Which hormone inhibits gluconeogenesis by downregulating key gluconeogenic enzymes?
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What is the primary action of glucagon in gluconeogenesis?
What is the primary action of glucagon in gluconeogenesis?
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Which of the following conditions is associated with increased gluconeogenesis?
Which of the following conditions is associated with increased gluconeogenesis?
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What is the main role of pyruvate carboxylase in gluconeogenesis?
What is the main role of pyruvate carboxylase in gluconeogenesis?
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What is a potential consequence of high levels of acetyl-CoA in the body?
What is a potential consequence of high levels of acetyl-CoA in the body?
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Which of the following amino acids are classified as non-glucogenic?
Which of the following amino acids are classified as non-glucogenic?
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What is the primary organ where gluconeogenesis occurs?
What is the primary organ where gluconeogenesis occurs?
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What is the main role of oxaloacetate in the first bypass of gluconeogenesis?
What is the main role of oxaloacetate in the first bypass of gluconeogenesis?
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Which of the following statements about bypass reactions in gluconeogenesis is true?
Which of the following statements about bypass reactions in gluconeogenesis is true?
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What catalyzes the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate in gluconeogenesis?
What catalyzes the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate in gluconeogenesis?
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In the process of gluconeogenesis, what is produced when malate is converted back to oxaloacetate?
In the process of gluconeogenesis, what is produced when malate is converted back to oxaloacetate?
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Which of the following statements about glycolysis and gluconeogenesis is correct?
Which of the following statements about glycolysis and gluconeogenesis is correct?
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Why is glucose 6-phosphatase not present in muscle and brain cells?
Why is glucose 6-phosphatase not present in muscle and brain cells?
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Study Notes
Carbohydrate Metabolism II
- Carbohydrate metabolism encompasses digestion, absorption, catabolism (glycolysis, pentose-phosphate pathway, glycogenolysis), and anabolism (gluconeogenesis, glycogenesis). Cellular respiration, specifically the citric acid cycle, is also crucial.
Gluconeogenesis
- Gluconeogenesis forms glucose from non-carbohydrate precursors like 3-carbon compounds (pyruvate, lactate, glycerol) and glucogenic amino acids.
- Primarily occurs in liver cells, with some activity in the kidneys.
Glucogenic Amino Acids
- Eighteen of the twenty amino acids are glucogenic.
- Leucine and Lysine are the two non-glucogenic amino acids.
- Glucogenic amino acids can be converted to citric acid cycle intermediates.
Gluconeogenesis - Bypass Reactions
- Gluconeogenesis is a reciprocal process to glycolysis.
- It bypasses three irreversible steps in glycolysis.
- The first bypass converts pyruvate to phosphoenolpyruvate through oxaloacetate.
1st Bypass
- Oxaloacetate is transported out of the mitochondria as malate.
- Malate is converted back to oxaloacetate.
- Oxaloacetate is further converted into phosphoenolpyruvate, a critical intermediate in gluconeogenesis.
- NADH is produced during this conversion, contributing to subsequent gluconeogenesis.
2nd and 3rd Bypass Reactions
- The second bypass involves converting fructose-1,6-bisphosphate to fructose-6-phosphate via fructose-1,6-bisphosphatase-1 enzyme.
- The third bypass converts glucose-6-phosphate to glucose via glucose-6-phosphatase.
How is Gluconeogenesis Regulated?
- Acetyl-CoA is a positive modulator of pyruvate carboxylase, pushing the pathway towards gluconeogenesis.
- It's a negative regulator of pyruvate dehydrogenase, which is essential for glycolysis.
Fructose 2,6-bisphosphate
- Fructose 2,6-bisphosphate is a potent regulator affecting both glycolysis and gluconeogenesis.
- High levels stimulate phosphofructokinase and inhibit fructose-1,6-bisphosphatase in favor of glycolysis. Low levels favor gluconeogenesis.
- Glucagon decreases F-2,6-BP levels through fructose-2,6-bisphosphatase enzyme, stimulating gluconeogenesis.
Hormonal Regulation
- Insulin inhibits gluconeogenesis when blood glucose levels are high.
- Glucagon stimulates gluconeogenesis when blood glucose levels are low.
- Cortisol and epinephrine promote gluconeogenesis during stress, ensuring a constant glucose supply for energy.
Pentose Phosphate Pathway (PPP)
- The PPP operates in parallel with glycolysis.
- It has two phases: oxidative and non-oxidative.
- G6PD is a rate-limiting step.
- Important products of the oxidative phase are ribose-5-phosphate and NADPH.
How is PPP Regulated?
- Regulation depends on the cell's needs and NADP+ concentration.
- When nucleotides need to be synthesized or cells are dividing, the PPP is favoured.
- When NADPH is required (e.g. for fatty acid, cholesterol, and steroid synthesis), the PPP is also favoured.
Glycogen Metabolism
- Glycogenesis involves glycogen synthesis.
- Glycogenolysis involves glycogen breakdown.
- Insulin promotes glycogen synthesis.
- Glucagon promotes glycogen breakdown.
Organs and Glycogen
- The liver and muscle are key organs for glycogen metabolism.
- Glycogenesis occurs during the 'fed' state when blood glucose levels are high.
- Insulin regulates blood glucose levels partially by influencing glycogen.
- Muscle glycogen is used for local energy during exercise whereas liver glycogen releases glucose into the bloodstream.
Glycogenolysis
- Glycogenolysis involves glycogen breakdown, releasing glucose units into the bloodstream.
Carbohydrate Metabolic Diseases
- Diabetes Mellitus (Type I and Type II) are examples of carbohydrate metabolic disorders.
Other Examples
- Lactose intolerance results from low lactase levels due to genetic deficiency or intestinal injury.
- Glucose-6-phosphate dehydrogenase (G6PD) deficiency impacts NADPH production, leaving cells vulnerable to oxidative damage.
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Description
Test your knowledge on carbohydrate metabolism, including key processes like gluconeogenesis and the role of glucogenic amino acids. This quiz covers the digestion, absorption, and catabolism of carbohydrates, along with their anabolic pathways. Challenge yourself on the critical bypass reactions in gluconeogenesis!
