27 Questions
What is the primary purpose of the preparatory phase of glycolysis?
To convert glucose into two three-carbon units
Which of the following is NOT a characteristic of the payoff phase of glycolysis?
It consumes two NADH
What is the net yield of ATP per molecule of glucose converted to pyruvate in glycolysis?
2 ATP
Which of the following is a target of tight regulation in glycolysis?
The irreversible reactions
Which of the following is a key step in the activation of the glycolytic pathway?
Phosphorylation of glucose
What is the end product of the hydrolysis of lactose by the enzyme lactase?
Glucose and galactose
Which of the following monosaccharides does NOT directly enter glycolysis?
Galactose
What is the main purpose of the Cori cycle in the context of glycolysis?
To transport lactate from peripheral tissues to the liver for gluconeogenesis
Under what conditions is pyruvate reduced to lactate instead of being oxidized to acetyl-CoA?
Under anaerobic conditions or low oxygen conditions (hypoxia)
What is the purpose of the isomerization of glucose-1-phosphate to glucose-6-phosphate before entering glycolysis?
To enable the molecule to be directly utilized in glycolysis
What is the primary purpose of the oxidation of pyruvate to acetyl-CoA under aerobic conditions?
To allow pyruvate to enter the citric acid cycle for further energy production
What is the primary purpose of the phosphorylation of glucose in the first step of glycolysis?
To trap the glucose molecule inside the cell and prevent it from leaving.
In the preparatory phase of glycolysis, what is the purpose of converting the six-carbon glucose molecule into two three-carbon units?
To prepare the molecules for the oxidation step in the payoff phase.
Which statement best describes the role of ATP in the preparatory phase of glycolysis?
ATP is required as an energy source for the phosphorylation of glucose in the first step.
Why are glucose and fructose predominantly present in their cyclized forms in solution, according to the text?
The cyclized forms are more readily recognized and processed by the enzymes in the glycolytic pathway.
Which of the following statements accurately describes the role of phosphofructokinase-1 in glycolysis?
It catalyzes an irreversible, highly exergonic reaction and is regulated by ATP, fructose-2,6-bisphosphate, and other metabolites.
What is the rationale behind the cleavage of fructose 1,6-bisphosphate in glycolysis?
To convert a six-carbon sugar into two three-carbon sugars, which are high-energy phosphate sugars.
Which type of catalysis is employed by animal and plant aldolases in the cleavage of fructose 1,6-bisphosphate?
Covalent catalysis
What is the rationale behind the interconversion of triose phosphates in glycolysis?
To allow glycolysis to proceed by one pathway, as aldolase creates two triose phosphates, but only one is the substrate for the next enzyme.
Which of the following statements accurately describes the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate in glycolysis?
This step uses NAD+ and produces NADH, marking the beginning of the payoff phase of glycolysis.
What is the significance of keeping the concentration of glyceraldehyde 3-phosphate (GAP) low in glycolysis?
It pulls the thermodynamically unfavorable interconversion of triose phosphates forward.
What is the primary role of lactic acid fermentation in skeletal muscle during strenuous exercise?
To regenerate NAD+ for continued glycolysis under anaerobic conditions
What is the primary reason for the acidification of muscle during strenuous exercise?
The accumulation of lactic acid from lactic acid fermentation
What is the primary purpose of the Cori cycle?
To provide a mechanism for metabolic cooperation between skeletal muscle and the liver
Which of the following statements about lactic acid fermentation in red blood cells (RBCs) is correct?
Lactic acid fermentation in RBCs is a continuous process due to the lack of mitochondria
What is the primary reason for the high oxygen consumption during the recovery phase after strenuous exercise?
To fuel the process of gluconeogenesis in the liver
Which of the following statements about fermentation in the production of food is correct?
Fermentation in food production involves the reduction of pyruvate to lactate or ethanol without consuming oxygen
Study Notes
Glycolysis Phases
- Glycolysis has two phases: a preparatory phase and a payoff phase.
- The preparatory phase (steps 1-5) converts six-carbon glucose into two three-carbon units, each phosphorylated.
- The payoff phase (steps 6-10) initiates the oxidation of the three-carbon units.
Preparatory Phase
- Input: 2 ATP, 2 NAD+
- Output: 2 glyceraldehyde 3-phosphate, pyruvate
- For each glucose molecule, 2 ATP are consumed in the preparatory phase and 4 ATP are produced in the payoff phase, giving a net yield of 2 ATP per molecule of glucose converted to pyruvate.
- For each glucose molecule, 2 NAD+ are consumed in step 6 and need to be regenerated for glycolysis to continue.
- Output: 4 ATP, 2 NADH
Irreversible Reactions
- Three irreversible reactions are the target of tight regulation.
- These reactions are bypassed during gluconeogenesis.
Feeder Pathways
- Glycogen is cleaved by glycogen phosphorylase, yielding glucose-1-phosphate, which is then isomerized into glucose-6-phosphate, entering glycolysis.
- Disaccharides are hydrolyzed by different enzymes into monosaccharides that enter glycolysis.
- Monosaccharides fructose, galactose, and mannose enter glycolysis at different points.
Catabolic Fates of Pyruvate
- Two possible catabolic fates of pyruvate:
- Under aerobic conditions, pyruvate is oxidized to acetyl-CoA.
- Under anaerobic conditions or low oxygen condition (hypoxia), pyruvate is reduced to lactate or ethanol.
- Pyruvate also serves as a precursor in many anabolic reactions.
Mitochondria and Cytosol
- Pyruvate can be oxidized to acetyl-CoA in mitochondria (high oxidative capacity tissues).
- Under anaerobic conditions, pyruvate is reduced to lactate or ethanol in the cytosol.
The Preparatory Phase of Glycolysis
- Requires ATP.
- Step 1: phosphorylation of glucose, using the energy of ATP, catalyzed by hexokinase.
- Step 4: cleavage of fructose 1-6-bisphosphate, rationale being the cleavage of a six-carbon sugar into two three-carbon sugars.
- Step 5: interconversion of the trioses phosphate, rationale being the conversion of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (GAP).
The Payoff Phase of Glycolysis
- Yields ATP and NADH.
- Step 6: oxidation of glyceraldehyde 3-phosphate to 1-3 bisphosphoglycerate, using NAD+ and producing NADH.
Fate of Pyruvate
- Anaerobic glycolysis fermentation: reduction of pyruvate to another product (lactate in animals, ethanol in yeast).
- Generation of energy (2 ATP) without consuming oxygen or NAD+.
- No net change in oxidation state of the sugars.
- Regenerates NAD+ for further glycolysis under anaerobic conditions.
Cori Cycle
- Metabolic cooperation between skeletal muscle and the liver.
- Extremely active muscles use glycogen as an energy source, generating lactate via glycolysis.
- During recovery, some of this lactate is transported to the liver and converted to glucose via gluconeogenesis.
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