Questions and Answers
What substance can change into 5, 4, 7, 3 carbon sugars?
Glucose
Only 5 molecules of glucose can be resynthesized for every 6 molecules of glucose.
True
What pathway does H can enter to form ATP?
Oxidative phosphorylation pathway
Which molecule does H combine with to form NADPH?
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When glucose is not immediately required for energy, what is it stored as?
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What hormone is important in the regulation of gluconeogenesis?
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ATP promotes the activity of phosphofructokinase.
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What is produced when there is a buildup of pyruvic acid and H atoms under anaerobic conditions?
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What happens to lactic acid when oxygen becomes available again?
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What is the normal blood glucose concentration of a person who has fasted for 3-4 hours?
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Which hormone increases blood glucose levels?
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What is the central role of glucose in carbohydrate metabolism?
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What are the final products of carbohydrate digestion?
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Fructose and galactose can be used directly by cells for energy.
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What is glycogenesis?
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What happens during glycolysis?
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Which of the following statements is true about the Krebs cycle?
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Glycogenolysis breaks down glycogen to form glucose.
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What is oxidative phosphorylation?
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The oxidation of hydrogen atoms during metabolism primarily occurs in the __________.
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What role does insulin play in glucose transport?
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What is the total net gain of ATP from one molecule of glucose metabolism?
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Study Notes
Carbohydrate Metabolism Overview
- Central to energy production in cells through oxidation of carbohydrates, fats, and proteins.
- Free energy from the complete oxidation of 1 mole of glucose: 686,000 calories.
- ATP serves as the energy currency of the body, continuously converted from ADP via energy from food oxidation.
Formation and Storage of Glycogen
- Glucose enters cells, phosphorylated by enzymes like glucokinase (in the liver) and hexokinase (in other cells) to capture glucose inside.
- Phosphorylation is irreversible except in liver, renal, and intestinal cells due to glucose phosphatase.
- Glycogenesis: conversion of glucose-6-phosphate to glucose-1-phosphate, which is polymerized to form glycogen for storage in liver and muscle.
- Glycogenolysis: breakdown of glycogen to release glucose while phosphorylated by the enzyme phosphorylase.
Hormonal Regulation
- Glucagon, secreted by alpha cells of the pancreas, stimulates glycogenolysis when blood glucose is low.
- Epinephrine increases glucose availability during sympathetic activation by stimulating glycogen breakdown in liver and muscles.
Metabolic Pathways
- Glycolysis: Splitting glucose into 2 pyruvic acid molecules through a series of enzyme-catalyzed reactions, yielding a net gain of 2 ATP.
- Conversion to Acetyl-CoA: Pyruvic acid is converted to Acetyl-CoA, releasing CO2 and producing no ATP.
- Krebs Cycle: Acetyl-CoA enters the cycle, resulting in the degradation of acetyl into CO2 and H+; produces 2 ATP per molecule of glucose metabolized.
- Oxidative Phosphorylation: Majority of ATP (approximately 90%) is synthesized here from H+ released in earlier steps through electron transport and chemiosmotic mechanisms.
Pentose Pathway
- Operates as an alternative pathway for glucose oxidation primarily in liver and fat cells, crucial when Krebs cycle enzymes are deficient.
- Converts glucose into 5-carbon sugars, facilitating the resynthesis of glucose and fat synthesis via NADPH production.
Control of Glycolysis
- Regulated by the concentrations of ATP and ADP; high ATP inhibits the enzyme phosphofructokinase, slowing glycolysis when energy is abundant.
- This mechanism ensures the energy needs of cells dictate the rate of carbohydrate oxidation.
Glucose Storage
- Excess glucose not needed immediately is stored as glycogen in cells, providing energy for 12-24 hours.
- Surplus glucose beyond storage capacity is converted into fat, stored in adipose tissues for long-term energy needs.### Glycolysis Regulation
- Excess ATP slows or stops glycolysis, halting carbohydrate metabolism.
- Increased ADP concentration from ATP usage boosts phosphofructokinase activity, initiating glycolysis.
- Citrate, a product of the citric acid cycle, inhibits phosphofructokinase and glycolytic processes.
Anaerobic Metabolism
- Oxygen deficiency halts oxidative phosphorylation, but glycolysis can still occur.
- Glycolysis converts glucose to pyruvic acid without oxygen, albeit inefficiently, providing crucial energy.
- Lactic acid is produced under anaerobic conditions when pyruvic acid and hydrogen ions accumulate.
Lactic Acid Formation and Effects
- Excess pyruvic acid and hydrogen ions slow glycolysis; their reaction forms lactic acid.
- Lactic acid diffuses to less active cells, allowing continued glycolysis in active ones.
- Upon oxygen reintroduction, lactic acid converts back to pyruvic acid and contributes to ATP production.
Utilization of Lactic Acid
- The heart can metabolize lactic acid into pyruvic acid for energy during heavy exercise.
- Muscle activity produces lactic acid, which is used as an additional energy source by the heart.
Gluconeogenesis
- Gluconeogenesis converts amino acids and glycerol into glucose when carbohydrate stores are low, especially during fasting.
- Ensures stable blood glucose levels, critical for brain energy needs, provided largely by the liver.
- The liver can synthesize glucose from lactate and amino acids, with 25% of liver glucose production derived from gluconeogenesis.
Amino Acid Conversion to Glucose
- Different amino acids undergo distinct chemical processes to convert to glucose, e.g., alanine through deamination to pyruvic acid.
- More complex amino acids can be converted to simple sugars, entering the phosphogluconate pathway to form glucose.
Regulation of Gluconeogenesis
- Stimulated by low carbohydrate availability and decreased blood sugar levels.
- Diminished carbohydrates enable conversion of amino acids and glycerol into carbohydrates, regulated by cortisol.
Hormonal Regulation
- Low carbohydrate availability triggers increased secretion of adrenocorticotropic hormone (ACTH) from the adenohypophysis.
- ACTH stimulates cortisol release from the adrenal cortex, mobilizing proteins for amino acid availability.
- Cortisol promotes gluconeogenesis by providing substrates for glucose conversion in the liver.
Blood Glucose Levels
- Normal fasting blood glucose is approximately 90 mg/dl. After high carbohydrate meals, levels can exceed 140 mg/dl.
- Regulation of blood glucose involves insulin (lowers levels) and glucagon (raises levels), managed by pancreatic hormones.
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Description
This quiz covers key aspects of carbohydrate metabolism, including the formation and storage of glycogen, and an in-depth comparison of metabolic pathways like glycolysis and the Krebs cycle. Learn about the regulatory mechanisms that control these processes, essential for understanding energy production in the body.
