Podcast
Questions and Answers
What is the first stage of glucose oxidation in the mitochondrial pathway?
What is the first stage of glucose oxidation in the mitochondrial pathway?
Which enzyme complex is responsible for the oxidative decarboxylation of pyruvate?
Which enzyme complex is responsible for the oxidative decarboxylation of pyruvate?
How many coenzymes are required by the pyruvate dehydrogenase complex for its function?
How many coenzymes are required by the pyruvate dehydrogenase complex for its function?
What is the primary role of the Krebs cycle?
What is the primary role of the Krebs cycle?
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Where does the citric acid cycle take place?
Where does the citric acid cycle take place?
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What initiates the digestion of carbohydrates in the mouth?
What initiates the digestion of carbohydrates in the mouth?
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What is the final product of lactose digestion?
What is the final product of lactose digestion?
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Where does glycolysis primarily occur?
Where does glycolysis primarily occur?
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What is the major byproduct of glycolysis in aerobic conditions?
What is the major byproduct of glycolysis in aerobic conditions?
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What happens to galactose and fructose once absorbed?
What happens to galactose and fructose once absorbed?
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Which pathway provides energy to muscle cells during contraction when oxygen is lacking?
Which pathway provides energy to muscle cells during contraction when oxygen is lacking?
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What is one of the roles of 2,3 bisphosphoglycerate (BPG) produced during glycolysis?
What is one of the roles of 2,3 bisphosphoglycerate (BPG) produced during glycolysis?
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Which enzyme is responsible for the digestion of maltose into glucose?
Which enzyme is responsible for the digestion of maltose into glucose?
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What is the function of the Krebs cycle in metabolism?
What is the function of the Krebs cycle in metabolism?
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Which substance is used in the synthesis of heme?
Which substance is used in the synthesis of heme?
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What is the primary site of the Hexose Monophosphate Shunt?
What is the primary site of the Hexose Monophosphate Shunt?
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Which enzyme deficiency is associated with Favism?
Which enzyme deficiency is associated with Favism?
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What effect does insulin have on blood glucose levels?
What effect does insulin have on blood glucose levels?
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Which of the following compounds reduces oxidative stress in the body?
Which of the following compounds reduces oxidative stress in the body?
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What is the significance of CO2 in the synthesis of fatty acids?
What is the significance of CO2 in the synthesis of fatty acids?
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What hormone is secreted from the α cells of the pancreas to increase blood glucose levels?
What hormone is secreted from the α cells of the pancreas to increase blood glucose levels?
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Which of the following is a product of gluconeogenesis?
Which of the following is a product of gluconeogenesis?
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Which hormone is responsible for inhibiting glucose uptake by tissues?
Which hormone is responsible for inhibiting glucose uptake by tissues?
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What is a common characteristic of Type I Diabetes Mellitus?
What is a common characteristic of Type I Diabetes Mellitus?
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Under fasting conditions, which processes are predominantly occurring in the liver?
Under fasting conditions, which processes are predominantly occurring in the liver?
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What is the renal threshold for glucose above which glucose appears in urine?
What is the renal threshold for glucose above which glucose appears in urine?
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Which symptom is associated with hyperglycemia?
Which symptom is associated with hyperglycemia?
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Which condition is associated with an abnormal low renal threshold for glucose?
Which condition is associated with an abnormal low renal threshold for glucose?
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What distinguishes a hyperglycemic coma from a hypoglycemic coma in terms of skin condition?
What distinguishes a hyperglycemic coma from a hypoglycemic coma in terms of skin condition?
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At what fasting blood glucose level is diabetes mellitus diagnosed?
At what fasting blood glucose level is diabetes mellitus diagnosed?
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What is the condition characterized by blood glucose levels greater than 126 mg/dl when fasting?
What is the condition characterized by blood glucose levels greater than 126 mg/dl when fasting?
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Which hormone is most likely to cause hypoglycemia if administered excessively?
Which hormone is most likely to cause hypoglycemia if administered excessively?
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Which type of diabetes commonly occurs in childhood?
Which type of diabetes commonly occurs in childhood?
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What is a symptom of hypoglycemia that can occur if not managed in time?
What is a symptom of hypoglycemia that can occur if not managed in time?
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What is a possible complication more common in Type I Diabetes Mellitus?
What is a possible complication more common in Type I Diabetes Mellitus?
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Which metabolic response occurs due to low insulin levels in diabetes?
Which metabolic response occurs due to low insulin levels in diabetes?
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What is the typical effect of oral drugs in Type I Diabetes Mellitus?
What is the typical effect of oral drugs in Type I Diabetes Mellitus?
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Study Notes
Carbohydrate Digestion
- Digestion starts in the mouth with salivary amylase which acts on cooked starch and glycogen.
- Pancreatic amylase acts in the small intestine at pH 7.1, breaking down cooked and uncooked starch.
- Final digestion is completed by intestinal enzymes: lactase, maltase, sucrase, and dextrinase.
- Cellulose is not digested by humans due to the lack of the enzyme cellulase.
Fate of Absorbed Sugar
- Absorbed sugar is primarily taken up by the liver where galactose and fructose are converted to glucose.
- Glucose can be utilized by tissues through oxidation, storage, or conversion.
- Oxidation: Glucose is oxidized for ATP production through glycolysis and the Krebs cycle. The hexose monophosphate (HMP) shunt also utilizes glucose to produce ribose, NADPH, and glucuronic acid.
- Storage: Glucose can be stored as glycogen through glycogenesis or converted to fat (lipogenesis).
- Conversion: Glucose can be converted to ribose and deoxyribose for DNA and RNA synthesis, lactose for milk, and various other compounds.
Glucose Oxidation
- All carbohydrate metabolic pathways occur in the cytoplasm except for the Krebs cycle, which takes place in the mitochondria.
- Complete glucose oxidation to CO2 and H2O involves both glycolysis (cytoplasm) and the Krebs cycle (mitochondria).
- Anaerobic glycolysis occurs in the absence of mitochondria (RBCs) or during oxygen deprivation (muscle exercise).
Glycolysis
- Also known as the Embden-Meyerhof pathway, glycolysis is the anaerobic phase of glucose oxidation.
- Site: Cytoplasm of all tissues but especially important in muscle during exercise due to oxygen lack and in RBCs due to the absence of mitochondria.
Function of Glycolysis
- It is the primary source of energy for muscle contraction and for red blood cells.
- Produces 8 or 6 ATP in the presence of oxygen.
- Generates 2,3-bisphosphoglycerate (BPG) which reduces hemoglobin's affinity for oxygen, facilitating oxygen delivery to tissues.
- Produces pyruvate, which can enter the Krebs cycle.
- DHAP can be converted to glycerol-3-phosphate, crucial for lipogenesis.
- Produces two amino acids: serine (from 3-phosphoglycerate) and alanine (from pyruvate).
Mitochondrial Pathway for Glucose Oxidation
- The complete oxidation of glucose to CO2 and H2O involves both glycolysis (cytoplasm) and the Krebs cycle (mitochondria).
- First stage: Oxidative decarboxylation of pyruvate to acetyl CoA.
- Second stage: Krebs cycle.
Oxidative Decarboxylation of Pyruvate to Acetyl CoA
- Catalyzed by the pyruvate dehydrogenase complex (PDC).
- Requires five coenzymes: TPP (thiamin pyrophosphate), lipoic acid, CoASH, FAD, and NAD.
- Pyruvate is converted into acetyl-CoA.
Citric Acid Cycle (CAC), Krebs Cycle or Tricarboxylic Acid Cycle (TCA)
- Site: Mitochondria
- Amphibolic role: Involved in both anabolic and catabolic reactions.
- Catabolic role: Produces 12 ATP for each acetyl-CoA. Involved in the complete oxidation of carbohydrates, lipids, and proteins.
- Anabolic role: Involved in heme synthesis (succinyl CoA + glycine), amino acid synthesis through transamination (glutamate to α-ketoglutarate and aspartate to oxaloacetate), fatty acid synthesis, gluconeogenesis, and the production of CO2 for various biosynthetic pathways.
Hexose Monophosphate Shunt (HMP Shunt), Pentose Shunt
- Site: Cytoplasm of the liver.
-
Functions:
- Production of pentoses (required for DNA and RNA synthesis).
- Production of NADPH (required for fatty acid and cholesterol synthesis, sphingosine and galactolipid synthesis, glucuronic acid synthesis, non-essential amino acid synthesis, and reduction of glutathione).
Favism
- Deficiency in glucose-6-phosphate dehydrogenase (G6PD) leads to hemolysis of red blood cells, especially following the intake of fava beans.
- Mechanism: Reduced G6PD leads to decreased NADPH production, resulting in a reduced ability to reduce glutathione. This leaves the cells vulnerable to oxidative stress, causing hemolysis.
Blood Glucose
- Fasting blood glucose levels: 70-110 mg/dl.
- One hour after a meal: 120-150 mg/dl.
- Regulation of blood glucose involves hormonal and hepatic mechanisms.
Hormonal Regulation of Blood Glucose
-
Insulin: Secreted by beta cells of the pancreas. Decreases blood glucose by:
- Facilitating glucose uptake into cells.
- Stimulating glycolysis (activation of hexokinase, PFK1, pyruvate kinase).
- Stimulating glycogenesis.
- Inhibiting glycogenolysis.
- Inhibiting gluconeogenesis.
- Stimulating lipogenesis.
- Stimulating protein synthesis.
-
Anti-Insulin Hormones:
- Glucagon (secreted from alpha cells of pancreas): Increases blood glucose by stimulating glycogenolysis and gluconeogenesis.
- Catecholamines (epinephrine and norepinephrine): Stimulate glycogenolysis and inhibit glucose uptake by the liver.
- Corticosteroids (glucocorticoids): Stimulate gluconeogenesis and inhibit glucose uptake by tissues.
- Growth hormone: Inhibits glucose uptake and reduces insulin sensitivity.
- Thyroid hormone: Decreases insulin sensitivity and stimulates glycogenolysis.
Hepatic Regulation of Blood Glucose
- Fasting: Glycogenolysis and gluconeogenesis dominate.
- After a meal: Glycogenesis and lipogenesis are favored.
Renal Regulation of Blood Glucose
- Renal Threshold: The blood glucose level above which glucose appears in urine (180 mg/dl).
- Abnormal Low Renal Threshold: Occurs when glucose appears in urine even with normal insulin levels due to kidney disease (e.g., 25% of pregnancies).
- Abnormal High Renal Threshold: Glucose does not appear in urine even in diabetic patients (e.g., older individuals).
Variation in Blood Glucose
-
Hyperglycemia: Elevated blood glucose levels (fasting >126 mg/dl, 1 hour after meal >200 mg/dl).
- Causes: Decreased insulin (diabetes mellitus or surgical removal of pancreas), increased anti-insulin hormones (adrenaline, cortisone, thyroid hormone, growth hormone).
-
Hypoglycemia: Low blood glucose level (< 40 mg/dl). More dangerous than hyperglycemia as the brain relies on glucose.
- Symptoms: confusion, dizziness, tremors, weakness, tachycardia, coma if untreated.
- Causes: Increased insulin (excessive insulin dosage, missed meals, insulinoma), decreased anti-insulin hormones (glucocorticoid deficiency, pituitary hormone deficiency, glycogen storage diseases, fructosemia, and galactosemia).
Diabetes Mellitus
- Definiton: Hyperglycemia and glucosuria.
- Type 1: Insulin-dependent diabetes mellitus (IDDM). Often diagnosed during childhood. Usually underweight. Has an autoimmune component. Insufficient insulin production. Requires insulin therapy. More prone to ketoacidosis.
- Type 2: Non-insulin dependent diabetes mellitus (NIDDM). Diagnosed after 35 years. Usually obese. Moderate genetic predisposition. May have normal insulin levels but exhibit insulin resistance. Oral drugs may be effective. Insulin therapy may not be required. Less prone to ketoacidosis.
Manifestation of Diabetes Mellitus
-
Carbohydrate Metabolism:
- Decreased insulin leads to decreased glucose uptake by cells, resulting in polyphagia (excessive eating)
- Hyperglycemia (due to decreased glucose oxidation and increased gluconeogenesis) leads to increased blood osmolarity, resulting in dehydration and polydipsia (excessive drinking).
- Glucosuria (due to hyperglycemia) leads to increased osmotic diuresis, causing polyuria (excessive urination) and loss of water-soluble vitamins.
-
Protein Metabolism:
- Decreased insulin leads to increased protein catabolism, resulting in muscle wasting, decreased antibody formation, increased susceptibility to infections, and poor wound healing.
-
Lipid Metabolism:
- Decreased insulin increases lipolysis in adipose tissue, resulting in weight loss, fatty liver, increased free fatty acids, hypercholesterolemia, and atherosclerosis.
-
Microangiopathy: Degeneration of small blood vessels.
- Retinopathy: Affects the retina, leading to blindness.
- Nephropathy: Affects the kidney, leading to renal failure.
Most Frequent Symptoms of Diabetes
- Polyphagia (excessive hunger)
- Polyuria (excessive urination)
- Polydipsia (excessive thirst)
Difference Between Hypoglycemia and Hyperglycemic Coma
| Coma Type | Odour | Dehydration | Pulse | History | Treatment |
|---|---|---|---|---|---|
| Hyperglycemic Coma | Acetone odour in nose | Present (dry skin, sunken eye) | Normal | Diabetes Mellitus | Insulin and potassium |
| Hypoglycemic Coma | No odour | Absent | Rapid and weak | Insulin injection | Glucose injection |
Diagnosis of Diabetes Mellitus
| Condition | Fasting Blood Glucose (mg/dl) | 1 Hour After Meal Blood Glucose (mg/dl) |
|---|---|---|
| Normal | < 110 | < 140 |
| Impaired Glucose Tolerance | 110 - 125 | > 140 - 200 |
| Diabetes Mellitus | > 126 | > 200 |
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Description
Explore the intricacies of carbohydrate digestion and the metabolic fate of absorbed sugars. This quiz covers the enzymatic breakdown of carbohydrates and the physiological processes involving glucose, including oxidation, storage, and conversion. Test your knowledge on the importance of carbohydrates in human metabolism.
