CHO Metabolism and Metabolic Pathways

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Questions and Answers

What is the primary function of anabolic pathways in metabolism?

To break down complex molecules

To synthesize complex molecules from simpler ones (correct)

To release energy during the breakdown of glucose

To store energy in the form of fat

Which of the following is a characteristic of catabolic pathways?

They consume energy overall

They synthesize complex molecules

They only occur in the presence of oxygen

They release energy upon breakdown (correct)

What is the role of amphibolic pathways?

To synthesize fatty acids from glucose

To act as a link between anabolic and catabolic pathways (correct)

To entirely break down food molecules

To enhance the absorption of carbohydrates

Which enzyme is responsible for converting starch and glycogen into dextrins?

Salivary amylase Signup and view all the answers

Complete oxidation of 1 gram of carbohydrates yields how many kilocalories?

4 kcal Signup and view all the answers

Which of the following food sources contributes most significantly to dietary carbohydrates?

Starch from potatoes Signup and view all the answers

Lactose intolerance is caused by a deficiency in which enzyme?

Lactase Signup and view all the answers

What must happen to polysaccharides and disaccharides before they can be absorbed?

They must be converted to monosaccharides Signup and view all the answers

What is the net energy gain in aerobic glycolysis after accounting for ATP consumed?

6-8 ATP Signup and view all the answers

Which reaction illustrates substrate level phosphorylation?

1,3-biphosphoglycerate + ADP → 3-phosphoglycerate + ATP Signup and view all the answers

How do RBCs ensure energy production in the absence of mitochondria?

Through anaerobic glycolysis only Signup and view all the answers

Which enzyme is NOT one of the three key irreversible enzymes in glycolysis?

Pyruvate dehydrogenase Signup and view all the answers

What regulates the synthesis of key enzymes in glycolysis in response to hormones?

Insulin stimulates synthesis of all three key enzymes Signup and view all the answers

Which compound increases glycolysis by stimulating phosphofructokinase-1?

Fructose 1,6-bisphosphate Signup and view all the answers

Which effect do ATP and ADP have on phosphofructokinase-1 in glycolysis regulation?

ATP inhibits it; ADP stimulates it Signup and view all the answers

What is the primary end product of glycolysis in mature red blood cells?

Lactate Signup and view all the answers

What is the primary consequence of the absence of intestinal lactase?

Lactose accumulates and is fermented by intestinal bacteria. Signup and view all the answers

Which metabolic pathway is involved in the synthesis of glucose from non-carbohydrate precursors?

Gluconeogenesis Signup and view all the answers

What occurs during the energy-requiring stage of glycolysis?

Conversion of glucose to glyceraldehyde-3-P. Signup and view all the answers

What is one of the key functions of glycolysis in the metabolism of carbohydrates?

It helps in the metabolism of fructose and galactose. Signup and view all the answers

How much ATP is generated from aerobic glycolysis?

6-8 ATP Signup and view all the answers

What role does 2,3-bisphosphoglycerate play in glycolysis?

It decreases hemoglobin's oxygen affinity. Signup and view all the answers

What is the net ATP consumed during the initial steps of glycolysis?

2 ATP Signup and view all the answers

Which of the following statements about anaerobic glycolysis is correct?

It yields 2 ATP per glucose molecule. Signup and view all the answers

Study Notes

CHO Metabolism

Presented by Dr. Yasser Elghobashy, Medical Biochemistry and Molecular Biology
Focuses on the fate of food molecules after digestion and absorption.

Introduction to Metabolism

Metabolism encompasses the chemical enzymatic reactions within the body.
These reactions involve the synthesis and breakdown of various substances.

Metabolic Pathways

Anabolic pathways: These pathways involve the synthesis of complex molecules from simpler ones. This process requires energy input (endergonic).
- Example: Synthesis of proteins.
Catabolic pathways: These pathways involve the breakdown of complex molecules into simpler ones. This process releases energy (exergonic).
- Example: Oxidative processes releasing energy.
Amphibolic pathways: These act as a link between anabolic and catabolic pathways.
- Example: Citric acid cycle

Diagrammatic Representation

Food molecules undergo digestion and absorption to form simpler molecules.
The simpler molecules enter the amphibolic pathways.
Branches lead to anabolic pathways (protein, carbohydrates, lipids and nucleic acids) and catabolic pathways (CO2 and water).

Carbohydrate Metabolism

Carbohydrates contribute 50% of daily caloric intake.
Complete oxidation of 1 gram of carbohydrate yields 4 kcal.
Sources of carbohydrates in food:
- Starch (e.g., potatoes): approximately 50% of dietary carbohydrates.
- Sucrose and lactose: significant portion of the rest.
- Fructose and glucose (fruits, honey): additional sources.

Digestion of Carbohydrates

Polysaccharides and disaccharides require conversion to monosaccharides for absorption.
Enzymes involved in the process:
- Salivary amylase: breaks down starch/glycogen into dextrins.
- Pancreatic amylase: breaks down dextrins into maltose.
- Intestinal disaccharidases (maltase, sucrase, lactase) further break down disaccharides into monosaccharides:
  - Maltase: maltose to glucose
  - Sucrase: sucrose to glucose and fructose
  - Lactase: lactose to glucose and galactose

Lactose Intolerance

Definition: A disease resulting from lactase enzyme deficiency (congenital or acquired).
Cause: Deficiency of lactase enzyme.
Effects: Undigested lactose accumulates in the intestine leading to fermentation by intestinal bacteria, producing acids and gases.
Symptoms: Abdominal distension, cramps, and diarrhea.
Treatment: Lactose-free milk formula

Metabolic Pathways of Carbohydrates

Catabolic pathways: involve oxidative pathways;
- Glycolysis
- Pentose phosphate shunt (Hexose monophosphate)
- Uronic acid pathway
- Glycogenolysis
Anabolic pathways:
- Gluconeogenesis
- Glycogenesis
Amphibolic pathways:
- Citric acid cycle

Glycolysis

Definition: Glucose oxidation to pyruvate (with oxygen) or lactate (without oxygen).
Site: Cytoplasm of all cells; crucial in cells lacking mitochondria (e.g., red blood cells) and cells with frequent oxygen deficiency (e.g., muscle cells during exercise).
Stages:
- Stage 1: Energy-requiring step—glucose conversion to glyceraldehyde-3-phosphate.
- Stage 2: Energy-producing step—glyceraldehyde-3-phosphate conversion to pyruvate/lactate.

Importance of Glycolysis

Energy production: Anaerobic - 2 ATP; Aerobic - 6-8 ATP.
Main pathway: for fructose and galactose metabolism from diet.
Oxygenation of tissues: through 2,3 biphosphoglycerate (affecting Hb O2 affinity).
Precursors for other molecules: Dihydroxyacetone phosphate (glycerol-3-P for lipogenesis) and pyruvate (alanine synthesis).

Energy Production in Glycolysis

ATP consumption: 2 ATP (conversion of glucose to fructose-1,6-bisphosphate).
ATP production (aerobic): 4 ATP from substrate-level phosphorylation and additional 6-8 ATP from oxidative phosphorylation of NADH.
ATP production (anaerobic): 2 ATP (from substrate-level phosphorylation only).

Net Energy Gain

Aerobic glycolysis: 8-10 ATP.
Anaerobic glycolysis: 2 ATP.

Glycolysis in Red Blood Cells

Mitochondria absent: RBCs rely entirely on glycolysis.
End-product: Lactate.
Net energy: 2 ATP.
Glucose uptake: Independent of insulin.
2,3-biphosphoglycerate production: a crucial metabolic feature.

Regulation of Glycolysis

Key enzymes (irreversible): Hexokinase/glucokinase; Phosphofructokinase-1; Pyruvate kinase
Hormonal regulation: Insulin promotes enzyme synthesis, glucagon inhibits.
Allosteric regulation: G-6-P inhibits hexokinase; fructose-2,6-bisphosphate stimulates phosphofructokinase-1; citrate inhibits phosphofructokinase-1; fructose-2,6-bisphosphate stimulates pyruvate kinase.
Covalent modification: Pyruvate kinase is inactivated by phosphorylation.
Energy regulation: ATP and AMP affect PFK-1 and pyruvate kinase.
Note: Fructose-2,6-bisphosphate is an important allosteric regulator for both glycolysis and gluconeogenesis.

In Vitro Inhibition of Glycolysis

Arsenate: Inhibits glyceraldehyde-3-phosphate dehydrogenase.
Iodoacetate: Inhibits glyceraldehyde-3-phosphate dehydrogenase.
Fluoride: Inhibits enolase.

Important Note

Hemolytic anemia can occur due to pyruvate kinase deficiency. This deficiency negatively affects RBC glycolysis

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Description

Explore the intriguing world of metabolism with a focus on carbohydrate (CHO) metabolism. This quiz covers essential metabolic pathways, including anabolic, catabolic, and amphibolic processes, and the fate of food molecules post-digestion. Test your knowledge on the biochemical reactions that sustain life and energy production in the body.