Krebs Cycle and Cellular Respiration Quiz

Metabolic Pathways for Energy Conversion

• Major nutrients like carbohydrates, fats, and proteins are broken down to usable energy.

Learning Objectives

• ATP, a crucial energy source, is renewable and stored in cells.
• Cells absorb essential nutrients such as glucose, amino acids, fatty acids, triacylglycerols, and cholesterol.
• Glucose metabolism occurs in both anaerobic and aerobic processes.
• Fatty acid oxidation results in either ketone body formation or immediate ATP production.
• The citric acid cycle is a combined pathway for nutrient breakdown into energy. It also provides intermediates for the glycolytic and citric acid pathways to function properly.
• The citric acid cycle facilitates the breakdown of nutrients into energy and regulates the deamination of amino acids, delivering essential intermediates to support the glycolytic and citric acid cycles.

Metabolism = Anabolism and Catabolism

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy.

• Catabolic processes produce energy needed for anabolic reactions alongside producing heat regulation.

Energy

• Free energy (ΔG): The energy available for work in a chemical reaction.
• Exergonic reaction: A reaction releasing free energy (ΔG < 0).
• Endergonic reaction: A reaction requiring free energy input (ΔG > 0).

Stages of Energy Extraction from Fuel Molecules

• Stage I: Digestion, absorption, and transport of nutrients.
• Stage II: Converting various molecules into smaller key components and metabolites.
• Stage III: Complete energy transfer for cellular use.

Adenosine 5'-triphosphate (ATP)

• ATP is the cellular energy currency.
• ATP is constantly recycled, releasing the stored energy and converting itself into ADP and phosphate.
• The conversion of ATP to ADP releases 7.3 kcal/mol of free energy.

ATP Turnover

• ATP turnover rate is high, about 40 kg per day.
• ATP is crucial for various cellular functions, making its continuous replenishment essential.

Nicotinamide Adenine Dinucleotide (NAD)

• NAD acts as an electron donor and acceptor during crucial biochemical processes.
• NAD exists in reduced (NADH) and oxidized (NAD+) states, facilitating electron transfer.
• Nicotinamide is a part of Vitamin B3.

Flavine Adenine Dinucleotide (FAD)

• FAD is an electron carrier involved in redox reactions.
• FAD exists in reduced (FADH₂) and oxidized (FAD) forms.
• Riboflavin is a part of Vitamin B2.

Coenzyme A

• Coenzyme A is essential for the transport of fatty acids and acetyl groups.

Bioenergetics

• The production of ATP from carbohydrates, fats, and proteins.

Major Nutrient Metabolism: Overview

• Fats are broken down into fatty acids and glycerol.
• Polysaccharides are broken down into glucose and other sugars.
• Proteins are broken down into amino acids.
• Each nutrient flows to their respective metabolic pathways for energy extraction.

Carbohydrate Metabolism: Glycolysis and Krebs Cycle

• Glycolysis: The breakdown of glucose into pyruvate.
• It is a series of enzymatic reactions in the cytoplasm.
• Results in a small yield of ATP under anaerobic conditions.
• Pyruvate: A crucial intermediate for glucose metabolism and is either converted into lactate under anaerobic conditions or into acetyl-CoA under aerobic conditions to enter the Krebs cycle.
• Krebs cycle (Citric acid cycle): A crucial metabolic pathway for extracting energy from acetyl CoA.
• The Krebs cycle occurs in the mitochondria.
• Releases carbon dioxide, generating ATP, NADH, and FADH₂.

Features of Glycolysis

• A multi-step, cytoplasmic process.
• Breaks down 6-carbon glucose into 3-carbon compounds (pyruvate or lactate).
• Provides considerable amounts of ATP without oxygen.
• Intermediates can be used for other metabolic pathways.

The Overall Glycolysis Reaction

• Glucose is converted to pyruvate, producing ATP, NADH, and hydrogen ions.

Krebs Cycle

• The citric acid cycle, also known as the tricarboxylic acid cycle.
• It occurs in the mitochondria.
• Acetyl CoA enters the cycle.
• The cycle results in CO₂, NADH, FADH₂, one GTP.

Krebs Cycle: The Net Reaction

• Acetyl CoA, NAD⁺, FAD, GDP, P₁, and H₂O yield CoA, CO₂ , NADH, FADH₂, GTP, and H⁺.

Energy Yield via Glycolysis

• Four ATPs are generated per glucose molecule.
• Two ATPs are consumed during the initial glycolytic steps.
• Two ATPs are net generated per glucose.

Pyruvate Dehydrogenase Links Glycolysis to the Citric Acid Cycle

• Pyruvate, a product of glycolysis, is converted to acetyl CoA.
• This conversion is mediated by the enzyme pyruvate dehydrogenase complex in the mitochondria.
• Thiamine pyrophosphate is crucial for this conversion.

Thiamine (Vitamin B₁)

• A precursor for the coenzyme thiamine pyrophosphate, important for the conversion of pyruvate to acetyl-CoA.

Conditions for Mitochondrial Pyruvate Metabolism

• Oxygen availability
• Adequate NAD+ levels
• Adequate thiamine (Vitamin B₁) levels.

Clinical Features of Lactic Acidosis

• Low blood pH
• High serum lactate
• Increased respiratory rate
• Muscle pain

Causes of Lactic Acidosis

• Low oxygen levels
• Thiamine deficiency
• Pyruvate dehydrogenase deficiency
• Certain drugs

Carbohydrates: Simple and Complex Sugars

• Monosaccharides: Simple sugars like glucose, galactose, and fructose.
• Disaccharides: Two monosaccharides linked together (e.g., sucrose, lactose, maltose).

Glycolysis (for Galactose and Fructose Metabolism)

• The metabolic pathways for converting galactose and fructose to glycolytic intermediates.

Metabolism of Galactose and Galactosemia

• Galactosemia: A condition where galactose can't be converted to glucose. Galactose builds up in cells and it is converted into galactitol.
• This inability causes serious issues.