Metabolism, Catabolism, and Anabolism

Definitions and Overview

Metabolism encompasses all chemical reactions that occur within living organisms to maintain life. It includes both catabolic (breakdown) and anabolic (synthesis) processes, which are essential for energy production and cellular function.

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

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

• Nutrients for Fuel: The body uses carbohydrates, fats, and proteins as sources of energy.

Example: Glucose catabolism during cellular respiration provides ATP for cellular activities.

Energy Transfer and ATP Hydrolysis

• Endergonic Reactions: Require energy input (e.g., synthesis of macromolecules).

• Exergonic Reactions: Release energy (e.g., breakdown of glucose).

• ATP Hydrolysis:

• Electron Transfer: In oxidation-reduction reactions, electrons are transferred between molecules, often involving NADH and FADH2.

Example: NAD+ is reduced to NADH during glycolysis, carrying electrons to the electron transport chain.

Glucose Catabolism and ATP Synthesis

Overview of Glucose Catabolism

Glucose catabolism is the process by which glucose is broken down to produce ATP, the cell's energy currency. This involves several key pathways:

• Glycolysis: Occurs in the cytoplasm; converts glucose to pyruvate, producing ATP and NADH.

• Formation of Acetyl CoA: Pyruvate is converted to acetyl CoA, which enters the citric acid cycle.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, driving ATP synthesis.

• Chemiosmosis: The movement of protons across the mitochondrial membrane generates ATP via ATP synthase.

Example: Each glucose molecule can yield up to 36-38 ATP molecules through complete aerobic respiration.

Energy Yield of Glucose Catabolism

• Glycolysis: 2 ATP (net), 2 NADH

• Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2

• Electron Transport Chain: Majority of ATP produced here via oxidative phosphorylation

Example: The overall energy yield from one glucose molecule is summarized as:

• Glycolysis:

• Krebs Cycle:

• ETC:

Fatty Acid and Amino Acid Catabolism

Fatty Acid Catabolism

Fatty acids are broken down through lipolysis and beta-oxidation to produce ATP, especially during fasting or prolonged exercise.

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids.

• Beta-oxidation: Fatty acids are converted to acetyl CoA, which enters the citric acid cycle.

• Ketogenesis: Excess acetyl CoA leads to the formation of ketone bodies, especially during carbohydrate deprivation.

Example: During starvation, the liver produces ketone bodies for use by the brain and muscles.

Amino Acid Catabolism

• Deamination: Removal of the amino group from amino acids, producing ammonia and a carbon skeleton.

• Transamination: Transfer of amino groups between amino acids and keto acids.

• Ammonia and Urea Production: Ammonia is converted to urea in the liver for excretion.

Example: Excess protein intake leads to increased urea production, which is excreted in urine.

Anabolic Pathways

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Anabolic pathways synthesize complex molecules from simpler ones, storing energy for future use.

• Glycogenesis: Formation of glycogen from glucose for storage in liver and muscle.

• Glycogenolysis: Breakdown of glycogen to release glucose.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (e.g., amino acids, glycerol).

Example: During fasting, gluconeogenesis maintains blood glucose levels.

Fatty Acid Synthesis and Storage

• Triglyceride Formation: Fatty acids are synthesized and stored as triglycerides in adipose tissue.

Example: Excess dietary carbohydrates are converted to fatty acids and stored as fat.

Absorptive and Postabsorptive States

Comparison of States

The body alternates between the absorptive state (after eating) and the postabsorptive state (between meals) to regulate energy storage and usage.

• Absorptive State: Nutrients are absorbed and stored; insulin promotes glucose uptake and storage.

• Postabsorptive State: Stored nutrients are mobilized; glucagon stimulates glycogenolysis and gluconeogenesis.

Example: Blood glucose levels are maintained by switching between these states.

Hormonal Regulation

• Insulin: Promotes glucose uptake, glycogenesis, and fat synthesis.

• Glucagon: Stimulates glycogen breakdown and glucose synthesis.

Example: After a meal, insulin levels rise; during fasting, glucagon levels increase.

Glucose Sparing and Feeding Behaviors

• Glucose Sparing: During fasting, tissues use fatty acids for energy, preserving glucose for the brain.

• Feeding Regulation: Controlled by hypothalamic centers and hormones (e.g., leptin, ghrelin).

Example: Leptin suppresses appetite, while ghrelin stimulates hunger.

Metabolic Rate and Thermoregulation

Metabolic Rate

Metabolic rate is the rate at which the body uses energy. The basal metabolic rate (BMR) is the energy expended at rest.

• Factors Affecting Metabolic Rate: Age, sex, genetics, body composition, and activity level.

Example: Muscle tissue increases BMR compared to fat tissue.

Thermoregulation

• Heat Transfer Mechanisms: Radiation, conduction, convection, and evaporation.

• Importance: Maintains optimal enzyme activity and physiological function.

Example: Sweating cools the body through evaporation.

Nutrition and Body Mass

Dietary Nutrients and Energy Yields

Nutrition involves the intake of essential nutrients, including macronutrients (carbohydrates, proteins, fats) and micronutrients (vitamins, minerals).

• Carbohydrates: 4 kcal/g

• Proteins: 4 kcal/g

• Fats: 9 kcal/g

Example: A balanced diet provides all essential nutrients for health and energy.

Dietary Sources and Balanced Diet

• Major Sources: Fruits, vegetables, grains, meats, dairy, oils.

• Balanced Diet: Includes recommended amounts of vitamins, minerals, and energy.

Example: Dietary guidelines recommend limiting saturated fats and increasing fiber intake.

Lipoproteins and Cholesterol

• Lipoproteins: Transport cholesterol and fats in the blood (e.g., LDL, HDL).

• Cholesterol Metabolism: The liver regulates cholesterol synthesis and breakdown.

Example: High LDL levels are associated with increased risk of cardiovascular disease.

Body Mass Determination

• Body Mass Index (BMI): Used to assess healthy body weight.

Example:

Additional info: These notes expand on the provided learning outcomes with academic context and examples for self-contained study.