Metabolism

Metabolism Overview

  • Definition: Metabolism is the sum of all chemical changes in the body.

  • Reactions: There are two classes of metabolic reactions:

    • Anabolism: Requires energy to build complex molecules.

    • Catabolism: Releases energy by breaking down molecules.

Carbohydrates

  • Glucose Catabolism:

    • Most dietary carbohydrates are burned within a few hours of absorption.

    • Glucose is the primary product of carbohydrate digestion.

    • Overall reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O, occurring through a series of enzyme-controlled steps aimed at producing ATP.

  • Major Pathways:

    • Glycolysis: Splits glucose into two pyruvate molecules; occurs in the cytoplasm.

    • Anaerobic Fermentation: Processes pyruvate to continue glycolysis without oxygen, also in the cytoplasm.

    • Aerobic Respiration: Occurs in mitochondria, oxidizing pyruvate to CO2 and water.

Coenzymes

  • Key Coenzymes:

    • NAD+: Derived from niacin (vitamin B3).

    • FAD: Derived from riboflavin (vitamin B2).

    • Enzymes remove electrons from glucose metabolites and transfer them to coenzymes, acting as temporary energy carriers.

    • Essential for glucose catabolism; enzymes cannot function without coenzymes.

Glycolysis

  • Process Summary:

    • Converts glucose into two pyruvate molecules.

    • Requires an initial investment of 2 ATP; generates 4 ATP (net yield of 2 ATP).

    • Requires 2 NAD+ which is converted to NADH during the process.

    • Results in 2 ATP, 2 NADH, and 2 pyruvate.

    • Fate of pyruvate depends on the presence of oxygen.

Anaerobic Fermentation

  • In the absence of oxygen, pyruvate is converted to lactic acid, generating NAD+ essential for recycling in glycolysis.

  • Produces only 2 ATP; useful only for short-term energy (e.g., during intense exercise).

Aerobic Respiration

  • Process Overview: Occurs when oxygen is available, with pyruvate entering the mitochondria for further oxidation.

  • Matrix Reactions: Part of the citric acid cycle; processes each pyruvate to produce 2 ATP, 6 CO2, 8 NADH, and 2 FADH2.

  • Membrane Reactions: Oxidize NADH and FADH2 via the electron transport chain, where electrons transfer leads to ATP production and water formation.

ATP Production

  • Summary of Yield:

    • Glycolysis produces 2 ATP.

    • Matrix reactions yield 2 ATP.

    • Membrane reactions yield up to 28 ATP.

    • Total yield from one glucose molecule = 32 ATP.

Glucose Production and Utilization

  • Glycogen and Glucose Hydrolysis:

    • Excess glucose is converted to glycogen or fat.

    • Processes:

      • Glycogenesis: Synthesis of glycogen, stimulated by insulin.

      • Glycogenolysis: Hydrolysis of glycogen, stimulated by glucagon.

      • Gluconeogenesis: Conversion of non-carbohydrates into glucose.

Lipids

  • Lipogenesis: Formation of triglycerides from excess glucose and amino acids.

  • Lipolysis: Breakdown of triglycerides into fatty acids and glycerol, especially when glucose is low.

  • Fatty acids yield more ATP (129 ATP) compared to glucose due to their oxidation to acetyl groups.

Proteins

  • Amino acids primarily used for protein synthesis.

  • Can be converted to glucose or fat; also have the potential to be used directly for energy after deamination (removal of the amine group produces ammonia, which is converted to urea).

Metabolic States

  • Absorptive (Fed) State:

    • Lasts about 4 hours post meal; body absorbs nutrients.

    • Carbohydrates primarily transported to the liver, fats distributed to tissues, and amino acids used for protein synthesis.

  • Regulation of Absorptive State:

    • Largely driven by insulin.

    • Insulin promotes glucose uptake, substrate storage, and protein synthesis.

  • Postabsorptive (Fasting) State:

    • Body utilizes stored fuels; involves glycogenolysis and gluconeogenesis to maintain blood glucose levels critical for brain function.

    • Protein usage increases when glycogen and fat stores deplete.

Metabolic Rate

  • Definition: Energy use in the body measured in kcal/hour.

  • Influenced by activity level, mental state, and hormonal levels.

  • Basal Metabolic Rate (BMR): Energy used at rest; average is ~2000 kcal/day. Total Metabolic Rate (TMR) = BMR + voluntary activities.

Factors Affecting Metabolic Rate

  • Increase in TMR: Physical activity, pregnancy, environmental temperature change (fever).

  • Decrease in TMR: Prolonged fasting, depression, weight-loss diets (slow metabolic adaptation).

Appetite Regulation

  • Energy balance affects weight; determined by energy intake vs. output.

  • Short-term appetite regulation involves hormones:

    • Ghrelin: Hunger hormone stimulating appetite.

    • Peptide YY and CCK: Signals satiety and regulate intake.

  • Long-term regulation by:

    • Leptin: Reflects fat stores; inhibits appetite and promotes fat breakdown.

    • Insulin: Lowers blood glucose and encourages fat storage.

  • Hypothalamus plays a key role in processing signals from these hormones to regulate appetite effectively.