Metabolism, Energy Balance, and Nutrition Flashcards

Introduction

• Food is the body's only energy source.
• Nutrients are used for energy, structural/functional molecules, or stored as fat/glycogen.

Metabolic Reactions

• Metabolism: all chemical reactions in the body.
• Catabolism: breaks down complex molecules.
• Anabolism: combines simple molecules to form complex ones.
• ATP transfers energy between molecules.

Energy Transfer

• Molecules store energy in bonds.
• Oxidation: Removal of electrons, releases energy (often dehydrogenation).
  • NAD and FAD carry hydrogen atoms.
• Reduction: Addition of electrons, increases energy.
• Oxidation is usually energy-releasing.

Mechanisms of ATP Generation

• Substrate-level phosphorylation
• Oxidative phosphorylation
• Photophosphorylation

Carbohydrate Metabolism

• Polysaccharides/disaccharides convert to glucose.
• Liver converts fructose/galactose to glucose.
• Glucose is the body's preferred ATP source.
• Fate of Glucose:
  • Immediate energy (ATP production)
  • Amino acid formation
  • Glycogen storage (glycogenesis).
  • Conversion to glycerol and fatty acids (lipogenesis).
• Glucose Movement into Cells:
  • GI tract: secondary active transport (Na+ - glucose symporters).
  • Other cells: facilitated diffusion (Gly-T molecules), insulin increases Gly-T insertion.
  • Phosphorylation traps glucose inside cells.

Glucose Catabolism

• Also known as cellular respiration, happens in every cell except red blood cells.
• Four stages: glycolysis, acetyl coenzyme A formation, Krebs cycle, electron transport chain.
• Glycolysis:
  • Glucose (6-carbon) breaks down into two pyruvic acid (3-carbon) molecules.
  • Net gain of 2 ATP molecules.
• Fate of pyruvic acid:
  • Low oxygen: reduced to lactic acid.
  • Aerobic: converted to acetyl coenzyme A, enters Krebs cycle.
• Acetyl Coenzyme A Formation:
  • Pyruvic acid converted to acetyl group, then acetyl CoA.
  • Coenzyme A from pantothenic acid (B vitamin).
• Krebs Cycle:
  • Also called citric acid cycle or TCA cycle, occurs in mitochondrial matrix.
  • Releases energy stored in pyruvic acid derivatives.
  • Involves decarboxylations, oxidations, and reductions.
  • For every two acetyl CoA molecules:
    • 6 NADH, 6 H+, 2 FADH2 produced.
    • 2 ATP generated by substrate-level phosphorylation.
  • Energy primarily in NADH + H+ and FADH2.
• Electron Transport Chain:
  • Sequence of electron carrier molecules on inner mitochondrial membrane.
  • Electrons release energy for ATP generation.
  • Final electron receptor is molecular oxygen (O2).
  • Chemiosmosis links electron transport to H+ pumping and ATP generation.
  • Carrier molecules: flavin mononucleotide, cytochromes, iron-sulfur centers, copper atoms, ubiquinones.
  • Three complexes act as proton pumps.
• Summary of Cellular Respiration:
  • C6H{12}O6 + 6O2 \rightarrow 36 \text{ or } 38 ATP + 6CO2 + 6H2O
  • 36-38 ATPs generated per glucose molecule during aerobic respiration.

Glucose Anabolism

• Glycogenesis: glucose to glycogen (stimulated by insulin).
• Glycogenolysis: glycogen back to glucose (stimulated by glucagon, epinephrine).
• Gluconeogenesis: protein or fat to glucose.
  • Stimulated by cortisol, thyroid hormone, epinephrine, glucagon, human growth hormone.

Lipid Metabolism

• Lipids transported as lipoproteins:
  • Chylomicrons: dietary lipids, transport to adipose tissue.
  • VLDLs: endogenous triglycerides, from hepatocytes to adipocytes, convert to LDLs.
  • LDLs: carry cholesterol to cells, deposit in arteries if excessive.
  • HDLs: remove cholesterol, transport to liver for elimination.
• Cholesterol sources: food, liver synthesis.
• Desirable levels: TC < 200 mg/dl, LDL < 130 mg/dl, HDL > 40 mg/dl, Triglycerides: 10-190 mg/dl.
• Lipid fates: ATP production, storage in adipose tissue, structural/essential molecules.
• Triglyceride Storage:
  • Adipose tissue contains lipases.
  • Fats are constantly catabolized and mobilized.

Lipid Catabolism: Lipolysis

• Triglycerides split into fatty acids and glycerol (lipolysis) via hormones.
  • Glycerol can be converted to glucose.
  • Fatty acids undergo beta oxidation to form acetyl CoA, which enters the Krebs cycle.
  • Ketogenesis: Acetyl CoA forms ketone bodies (acetoacetic acid, beta-hydroxybutyric acid, acetone).

Lipid Anabolism: Lipogenesis

• Glucose/amino acids converted to lipids, stimulated by insulin.
• Intermediates: glyceraldehyde-3-phosphate, acetyl coenzyme A.

Protein Metabolism

• Amino acids absorbed into liver via hepatic portal vein.
• Amino Acid Fate:
  • Synthesized into proteins (enzymes, transport molecules, etc.).
  • Stored as fat/glycogen or used for energy.
• Protein Catabolism:
  • Amino acids converted to Krebs cycle intermediates via deamination, decarboxylation, hydrogenation.
  • Amino acids can be converted into glucose, fatty acids, ketone bodies.
• Protein Anabolism:
  • Peptide bonds form between amino acids.
  • Stimulated by human growth hormone, thyroxine, insulin.
  • Essential amino acids (10) must be obtained from diet.
  • Nonessential amino acids synthesized via transamination.
• Phenylketonuria (PKU): genetic error, elevated phenylalanine levels.

Key Molecules at Metabolic Crossroads

• Glucose-6-phosphate, pyruvic acid, acetyl CoA.
  • Glucose-6-phosphate: glycogen/glucose synthesis, ribose-5-phosphate, pyruvate via glycolysis.
  • Pyruvic acid: lactic acid, alanine, gluconeogenesis.
  • Acetyl CoA: Krebs cycle, fatty acids, ketone bodies, cholesterol.

Metabolic Adaptations

• Absorptive state: nutrients enter blood/lymph, glucose available for ATP.
• Postabsorptive state: absorption complete, energy from stored nutrients.

Metabolism During the Absorptive State

• Cells produce ATP by oxidizing glucose.
• Liver converts glucose to glycogen/triglycerides.
• Lipids stored in adipose tissue.
• Amino acids converted to carbohydrates, fats, proteins.

Regulation of Metabolism During the Absorptive State

• Insulin stimulates absorptive state metabolism.

Metabolism During the Postabsorptive State

• Maintain blood glucose (70-110 mg/100 ml).
• Reactions that produce glucose:
  • Breakdown of liver glycogen.
  • Gluconeogenesis (lactic acid).
  • Gluconeogenesis (amino acids).
• Reactions that produce ATP without glucose:
  • Oxidation of fatty acids, lactic acid, amino acids, ketone bodies.
  • Breakdown of muscle glycogen.

Regulation of Metabolism During the Postabsorptive State

• Glucagon and sympathetic ANS activation.

Metabolism During Fasting and Starvation

• Nervous tissue/red blood cells use glucose.
• Gluconeogenesis from amino acids.
• Increased ketone body formation.

Energy Balance

• Balance between heat production/loss.
• Calorie: heat to raise 1g of water from 14 to 15°C.
• Kilocalorie/Calorie: 1000 calories

Metabolic Rate

• Overall rate of heat production.
• Basal metabolic rate (BMR): rate under basal conditions
• Total Metabolic Rate (TMR): energy expenditure per unit time.

Regulation of Food Intake

• Hypothalamus: feeding and satiety centers.
• Leptin inhibits eating, increases energy expenditure.

Regulation of Body Temperature

• Maintain core temperature near 37°C (98.6°F).
• Heat Production: metabolic rate, exercise, hormones, nervous system, etc.
• Heat Transfer: radiation, evaporation, conduction, convection.
• Hypothalamic thermostat: preoptic area.

Nutrition

• Guidelines: variety, healthy weight, low fat/cholesterol, vegetables/fruits/grains, moderate sugar/salt/alcohol.
• Minerals: regulate body processes.
• Vitamins: organic nutrients, often coenzymes.

Disorders: Homeostatic Imbalances

• Fever: elevated body temperature due to reset hypothalamus.
• Obesity: body weight >20% above standard, risk factor for various diseases.