Metabolism and Energetics: Study Guide

Metabolism and Energetics

Overview of Cellular Metabolism

Cellular metabolism refers to all the chemical reactions that occur within cells to maintain life. These reactions are organized into metabolic pathways that either build up (anabolic) or break down (catabolic) molecules, providing energy and building blocks for the cell.

• Metabolism: The sum of all chemical reactions in the body.

• Anabolism: Synthesis of complex molecules from simpler ones; requires energy.

• Catabolism: Breakdown of complex molecules into simpler ones; releases energy.

• Nutrient Pool: The available reserves of organic substrates in the body, used for energy production and biosynthesis.

Example: Glucose catabolism provides ATP for cellular work, while amino acids from protein catabolism can be used for new protein synthesis (anabolism).

Key Metabolic Pathways

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Electron Transport System (ETS): Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to generate a proton gradient, producing ATP via oxidative phosphorylation.

Net Energy Yield from Glucose Catabolism:

• Glycolysis: 2 ATP (net), 2 NADH

• Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2 (per glucose)

• Electron Transport System: Up to 34 ATP (from NADH and FADH2)

• Total ATP per glucose: Up to 38 ATP (theoretical maximum)

Equation for Glucose Catabolism:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + 38\ ATP}$

Carbohydrate Metabolism

• Glycolysis: Breakdown of glucose to pyruvate.

• Glycogenesis: Formation of glycogen from glucose (storage form in liver and muscle).

• Glycogenolysis: Breakdown of glycogen to release glucose.

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

Enzymes involved: Hexokinase, phosphofructokinase, glycogen synthase, glycogen phosphorylase, glucose-6-phosphatase.

Lipid Metabolism

• Lipid Digestion: Begins in the small intestine with bile salts and pancreatic lipase.

• Transport: Lipids are transported as chylomicrons in the lymph and blood.

• Beta Oxidation: Fatty acids are broken down in mitochondria to acetyl-CoA, which enters the citric acid cycle.

• Lipogenesis: Synthesis of fatty acids from acetyl-CoA, mainly in the liver and adipose tissue.

Example: During fasting, triglycerides are broken down to fatty acids and glycerol; fatty acids undergo beta oxidation for energy.

Protein Metabolism

• Digestion: Proteins are broken down into amino acids by proteases in the stomach and small intestine.

• Deamination: Removal of amino group from amino acids, producing ammonia (converted to urea in the liver).

• Use as Energy: Amino acids can be converted to pyruvate, acetyl-CoA, or enter the citric acid cycle.

Example: In starvation, muscle proteins are catabolized for energy.

Metabolic States

• Absorptive State: Occurs during and shortly after eating; nutrients are absorbed and stored. Insulin is the main hormone.

• Postabsorptive State: Occurs when the gut is empty; energy is supplied by stored nutrients. Glucagon, epinephrine, and cortisol are key hormones.

Vitamins and Nutrition

• Fat-Soluble Vitamins: A, D, E, K; stored in body fat, can accumulate to toxic levels.

• Water-Soluble Vitamins: B-complex, C; not stored, excess excreted in urine.

• Functions: Each vitamin has specific roles, e.g., vitamin C for collagen synthesis, vitamin D for calcium absorption.

Balanced Diet and Food Groups

• Balanced Diet: Provides all essential nutrients in appropriate amounts for health and growth.

• Five Major Food Groups:

  • Grains: Carbohydrates, fiber

  • Vegetables: Vitamins, minerals, fiber

  • Fruits: Vitamins, fiber

  • Proteins: Amino acids, iron

  • Dairy: Calcium, protein

Metabolic Disorders

Disorder	Description
Anorexia / Anorexia Nervosa	Self-induced starvation; psychological disorder leading to severe weight loss.
Bulimia	Binge eating followed by purging; can cause electrolyte imbalance.
Obesity	Excess body fat; can be due to regulatory (hormonal) or metabolic causes.
Elevated Cholesterol	High blood cholesterol; risk factor for cardiovascular disease.
Phenylketonuria (PKU)	Genetic disorder; inability to metabolize phenylalanine, leading to intellectual disability if untreated.
Protein Deficiency Disease	General term for diseases caused by insufficient protein intake.
Kwashiorkor	Severe protein deficiency in children; causes edema, liver enlargement.
Ketosis	Elevated ketone bodies due to fat metabolism; occurs in fasting or diabetes.
Ketoacidosis	Dangerous acidification of blood due to high ketone levels.
Gout / Gouty Arthritis	Uric acid crystal accumulation in joints; causes inflammation and pain.

Basal Metabolic Rate (BMR) and Thermogenesis

• BMR: The rate of energy expenditure at rest, necessary to maintain vital body functions.

• Thermogenesis: Production of heat, especially after eating (diet-induced thermogenesis) or through shivering/non-shivering mechanisms.

Equation for BMR (Harris-Benedict Equation):

Additional info: This is the equation for males; a different constant is used for females.

Regulation of Appetite

• Short-term Regulation: Controlled by neural and hormonal signals (e.g., ghrelin stimulates hunger, CCK and insulin promote satiety).

• Long-term Regulation: Leptin from adipose tissue decreases appetite; hypothalamic centers (satiety and feeding centers) integrate signals.

Heat Transfer and Thermoregulation

• Four Mechanisms of Heat Transfer:

  • Radiation: Emission of infrared heat waves

  • Conduction: Direct transfer of heat between molecules

  • Convection: Transfer of heat by movement of air or liquid

  • Evaporation: Loss of heat as water vaporizes from skin or respiratory tract

• Homeostatic Mechanisms: Hypothalamus acts as the body's thermostat; responses include sweating, shivering, vasodilation, and vasoconstriction to maintain constant body temperature.