Metabolism and Nutrition: Foundations for Anatomy & Physiology

Cellular Metabolism

Overview of Metabolism

Metabolism encompasses all chemical events within an organism or cell, essential for maintaining life and supporting growth, repair, and maintenance.

• Catabolism: Breakdown of organic substrates in the body to release energy.

• Anabolism: Synthesis of new organic molecules, supporting growth and repair.

• Metabolic Turnover: Continuous replacement of organic components (except DNA) using substances from catabolic reactions and nutrients absorbed from interstitial fluid.

• Nutrient Pool: The collective pool of organic substances available for cellular metabolism.

Metabolism of Organic Nutrients and Nutrient Pools

Major Nutrient Pathways

Organic nutrients from the diet are processed through interconnected metabolic pathways, forming pools that supply energy and building blocks for cellular functions.

• Fats: Broken down into fatty acids and glycerol, used for energy or stored in adipose tissue.

• Carbohydrates: Converted to glucose, stored as glycogen, or used directly for energy.

• Proteins: Broken down into amino acids, used for protein synthesis or energy production.

Pathway	Process	Storage Form
Fats	Lipogenesis, Lipolysis	Triglycerides
Carbohydrates	Glycogenesis, Glycogenolysis, Gluconeogenesis	Glycogen
Proteins	Protein synthesis, Proteolysis	Body protein

Calories and Energy Yield

Definition and Biological Significance

Calories measure the capacity to do biological work, representing the amount of heat required to raise the temperature of 1 g of water by 1°C.

• Kilocalorie (kcal): 1,000 calories; used in physiology and dietetics.

• Energy Yield:

  • Carbohydrates and proteins: ~4 kcal/g

  • Fats: ~9 kcal/g

  • Sugar and alcohol: ~7.1 kcal/g ("empty" calories, provide few nutrients)

Obesity and Body Mass Index (BMI)

Definitions and Prevalence

Obesity is defined as weight more than 20% above the recommended norm for one's age, sex, and height. BMI is a key indicator of overweight and obesity.

• BMI Formula: $BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

• Categories:

  • 20–25: Optimal

  • Over 27: Overweight

  • Above 30: Obese

• Prevalence in U.S.: 30% obese; another 35% overweight

Nutrients: Classification and Functions

Major Classes of Nutrients

Nutrients are chemicals ingested for growth, repair, or maintenance of the body, classified as macronutrients and micronutrients.

• Macronutrients: Water, carbohydrates, lipids, proteins (required in large quantities)

• Micronutrients: Vitamins and minerals (required in small quantities)

Minerals and Vitamins

Roles in Metabolism

Minerals and vitamins are essential for metabolic processes but are not used as fuel.

• Minerals: Inorganic elements extracted from soil or water, introduced into the food web. Examples: calcium, phosphorus, iron, magnesium.

• Vitamins: Small organic compounds necessary for metabolism, often functioning as coenzymes.

• Both are essential for the utilization of other nutrients.

Krebs Cycle (Citric Acid Cycle)

Central Role in Cellular Respiration

The Krebs cycle is a series of eight reactions occurring in the mitochondrial matrix, crucial for energy production through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

• Main Steps:

  1. Acetyl-CoA combines with oxaloacetic acid to form citric acid.

  2. Citric acid is progressively oxidized, releasing CO2, transferring electrons to NAD+ and FAD, and regenerating oxaloacetic acid.

• Electron Carriers: NADH and FADH2 transport electrons to the electron transport chain for ATP synthesis.

Metabolic Pathways Diagram

Summary Table: Key Pathways and Molecules

Pathway	Key Molecule	Function
Glycolysis	Glucose	Initial breakdown for energy
Krebs Cycle	Acetyl-CoA	Central energy-yielding cycle
Electron Transport Chain	NADH, FADH2	ATP generation via oxidative phosphorylation
Lipogenesis	Fatty acids, glycerol	Fat storage
Gluconeogenesis	Amino acids, glycerol	Glucose synthesis from non-carbohydrate sources

Additional info:

• Metabolic turnover ensures cellular adaptation to changing nutrient availability.

• Minerals and vitamins act as cofactors and coenzymes, facilitating enzymatic reactions.

• The Krebs cycle is a metabolic hub, integrating carbohydrate, fat, and protein metabolism.