Metabolism, Nutrition, and Energetics

Introduction

This chapter explores the biochemical processes that sustain life, focusing on metabolism, nutrient utilization, and energy balance. Understanding these concepts is essential for comprehending how the body maintains homeostasis and supports growth, repair, and activity.

Metabolism and Energetics

Definitions and Concepts

• Metabolism: The sum of all chemical and physical changes occurring in body tissues, encompassing both anabolic and catabolic reactions.

• Catabolism: The breakdown of large molecules into smaller ones, releasing energy used to synthesize ATP.

• Anabolism: The synthesis of new organic compounds from smaller molecules, requiring energy input.

• Nutrient Pool: The collection of all available nutrient molecules distributed in the blood.

• Energetics: The study of energy flow and transformation within the body, including heat balance.

Key Functions of Anabolism:

• Structural maintenance and repair

• Support of growth

• Production of secretions

• Storage of nutrient reserves

Major Nutrient Reserves:

• Triglycerides: Most abundant storage lipids, primarily fatty acids.

• Glycogen: Most abundant storage carbohydrate, a branched chain of glucose molecules.

• Proteins: Most abundant organic components, vital for cellular functions.

Oxidation and Reduction

• Oxidation: Loss of electrons (LEO: Lose Electron Oxidize).

• Reduction: Gain of electrons (GER: Gain Electron Reduce).

• Cellular breakdown of molecules relies on these reactions.

Carbohydrate Metabolism

Cellular Respiration

Carbohydrate catabolism generates ATP and other high-energy compounds through a series of metabolic pathways.

• Glycolysis: The breakdown of glucose into pyruvate, yielding 2 ATP (net) and 2 NADH.

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

• Citric Acid Cycle (Krebs Cycle): Acetyl-CoA is oxidized, producing NADH, FADH2, and GTP (converted to ATP).

• Electron Transport Chain: Electrons from NADH and FADH2 are transferred, generating most of the cell's ATP.

Overall Reaction for Glucose Catabolism:

Energy Yield: One glucose molecule provides a net gain of 30–32 ATP molecules.

Gluconeogenesis, Glycogenesis, and Glycogenolysis

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

• Glycogenesis: Formation of glycogen from excess glucose, requiring uridine triphosphate (UTP).

• Glycogenolysis: Rapid breakdown of glycogen to glucose monomers via a single enzymatic step.

Key Regulatory Note: Many reactions are reversible, but pyruvate to acetyl-CoA is not.

Lipid Metabolism

Lipolysis and Lipogenesis

• Lipids: Organic molecules containing carbon, hydrogen, and oxygen; triglycerides are the most abundant.

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids. Glycerol is converted to pyruvate; fatty acids undergo beta-oxidation to yield acetyl-CoA.

• Beta-Oxidation: Fatty acid carbon chains are broken down to acetyl-CoA, donating hydrogen atoms to coenzymes for the electron transport chain.

• Lipogenesis: Synthesis of lipids from acetyl-CoA; can use various organic substrates.

Energy Yield: One 18-carbon fatty acid can produce 120 ATP, more than three 6-carbon glucose molecules.

Essential and Nonessential Fatty Acids

• Essential Fatty Acids: Cannot be synthesized by the body; must be obtained from diet (e.g., linoleic and linolenic acids).

• Nonessential Fatty Acids: Synthesized from acetyl-CoA.

Lipid Transport and Distribution

• Most lipids circulate as lipoproteins due to their insolubility in water.

• Free Fatty Acids (FFAs): Diffuse readily, bound to albumin in plasma; important during starvation.

Lipoprotein Types

Protein Metabolism

Protein Synthesis and Catabolism

• Body synthesizes 100,000–140,000 proteins from 20 amino acids.

• Proteins serve as enzymes, hormones, structural elements, and neurotransmitters.

• Protein is rarely used for energy except during starvation.

Amino Acid Catabolism

• Transamination: Transfer of an amino group to a keto acid, forming a new amino acid.

• Deamination: Removal of an amino group, producing toxic ammonium ions.

• Urea Cycle: Liver converts ammonium ions and CO2 to urea, which is excreted in urine.

Protein Catabolism Limitations

• Proteins are harder to break down than carbohydrates or lipids.

• Ammonium ions are toxic.

• Proteins are essential structural and functional cell components.

Essential vs. Nonessential Amino Acids

• Essential Amino Acids: Must be obtained from diet (8 not synthesized, 2 insufficiently synthesized).

• Nonessential Amino Acids: Synthesized by the body as needed via amination.

Nutrition

Balanced Diet and Homeostasis

• Digestive tract must absorb fluids, organic nutrients, minerals, and vitamins.

• Nutrition: Absorption of nutrients from food; requirements vary by nutrient.

• Balanced Diet: Contains all ingredients needed for homeostasis.

Dietary Guidelines

• USDA's MyPlate recommends proportions from five food groups: fruits, vegetables, grains, protein, dairy.

• Malnutrition: Unhealthy state from nutrient imbalance.

Proteins in Diet

• Complete Proteins: Provide all essential amino acids (beef, fish, poultry, eggs, milk).

• Incomplete Proteins: Lacking one or more essential amino acids (most plant sources, except soy).

Minerals

• Bulk Minerals: Sodium, potassium, chloride, calcium, phosphorus, magnesium.

• Trace Minerals: Iron, zinc, copper, manganese, cobalt, selenium, chromium.

• Roles: Osmotic concentration, physiological processes, cofactors in enzymatic reactions.

Vitamins

• Fat-Soluble Vitamins: A, D, E, K

• Water-Soluble Vitamins: Rapidly exchanged and excreted; hypervitaminosis is rare.

Fat-Soluble Vitamin Functions

Metabolic Rate and Thermoregulation

Metabolic Rate

• Average caloric expenditure varies with activity, age, sex, hormones, and climate.

• Measured in kcal (energy to heat 1 kg water by 1°C).

• Clinicians assess metabolic state as calories per hour, day, or per unit body weight.

Basal Metabolic Rate (BMR)

• Minimum resting energy expenditure of an awake, alert person.

• Measured via respiratory activity; proportional to oxygen consumption.

Energy Balance

• Excess intake: Energy stored as triglycerides in adipose tissue.

• Deficit: Body uses energy reserves, resulting in weight loss.

Thermoregulation

• Homeostatic mechanisms maintain body temperature within limits.

• Enzymes function optimally within a narrow temperature range.

Concept Checks and Examples

• Vitamin C deficiency: Scurvy (piratical disorder).

• Vitamin A deficiency: Night blindness.

• Vitamin K deficiency: Problems with blood clotting.

• Most important minerals: Sodium and potassium.

• Order of energy use: Carbohydrates → Lipids → Proteins.

• Difficulty breaking down fat reserves: Fats are not water-soluble, making enzyme access slow and difficult.

Example: During prolonged fasting, the body shifts from using glucose to fatty acids and eventually proteins for energy, illustrating the hierarchy of nutrient utilization.

Additional info: Some details about metabolic pathways and vitamin functions were expanded for clarity and completeness.