Metabolism, Nutrition, and Energetics

Overview of Metabolic Reactions

Metabolism encompasses all chemical reactions occurring within the body, divided into two main categories: catabolism and anabolism. These reactions are essential for maintaining life, growth, and energy balance.

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells, driving all bodily functions.

• Nutrient Pool: Cells utilize a pool of organic building blocks (glucose, fatty acids, amino acids) for energy and biosynthesis.

• Carbohydrates: Most common energy source; glucose is central to cellular metabolism.

• Triglycerides: Used for energy via β-oxidation.

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

• Nucleic Acids: Digested into nucleotides for cellular use.

Hormonal Regulation of Metabolism

Metabolic processes are regulated by hormones, which can be either catabolic (promoting breakdown) or anabolic (promoting synthesis).

• Insulin: Stimulates glucose uptake and storage.

• Glucagon: Promotes glucose release from storage.

• Thyroid Hormones, Growth Hormone, Epinephrine: Influence metabolic rate and energy utilization.

Oxidation-Reduction Reactions

Energy is transferred in cells through oxidation-reduction (redox) reactions, which involve the transfer of electrons.

• Oxidation: Loss of electrons (often hydrogen atoms), resulting in energy release.

• Reduction: Gain of electrons, increasing energy content.

• Redox reactions are always coupled; one molecule is oxidized while another is reduced.

Carbohydrate Metabolism

Glucose Catabolism (Cellular Respiration)

Glucose catabolism is the process by which cells extract energy from glucose. It occurs in several stages:

• Glycolysis: Occurs in the cytosol; glucose is converted to pyruvate, yielding 2 ATP.

• Anaerobic Respiration: In absence of oxygen, pyruvate is converted to lactate.

• Aerobic Respiration: In presence of oxygen, pyruvate enters mitochondria, yielding up to 34 ATP via the citric acid cycle and electron transport chain.

• Total ATP Yield: Glycolysis (2 ATP) + Aerobic Respiration (34 ATP) = 36 ATP per glucose molecule.

Equation for Complete Glucose Oxidation:

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle occurs in the mitochondria and is central to aerobic metabolism. Its main function is to transfer hydrogen atoms to coenzymes (NAD and FAD), which carry electrons to the electron transport chain.

• Pyruvate: Converted to Acetyl-CoA, which enters the cycle.

• NAD and FAD: Electron carriers; become NADH and FADH2.

• ATP Production: Each turn of the cycle produces ATP, NADH, and FADH2.

Oxidative Phosphorylation and Electron Transport Chain (ETC)

Oxidative phosphorylation is the process of generating ATP in mitochondria, using oxygen as the final electron acceptor. The ETC consists of protein complexes that transfer electrons, creating a proton gradient used by ATP synthase to produce ATP.

• Electron Transport Chain: Transfers electrons from NADH and FADH2 to oxygen, forming water.

• ATP Synthase: Utilizes proton gradient to synthesize ATP (chemiosmosis).

• Water Formation:

Summary of ATP Production

• 2 ATP from glycolysis

• 3–5 ATP from NADH generated in glycolysis (via ETC)

• 2 ATP from citric acid cycle (via GTP)

• 23 ATP from ETC

• Most ATP is produced in mitochondria

Glucose Anabolism

Glucose can be synthesized or stored through several processes:

• Glycogenesis: Conversion of glucose to glycogen for storage (stimulated by insulin).

• Glycogenolysis: Breakdown of glycogen to glucose (stimulated by glucagon and epinephrine).

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (stimulated by cortisol, thyroid hormone, epinephrine, glucagon, and growth hormone).

Lipid Metabolism

Lipid Catabolism (Lipolysis and Beta-Oxidation)

Lipids are broken down to provide energy, especially when glucose is scarce.

• Lipolysis: Hydrolysis of triglycerides into fatty acids and glycerol.

• Beta-Oxidation: Fatty acids are broken down in mitochondria into 2-carbon fragments (Acetyl-CoA), which enter the citric acid cycle.

• ATP Yield: One 18-carbon fatty acid can yield up to 120 ATP.

Lipid Anabolism (Lipogenesis)

Lipogenesis is the synthesis of lipids from Acetyl-CoA, stimulated by high carbohydrate intake and insulin.

• Glycerol: Synthesized from dihydroxyacetone phosphate (intermediate of glycolysis).

• Nonessential Fatty Acids and Steroids: Synthesized from Acetyl-CoA.

Lipid Transport and Distribution

Because lipids are not water-soluble, they are transported in the blood as lipoproteins.

• Chylomicrons: Carry dietary lipids from intestines to lymph and blood.

• VLDL (Very Low-Density Lipoproteins): Transport triglycerides from liver to tissues.

• LDL (Low-Density Lipoproteins): Carry cholesterol to cells; excess can cause arterial plaques.

• HDL (High-Density Lipoproteins): Remove excess cholesterol from cells and transport it to the liver for elimination.

• Free Fatty Acids: Bound to albumin in blood; used by liver, cardiac, and skeletal muscle for energy.

Protein Metabolism

Protein Digestion and Absorption

Proteins are digested into amino acids, which are absorbed and used for protein synthesis or energy production.

• Digestion: Begins in the stomach (pepsin), continues in the small intestine (pancreatic enzymes).

• Absorption: Amino acids are transported across intestinal mucosa.

• Excess Proteins: Converted to glucose or triglycerides.

Amino Acid Catabolism

For proteins to be used as energy, amino acids must be converted into substances that can enter the citric acid cycle.

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

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

• Urea Cycle: Converts ammonium ions and CO2 into urea, which is excreted in urine.

Protein Catabolism and Disorders

• Protein catabolism is less efficient and produces toxic by-products (ammonium ions).

• Proteins are essential for structural and functional roles; excessive catabolism threatens homeostasis.

• Phenylketonuria (PKU): Genetic disorder affecting protein metabolism; requires dietary management.

Absorptive and Postabsorptive States

Absorptive State

Occurs after eating, when nutrients are absorbed and used for growth and energy storage.

• Insulin: Primary hormone; stimulates glucose uptake, glycogenesis, protein synthesis, and triglyceride synthesis.

• Most dietary lipids are stored in adipose tissue.

• Amino acids are converted to carbohydrates, fats, and proteins in the liver.

Postabsorptive State

Occurs during fasting or starvation; the body relies on internal energy reserves.

• Glucagon: Stimulates glycogenolysis and gluconeogenesis.

• Catabolism of stored triglycerides and proteins provides energy.

• Formation of ketone bodies increases as fatty acids are catabolized.

• Prolonged starvation leads to ketoacidosis, which can be life-threatening.

Metabolic Rate and Thermoregulation

Metabolic Rate

Metabolic rate is the rate at which the body expends energy, measured in calories per hour or day. It is affected by exercise, age, sex, hormones, and climate.

• Basal Metabolic Rate (BMR): Energy expended at rest to maintain vital functions; average is 70 kcal/hour.

• About 70% of daily energy expenditure is for basic organ functions, 20% for physical activity, and 10% for thermoregulation.

Thermoregulation

The body maintains a constant core temperature (36.5–37.5°C) through homeostatic mechanisms controlled by the hypothalamus.

• Heat Exchange Mechanisms: Radiation, convection, conduction, and evaporation.

• Heat Loss: Peripheral vasodilation, sweating, increased respiration.

• Heat Gain: Vasoconstriction, shivering, nonshivering thermogenesis (hormonal stimulation).

Table: Lipoprotein Classes and Functions

Table: Mechanisms of Heat Exchange

Additional info: These notes expand on the original lecture slides and textbook content, providing definitions, examples, and tables for clarity and completeness. All images included are directly relevant to the adjacent content and reinforce key concepts in metabolism, nutrition, and thermoregulation.