Metabolism and Bioenergetics Review

Metabolism and Bioenergetics

Overview of Metabolism

• Metabolism: Sum of all chemical reactions in an organism.
  • Includes:
  • Catabolism: Breakdown of organic substrates.
  • Anabolism: Synthesis of new organic molecules.
  • Cellular Metabolism: Encompasses chemical reactions within cells.
  • Metabolic Turnover: Continuous breakdown and replacement of cellular components, except DNA.
• Nutrient Pool: Resultant source of organic substrates from catabolic reactions and absorption.

Cellular Metabolism

Catabolism and Energy Production

• Cellular Catabolism (Aerobic Metabolism):
  • Occurs in mitochondria.
  • 40% of energy used to convert ADP to ATP for cellular work.
  • 60% of energy is lost as heat, which helps maintain body temperature.

Processes

• Major substrate types:
  • Amino Acids: From proteins.
  • Lipids: Energy storage.
  • Simple Sugars: Carbohydrate component.
• ATP Use: Required for processes like muscle contraction, intracellular transport, and secretion.

Nutrient Pool Management

Inadequate Nutrient Availability

• Mobilize Metabolic Reserves:
  • Breakdown of triglycerides and glycogen to release fatty acids and glucose.
  • Skeletal muscles can break down proteins to amino acids when needed.

Excess Nutrient Availability

• Restore Metabolic Reserves:
  • Storage of triglycerides and glycogen.
  • Excess amino acids can be converted into proteins.

Utilization of Nutrients

• Most tissues continuously absorb glucose for ATP production; the brain requires a constant supply.
• During starvation, other tissues may switch to catabolizing fatty acids or amino acids to conserve glucose for nerve tissues.

Cellular Respiration

Overview

• Cellular Respiration: Set of reactions that produce ATP.
  • Glycolysis: First step in glucose catabolism (anaerobic process).
  • Breaks down glucose into 2 pyruvate.
  • Produces a net gain of 2 ATP and releases hydrogen ions used in the electron transport chain.
  • Citric Acid Cycle (Krebs Cycle): Converts acetyl-CoA to produce ATP and reduce equivalents (NADH, FADH2).
  • Electron Transport Chain: Major site of ATP production via oxidative phosphorylation.

Glycolysis Steps:

• Conversion of glucose to glucose-6-phosphate.
• Production of NADH and ATP during breakdown of pathway intermediates.
• Net gain from glycolysis: 2 ATP and 2 pyruvate molecules.

Citric Acid Cycle Summary

• Requires oxygen (aerobic).
• Produces NADH and FADH2, which are used in the electron transport chain.
• Generates 2 ATP per glucose (2 cycles for each glucose molecule).

Electron Transport Chain

Key Points

• Occurs at mitochondrial cristae and is aerobic.
• Uses NADH and FADH2 to create a proton gradient.
• Over 90% of cellular ATP is produced via oxidative phosphorylation.
• Oxygen serves as the final electron acceptor.
• Formation of water occurs with excess hydrogen.

Lipid Metabolism

Lipid Catabolism

• Breakdown into glycerol and fatty acids through lipolysis.
• Beta-oxidation of fatty acids yields acetyl-CoA, which enters the citric acid cycle.

Lipogenesis

• Synthesis of fatty acids from acetyl-CoA.
• Essential fatty acids must come from dietary sources.

Protein Metabolism

Protein Digestion

• Begins in stomach and continues in the small intestine via pancreatic enzymes.
• Absorbed amino acids transported to the liver for synthesis of required proteins.

Amino Acid Metabolism

• Excess amino acids can undergo deamination and transamination.
• Toxic ammonium ions are converted to urea for excretion by liver.

Energy Storage and Use

Absorptive vs. Postabsorptive State

• Absorptive State: Nutrient absorption occurs; insulin facilitates storage.
• Postabsorptive State: Body mobilizes energy reserves; hormone regulation (glucagon, epinephrine) activates reserves.

Vitamins and Nutrition

Importance of Vitamins

• Vital for metabolic activities: 2 categories: Fat-soluble (A, D, E, K) and Water-soluble (B vitamins, C).
• Deficiency leads to hypovitaminosis; excess leads to hypervitaminosis, impacting health.

Clinical Disorders

Metabolic Disorders

• Common conditions include anorexia nervosa, bulimia, obesity, and metabolic syndrome.
• Imbalance of nutrients can lead to diseases such as PKU, ketoacidosis, and gout.

Thermoregulation

• Mechanisms of heat gain/loss include radiation, evaporation, convection, and conduction.
• Homeostasis maintained by hypothalamic heat-loss and heat-gain centers to regulate body temperature effectively.

Summary of Nutritional Recommendations

• Balanced diet essential: includes adequate carbohydrates, proteins, fats, vitamins, minerals, and hydration.
• Aimed for overall well-being and energy balance to avoid malnutrition.