Carbohydrate, Lipid, and Protein Metabolism

Sources and Uses of Carbohydrates, Lipids, and Proteins

Understanding the dietary sources and physiological uses of macronutrients is essential for grasping human metabolism.

• Carbohydrates: Main dietary sources include starch (grains, vegetables), sugars (fruits), and cellulose (insoluble fiber; provides roughage).

• Lipids: Used for energy and to make ATP. Types include:

  • Triglycerides: Neutral fats; found in most vegetable oils.

  • Saturated fats: Animal source.

  • Unsaturated fats: Most vegetable oils.

  Uses in Body: Used as fuel; stored as adipose tissue.

• Proteins: Dietary sources: eggs, milk, nuts, meats, soybeans. Contain complete proteins (contain all essential amino acids); legumes, nuts, and cereals contain incomplete proteins (lack some essential amino acids).

• Uses in Body: Structural materials: keratin (collagen and elastic muscle proteins); functional molecules: enzymes, hormones, antibodies, fuel.

Substrates and Products of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose. It involves several metabolic pathways:

• Glycolysis: Occurs in the cytosol; anaerobic process. Substrate: glucose. Products: 2 pyruvic acid molecules, 2 ATP molecules, high energy electrons.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondria. Substrate: mostly pyruvic acid (converted to acetyl CoA). Products: CO2, ATP, NADH, FADH2, high energy electrons.

• Electron Transport Chain and Oxidative Phosphorylation: Occurs in the mitochondria. Substrate: high energy electrons picked up by coenzymes are transferred to the electron transport chain. Products: 32 ATP molecules.

ATP Generation Associated with Carbohydrate Metabolism

ATP is generated through glycolysis, the Krebs cycle, and oxidative phosphorylation. The process varies depending on the metabolic state:

• Glycolysis: In response to low levels of ATP, glucose is converted to pyruvic acid.

• Glycogenesis: When glucose levels are high, glucose is converted to glycogen for storage.

• Glycogenolysis: When blood glucose levels drop, glycogen is converted to glucose.

• Gluconeogenesis: Formation of new (neo) glucose from non-carbohydrate molecules.

Equation for ATP production from glucose:

Lipid and Protein Metabolism

Lipids and proteins are metabolized for energy, especially during fasting or low carbohydrate intake.

• Lipid Metabolism:

  • Glycerol and Fatty Acids: Converted to acetyl CoA.

  • Lipogenesis: Formation of triglycerides from glycerol and fatty acids.

  • Lipolysis: Breakdown of stored fats into glycerol and fatty acids.

  • Ketogenesis: Liver converts acetyl CoA molecules to ketone bodies.

• Protein Metabolism: The goal of amino acid degradation is to produce molecules that can be oxidized for energy in the Krebs cycle or converted to glucose (gluconeogenesis).

Absorptive and Post-Absorptive States

The body alternates between absorptive (fed) and post-absorptive (fasting) states to maintain energy balance.

• Absorptive State: (Controlled by insulin)

  • Carbohydrates: Glucose is the major energy fuel. Glucose is converted to glycogen and fat in liver.

  • Lipids: Stored in liver and adipose tissue as triglycerides.

  • Amino Acids: Used for protein synthesis or converted to energy.

• Post-Absorptive State: (Regulated by glucagon and other hormones)

  • Glycogenolysis in liver.

  • Glycogenolysis in skeletal muscle.

  • Lipolysis in adipose tissues and the liver.

  • Liver converts glycerol to glucose (gluconeogenesis).

  • Protein converted to glucose (gluconeogenesis).

Summary Table: Metabolic Pathways and Their Functions

Key Terms and Definitions

• Glycolysis: The breakdown of glucose to pyruvic acid, producing ATP.

• Krebs Cycle: A series of reactions that generate energy through the oxidation of acetyl CoA.

• Electron Transport Chain: A sequence of proteins that transfer electrons, producing ATP.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to release glucose.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.

• Lipogenesis: Formation of triglycerides from glycerol and fatty acids.

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

• Ketogenesis: Formation of ketone bodies from acetyl CoA.

Example: Metabolic Adaptation During Fasting

During fasting, the body shifts from using glucose to using fatty acids and ketone bodies for energy. Glycogen stores are depleted, and gluconeogenesis and lipolysis become the primary sources of glucose and energy.

Additional info: The above notes expand on the original brief points with academic context, definitions, and a summary table for clarity and completeness.