Chapter 23: Metabolism, Nutrition, and Energetics

Section 1: Introduction to Cellular Metabolism

Cellular metabolism encompasses all chemical reactions that occur within cells to maintain life. These reactions are essential for energy production, growth, and maintenance of cellular structures.

• Metabolism: The sum of all chemical reactions in an organism, including both catabolism (breakdown of molecules) and anabolism (synthesis of molecules).

• Catabolism: The breakdown of organic substrates, releasing energy.

• Anabolism: The synthesis of new organic molecules, requiring energy.

• Cellular metabolism: Chemical reactions within cells that use nutrients absorbed from the digestive tract.

• Nutrient pool: The source of organic substrates that have been absorbed and are available for cellular metabolism.

Example: Glucose catabolism provides energy for ATP synthesis, while amino acids from the nutrient pool can be used to build proteins.

Section 2: The Nutrient Pool and Mobilization of Metabolic Reserves

The nutrient pool supplies cells with organic molecules for energy production and biosynthesis. The body mobilizes reserves when dietary intake is insufficient.

• Mobilization of reserves: When absorption from the digestive tract is insufficient, the body uses stored nutrients.

• Liver cells: Break down triglycerides and glycogen to release fatty acids and glucose.

• Adipocytes: Release fatty acids from triglycerides.

• Skeletal muscle cells: Break down contractile proteins for amino acids.

• Utilization of resources: Nervous tissue requires a continuous supply of glucose; during starvation, other tissues may use fatty acids or amino acids.

Example: During fasting, the liver releases glucose to maintain blood glucose levels for the brain.

Section 3: Glycolysis – The First Step in Glucose Catabolism

Glycolysis is the anaerobic process by which glucose is broken down in the cytoplasm to produce ATP, pyruvate, and NADH.

• Glycolysis: Converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

• Products: 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.

• Location: Cytoplasm of the cell.

Equation:

Example: Glycolysis provides quick ATP for muscle contraction during intense exercise.

Section 4: The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of aerobic reactions in the mitochondria that further oxidize pyruvate derivatives, generating electron carriers and ATP.

• Location: Mitochondrial matrix.

• Main function: Remove hydrogen atoms from organic molecules and transfer them to coenzymes (NAD+ and FAD).

• Products per cycle: 3 NADH, 1 FADH2, 1 GTP (converted to ATP), and 2 CO2 per acetyl-CoA.

• Coenzymes: NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) carry electrons to the electron transport chain.

Equation:

Example: The citric acid cycle is central to aerobic metabolism, providing most of the cell's reducing power for ATP production.

Section 5: The Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane that use electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis.

• Oxidative phosphorylation: The process by which ATP is formed as electrons are transferred from NADH or FADH2 to O2 by a series of electron carriers.

• Key steps:

  • Electrons are passed along the ETC, releasing energy.

  • Energy is used to pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient.

  • Protons flow back through ATP synthase, driving the synthesis of ATP from ADP and Pi.

  • Oxygen acts as the final electron acceptor, forming water.

• Redox reactions: Oxidation is the loss of electrons; reduction is the gain of electrons.

• ATP yield: The majority of ATP from glucose catabolism is produced by oxidative phosphorylation (up to 34 ATP per glucose molecule).

Equation:

Example: Cells with high energy demands, such as neurons and muscle cells, rely heavily on the ETC for ATP production.

Summary Table: Major Steps of Cellular Respiration

Key Terms and Concepts

• Metabolic turnover: The continuous breakdown and replacement of organic molecules in cells.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• Redox reactions: Chemical reactions involving the transfer of electrons.

• Oxidative phosphorylation: The process of ATP formation driven by the transfer of electrons to oxygen.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP during glycolysis and the citric acid cycle.

Additional info: These notes expand on the provided slides by including definitions, equations, and a summary table for clarity and completeness.