Cellular Metabolism and Energy Production

Substrate-Level Phosphorylation

Substrate-level phosphorylation is a process by which ATP is formed directly in metabolic pathways by the transfer of a phosphate group from a substrate to ADP.

• Definition: The direct synthesis of ATP from ADP and a phosphorylated intermediate.

• Example Reaction:

• Key Pathways: Occurs during glycolysis and the Krebs cycle.

• Contrast: Differs from oxidative phosphorylation, which uses the electron transport chain.

Example: In glycolysis, the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate produces ATP via substrate-level phosphorylation.

ATP Production in Cellular Respiration

ATP is the primary energy currency in cells, produced through several metabolic pathways.

• Key Organelles: Mitochondria are the main site of ATP production via oxidative phosphorylation.

• Other Organelles: Endoplasmic reticulum and Golgi apparatus are not directly involved in ATP synthesis.

• Enzyme: ATP synthase catalyzes the formation of ATP in mitochondria.

Example: The electron transport chain in mitochondria uses energy from electrons to pump protons and drive ATP synthesis.

Glucose Oxidation Pathways

Glucose oxidation is the process by which glucose is broken down to produce ATP, CO2, and water.

• Major Steps:

  1. Glycolysis (aerobic and anaerobic)

  2. Linking Step (conversion of pyruvate to acetyl-CoA)

  3. Krebs Cycle (Citric Acid Cycle)

  4. Electron Transport Chain (ETC) & Oxidative Phosphorylation

• ATP Yield:

  • Glycolysis: 2 ATP (net)

  • Krebs Cycle: 2 ATP

  • ETC & Oxidative Phosphorylation: ~28 ATP

  • Total per glucose: ~32 ATP

• CO2 Production: Occurs in the linking step and Krebs cycle.

• O2 Usage: Required for ETC and oxidative phosphorylation.

• NADH/FADH2 Production: Electron carriers generated in glycolysis and Krebs cycle, used in ETC.

Equation:

Example: During aerobic respiration, one molecule of glucose yields up to 32 molecules of ATP.

Metabolism in Absence of Oxygen (Anaerobic Metabolism)

When oxygen is unavailable, cells rely on anaerobic pathways to produce ATP.

• Glycolysis: Continues in absence of oxygen, producing 2 ATP per glucose.

• Fermentation: Regenerates NAD+ from NADH, allowing glycolysis to continue.

• Limitation: No ATP is produced from NADH in the absence of oxygen.

Example: Muscle cells produce lactate during intense exercise when oxygen is limited.

Enzymes and Chemical Reactions

Enzymes are biological catalysts that speed up chemical reactions in the body.

• Role: Lower activation energy, increase reaction rate.

• Bidirectional Reactions: Many metabolic reactions are reversible (anabolic and catabolic).

• General Reaction:

Example: The enzyme hexokinase catalyzes the phosphorylation of glucose in glycolysis.

Body Water Content

Water is a major component of the human body, essential for physiological processes.

• Total Body Water: Typically about 60% of body weight in adults.

• Distribution: Intracellular and extracellular compartments.

Example: A 70 kg adult has approximately 42 liters of total body water.

Hydrophilic Substances

Hydrophilic substances are those that dissolve easily in water.

• Definition: "Hydro" means water; "philic" means loving.

• Examples: Salts, sugars, and many proteins are hydrophilic.

Example: Glucose is hydrophilic and readily dissolves in blood plasma.

Proteins: Mixed Proteins

Proteins can be classified based on their composition and function.

• Mixed Proteins: Contain both protein and non-protein components (e.g., glycoproteins, lipoproteins).

• Examples: Hemoglobin (contains heme group), glycoproteins (contain carbohydrate).

Example: Lipoproteins transport lipids in the blood and are mixed proteins.

Homeostasis and Negative Feedback

Homeostasis is the maintenance of stable internal conditions in the body.

• Negative Feedback: A control mechanism that reduces the output or activity of any organ or system back to its normal range.

• Importance: Essential for regulating variables such as temperature, pH, and blood glucose.

Example: Insulin secretion lowers blood glucose when it rises above normal.

Blood Glucose Regulation

Blood glucose levels are tightly regulated to ensure proper cellular function.

• Normal Range: Fasting blood glucose is typically 70-110 mg/dL.

• Regulation: Insulin and glucagon are key hormones involved.

Example: After a meal, insulin is released to promote glucose uptake by cells.

Summary Table: ATP Production in Cellular Respiration

Additional info: Table values inferred from standard textbook sources for one molecule of glucose.