Detailed Notes on ATP Yield and Cellular Respiration

ATP Yield and Cellular Respiration

Theoretical Yield of ATP from Glucose

• Glycolysis:
  • Total yield: 2 ATP
  • Produces:
  • 2 NADH (equivalent to 6 ATP)
• Pyruvate Oxidation:
  • Produces:
  • 2 NADH (equivalent to 6 ATP)
• Krebs Cycle (Citric Acid Cycle):
  • Produces:
  • 2 ATP
  • 6 NADH (equivalent to 18 ATP)
  • 2 FADH₂ (equivalent to 4 ATP)
• Total Theoretical Yield: 36 ATP per glucose molecule

Actual Yield of ATP

• Actual ATP Yield: Approximately 30 ATP per glucose
• Reasons for discrepancy:
  • Proton leakage across mitochondrial membrane
  • H⁺ ions utilized in other cellular processes
  • Energy cost for transporting substrates (pyruvate, phosphate, ADP) into mitochondria

Controlling Aerobic Respiration

• Regulated by Feedback Inhibition:
  • Phosphofructokinase (PFK): an allosteric enzyme that:
  • Catalyzes third glycolysis step
  • Inhibited by high ATP levels, stimulated by high ADP levels
• Pyruvate Decarboxylase:
  • Catalyzes removal of CO₂ from pyruvate during oxidation
  • Inhibited by high levels of NADH, indicating high ATP output

Other Macromolecule Digestion

• Carbohydrates: Metabolized first for energy.
• Other macromolecules (proteins, lipids, nucleic acids) metabolized when needed:
  • Proteins: Amino acids enter glycolysis/Krebs after deamination (removal of amino group).
  • Lipids: Triglycerides broken down into glycerol and fatty acids; glycerol can enter gluconeogenesis or glycolysis, fatty acids undergo β-oxidation to form acetyl-CoA.

Anaerobic Respiration

• Glycolysis: Occurs without O₂, allows for limited ATP production.
• Fermentation: NADH converted back to NAD⁺ without O₂ to continue glycolysis:
  • Ethanol Fermentation (Yeasts):
  • Produces ethanol and CO₂ from pyruvate.
  • Lactic Acid Fermentation (Humans):
  • Produces lactate from pyruvate during intense exercise, later converted back to pyruvate.

Energy Transfer in Cellular Respiration

• ATP Formation:
  • Substrate-Level Phosphorylation: Direct ATP synthesis from ADP.
  • Oxidative Phosphorylation: ATP synthesized through redox reactions where O₂ is final electron acceptor.
• Energy Carriers:
  • NAD⁺ ↔ NADH and FAD ↔ FADH₂: Low-energy oxidized forms that become high-energy when reduced.

Overview of Aerobic Respiration

• Overall Reaction:
  ext{C}6 ext{H}{12} ext{O}6 + 6 ext{O}2
  ightarrow 6 ext{H}2 ext{O} + 6 ext{CO}2 + ext{Energy (ATP)}
• Stages:
  1. Glycolysis (Cytoplasm)
  2. Pyruvate Oxidation (Mitochondrial matrix)
  3. Krebs Cycle (Citric Acid Cycle) (Mitochondrial matrix)
  4. Electron Transport Chain & Oxidative Phosphorylation (Inner mitochondrial membrane)

Key Features of the Krebs Cycle

• Energy Yield:
  • Produces:
  • 2 ATP
  • 6 NADH
  • 2 FADH₂
  • Releases 4 CO₂
• Recycling of Oxaloacetate: Essential for the cycle to continue.

ATP Yield Breakdown**

• Glycolysis: 2 net ATP
• Krebs Cycle: 2 ATP
• ETC:
  • 2 NADH from glycolysis (converted to FADH₂) yield 4 ATP
  • 6 NADH from Krebs yield 18 ATP
  • 2 FADH₂ yield 4 ATP
• Total ATP: 30-36 per glucose, depending on efficiency and conditions.

Importance of NAD⁺ and Regeneration

• Vital for continuous cellular respiration; must be regenerated during aerobic respiration to prevent stoppage.
• Without oxygen, NADH cannot be oxidized back to NAD⁺, halting glycolysis.

Mitochondrion Structure and Function

• Double membrane organelle where aerobic respiration occurs:
  • Outer Membrane: Smooth, conducts similar functions to cell membrane.
  • Inner Membrane: Highly folded (cristae), hosts electron transport chain components.
  • Matrix: Site of Krebs Cycle and pyruvate oxidation.

Key Chemistry of ATP

• ATP Structure: Composed of adenine, ribose, and three phosphate groups.
• ATP hydrolysis releases energy, regenerating ADP and inorganic phosphate (Pi) for reusability.
• Cycle: ATP ↔ ADP + Pi, linking energy-releasing processes with energy-consuming reactions in cells.