Overview
This lecture covers the process of cellular respiration, detailing how cells extract energy from glucose to produce ATP, and outlines the stages, key molecules, and outputs of each step.
Cellular Respiration Overview
- Cellular respiration is the process of extracting energy from glucose to synthesize ATP, the cell's energy currency.
- The general equation is: glucose + oxygen → carbon dioxide + water + ATP.
- Cellular respiration is aerobic, requiring oxygen for efficient ATP production.
- The process yields about 36-38 ATP molecules per glucose, with CO₂ and H₂O as waste products.
Major Stages of Cellular Respiration
- Four primary phases: glycolysis, preparatory reaction, citric acid (Krebs) cycle, and electron transport chain.
- Glycolysis splits glucose (6C) into two pyruvate (3C) molecules in the cytoplasm, producing a net gain of 2 ATP and 2 NADH.
- Preparatory reaction converts pyruvate into acetyl-CoA, produces CO₂, and generates NADH.
- Citric Acid Cycle (Krebs cycle) occurs in the mitochondrial matrix, further breaks down acetyl-CoA, producing CO₂, NADH, FADH₂, and 2 ATP.
- Electron Transport Chain (ETC) occurs in the mitochondrial cristae, uses NADH and FADH₂ to generate 32-34 ATP via chemiosmosis.
Glycolysis: Inputs and Outputs
- Inputs: 1 glucose, 2 NAD⁺, 2 ATP, 4 ADP, 4 phosphate groups.
- Outputs: 2 pyruvate, 2 NADH, 4 ATP (gross), 2 ATP (net), 2 H₂O, 2 ADP.
- Glycolysis does not require oxygen (anaerobic).
Anaerobic Respiration (Fermentation)
- When oxygen is absent, pyruvate undergoes fermentation.
- In animals, forms lactate (lactic acid); in yeast, forms ethanol and CO₂.
- Fermentation yields only 2 ATP per glucose—much less efficient than aerobic respiration.
Preparatory Reaction & Citric Acid Cycle
- Prep reaction: Pyruvate enters mitochondria, is converted to acetyl-CoA, producing CO₂ and NADH.
- Citric Acid Cycle: Each acetyl-CoA is oxidized, yielding 3 NADH, 1 FADH₂, 1 ATP per cycle turn (2 turns per glucose).
- Outputs: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP per glucose (for 2 turns).
- All NADH and FADH₂ produced transfer electrons to the ETC.
Electron Transport Chain & Chemiosmosis
- NADH and FADH₂ donate electrons to protein carriers in the ETC, pumping H⁺ across the cristae membrane.
- A proton gradient (proton motive force) is created; H⁺ flows back through ATP synthase, generating ATP.
- Oxygen is the final electron acceptor, forming water with H⁺.
- Most ATP (32-34 per glucose) is made here via chemiosmosis.
Summary of ATP Yield
- Glycolysis: 2 ATP
- Citric Acid Cycle: 2 ATP
- Electron Transport Chain: 32-34 ATP
- Total: 36-38 ATP per glucose
- Some energy (~60%) is lost as heat.
Key Terms & Definitions
- Cellular Respiration — Process of breaking down glucose to produce ATP.
- ATP (Adenosine triphosphate) — Main energy carrier in cells.
- Aerobic — Requires oxygen.
- Anaerobic — Occurs without oxygen.
- Glycolysis — Splitting of glucose into pyruvate.
- NAD⁺/NADH & FAD/FADH₂ — Electron carrier molecules.
- Citric Acid Cycle (Krebs cycle) — Cyclic pathway that breaks down acetyl-CoA.
- Electron Transport Chain (ETC) — Series of proteins that create a proton gradient for ATP synthesis.
- Chemiosmosis — Movement of H⁺ ions to drive ATP synthesis.
- Fermentation — Anaerobic process to generate ATP and byproducts like lactate or ethanol.
Action Items / Next Steps
- Review the inputs and outputs of each stage of cellular respiration.
- Prepare for a detailed breakdown of each glycolysis step in the next lecture.
- Be ready to compare cellular respiration and photosynthesis in Chapter 8.