Overview
This lecture provides a comprehensive overview of cellular respiration, including its stages, key reactions, and the production and role of ATP, as well as distinctions between aerobic and anaerobic processes.
Cellular Respiration Overview
- Cellular respiration converts glucose (C6H12O6) and oxygen (O2) into carbon dioxide (CO2), water (H2O), and energy (mainly ATP).
- The overall chemical equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
- Process is exergonic (releases energy); some energy is lost as heat, some stored in ATP.
Role and Structure of ATP
- ATP (adenosine triphosphate) is the main energy currency of the cell.
- ATP stores energy in unstable phosphate bonds, releasing energy when a phosphate group is removed (exergonic reaction).
- ATP is composed of adenine (nitrogenous base), ribose (sugar), and three phosphate groups.
- ATP formation from ADP is endergonic and occurs by phosphorylation.
Efficiency in Energy Use
- Direct use of glucose energy is inefficient; ATP allows controlled, smaller energy releases.
- Cells regenerate ATP from ADP using energy from glucose oxidation.
Stages of Cellular Respiration
- Four main stages: Glycolysis (cytosol), Pyruvate Oxidation (mitochondrial matrix), Krebs Cycle (mitochondrial matrix), Electron Transport Chain (inner mitochondrial membrane).
- Glycolysis: Glucose → 2 pyruvate; net gain of 2 ATP and 2 NADH; involves substrate-level phosphorylation.
- Pyruvate oxidation: Pyruvate → acetyl CoA; generates CO2 and NADH; catalyzed by pyruvate dehydrogenase.
- Krebs Cycle: Acetyl CoA → 2 CO2; per glucose: 6 NADH, 2 FADH2, 2 ATP produced.
- Electron Transport Chain: NADH and FADH2 donate electrons; O2 is the final electron acceptor; ATP produced by oxidative phosphorylation via chemiosmosis.
Enzymes & Reactions in Cellular Respiration
- Kinase: transfers phosphate groups.
- Isomerase: catalyzes molecular rearrangements.
- Dehydrogenase: removes hydrogen atoms and transfers electrons.
ATP Yield & Calculation
- Glycolysis: 2 ATP, 2 NADH (net, adjust for mitochondrial transport).
- Pyruvate oxidation: 2 NADH.
- Krebs Cycle: 2 ATP, 6 NADH, 2 FADH2 per glucose.
- Each NADH yields 3 ATP; each FADH2 yields 2 ATP in the electron transport chain.
- Maximum theoretical ATP yield per glucose: 38 ATP (often lower in practice due to inefficiencies).
Anaerobic Respiration & Fermentation
- Without oxygen (anaerobic), cells use fermentation after glycolysis.
- Lactic acid fermentation (muscles): pyruvate → lactate, regenerating NAD+.
- Ethanol fermentation (yeast): pyruvate → acetaldehyde → ethanol + CO2, regenerating NAD+.
- Both fermentations yield 2 ATP per glucose (from glycolysis only).
Practice Questions & Key Concepts
- Products of cellular respiration: CO2, H2O, ATP (not NADH, FADH2, or glucose).
- Glycolysis yields 2 ATP net.
- Krebs Cycle generates the most NADH per glucose.
- O2 is the final electron acceptor in aerobic respiration.
- Complex II is succinate dehydrogenase in the electron transport chain.
- ATP synthase produces ATP in oxidative phosphorylation.
- Kinase enzymes transfer phosphate groups.
- Substrate-level phosphorylation occurs in glycolysis and Krebs Cycle, not the electron transport chain.
Key Terms & Definitions
- ATP (adenosine triphosphate) — main energy carrier in cells.
- Oxidation — loss of electrons, gain of oxygen, or loss of hydrogen.
- Reduction — gain of electrons, loss of oxygen, or gain of hydrogen.
- Phosphorylation — addition of a phosphate group.
- NAD+/NADH, FAD/FADH2 — electron carriers in respiration.
- Glycolysis — breakdown of glucose to pyruvate.
- Krebs Cycle — series of reactions oxidizing acetyl CoA to CO2.
- Electron Transport Chain — sequence of proteins transferring electrons to O2, generating ATP.
- Chemiosmosis — ATP synthesis driven by proton gradient across a membrane.
Action Items / Next Steps
- Review glycolysis, Krebs Cycle, and electron transport chain locations and outputs.
- Memorize net ATP, NADH, and FADH2 yields per stage.
- Practice identifying oxidation vs. reduction in reactions.
- Complete assigned practice questions from the lecture.