Overview
This lecture explains the process of cellular respiration, detailing its stages, involved organelles, major products, and how energy (ATP) is produced in the cell.
Cell Structure & Mitochondrion
- Plant cells have cell walls and large vacuoles; animal cells have only membranes and small vacuoles.
- Both plant and animal cells contain mitochondria, the main site of cellular respiration.
- The mitochondrion has an outer membrane, an inner folded membrane (cristae), and a fluid matrix.
Cellular Respiration: Definition & Equation
- Cellular respiration is the breakdown of glucose to create ATP, the cell’s main energy molecule.
- General equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~38 ATP.
- Cellular respiration is the reverse of photosynthesis.
Stages of Cellular Respiration
- Four major stages: Glycolysis, Link Reaction, Krebs Cycle (Citric Acid Cycle), Electron Transport Chain (ETC).
Glycolysis
- Occurs in the cytosol (cytoplasm); first step in both aerobic and anaerobic respiration.
- Splits one glucose (6C) into two pyruvate (3C each), making 2 net ATP and 2 NADH.
- Has energy investment (uses 2 ATP) and energy harvesting (makes 4 ATP) phases.
Link Reaction
- Occurs in the mitochondrial matrix.
- Converts two pyruvates into two acetyl-CoA (2C), producing 2 CO₂ and 2 NADH.
- Prepares molecules for entry into the Krebs cycle.
Krebs Cycle (Citric Acid/TCA Cycle)
- Takes place in the mitochondrial matrix.
- Each acetyl-CoA combines with oxaloacetate (4C) to form citric acid (6C).
- Produces 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂ per glucose after two cycles.
Electron Transport Chain (ETC)
- Located on the cristae (inner membrane folds).
- NADH and FADH₂ donate electrons to protein complexes, driving H⁺ into the intermembrane space.
- H⁺ flow back via ATP synthase (chemiosmosis), making 32–34 ATP.
- Oxygen acts as the final electron acceptor, forming water.
Anaerobic vs. Aerobic Respiration
- Without oxygen, pyruvate undergoes fermentation to make lactic acid (in humans) or ethanol + CO₂ (in yeast).
- Fermentation regenerates NAD⁺ but makes no extra ATP.
- With oxygen, respiration proceeds to the mitochondrion for maximum ATP yield.
Summary of Products (per glucose)
- Glycolysis: 2 ATP, 2 NADH
- Link Reaction: 0 ATP, 2 NADH
- Krebs Cycle: 2 ATP, 6 NADH, 2 FADH₂
- ETC: ~34 ATP
- Total: ~38 ATP (varies with literature, may be lower).
Key Terms & Definitions
- ATP (Adenosine Triphosphate) — main energy carrier in cells.
- Glycolysis — splitting glucose to make pyruvate, ATP, NADH.
- Pyruvate — 3-carbon end product of glycolysis.
- NADH/FADH₂ — electron carrier molecules used in ETC.
- Krebs Cycle (Citric Acid Cycle) — cycle that processes acetyl-CoA to CO₂, producing NADH, FADH₂, ATP.
- Electron Transport Chain (ETC) — series of proteins that use electrons to create ATP.
- Chemiosmosis — movement of H⁺ ions to generate ATP via ATP synthase.
- Oxidative Phosphorylation — process of making ATP using energy from electrons and oxygen.
Action Items / Next Steps
- Review mitochondrial structure and function.
- Practice writing and balancing the equation for cellular respiration.
- Study details of each respiration stage and the role of electron carriers.
- Prepare for questions on aerobic vs. anaerobic pathways.